Advances in Marine Biology, Volume 8

Advances in

MARINE BIOLOGY VOLUME 8

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Advances in

MARINE BIOLOGY VOLUME 8 Edited by

SIR FREDERICK S. RUSSELL Plymouth, England

and

SIR MAURICE YONGE Edinburgh, Scotland

Academic Press

London and New York

*

1970

ACADEMIC PRESS INC. (LONDON) LTD. BERKELEY SQUARE HOUSE BERKELEY SQUARE LONDON, W l X 6 B A

U S .Edition published by ACADEMIC PRESS INC.

111 FIFTH

AVENUE

NEW YORK, NEW YORK 10003

Copyright

0 1970 by

Academic Prass Inc. (London) Ltd.

All rights reserved

NO PART O F THIS BOOK MAY B E REPRODUCED I N ANY FORM BY PHOTOSTAT, MICROFILM, OR ANY OTHER MEANS, WITHOUT WRITTEN PERMISSION FROM T H E PUBLISHERS

S B N : 12-026108-1 Library of Congress Catalog Card Number: 63-1 4040

PRINTED IN GREAT BRITAIN B Y THE WHITEFRI4RS PRZSY LTJ). LONDON AND TONBRIIJGE

CONTRIBUTORS TO VOLUME 8 GORDONA. RILEY,Institute of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada. C. F. HICKLING, 95 Greenway, London N 2 0 , England.

A. NELSON-SMITH, Department of Zoology, University College of Swansea. Swansea. Wales. KOHMANY . ARAKAWA, Hiroshima Fi&eries E~perimentaZ Station, Ondo, Hiroshima, Japan.

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CONTENTS CONTRIBUTORSTO VOLUME8

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Particulate and Organic Matter in Sea Water GORDONA. RILEY

I. Introduction

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11. Distribution of Particulate Organic Matter in the Sea .. .. .. .. .. A. Methods .. B. Visual Examination of Non-living Particulate Matter C. Particulate Organic Carbon . . .. .. .. I .

111. Chemical Composition .. A. Elementary Composition B. Biochemical Composition

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C. Relationships of Organic and Inorganic Materials..

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IV. Experimental Studies of Non-living Particulate Matter. . A. Adsorption on Bubbles .. .. .. .. €3. I n situ Production and Utilization . . .. .. .. .. ,. .. C. Sinking Rates . . ..

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.. .. .. .. V. General Discussion . . .. A. Re-examination of the Aggregation Process .. B. Dynamics of Production and Consumption of Nonliving Particulate Matter in the Sea .. ..

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VI. References

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40 49 66

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79 112

viii

CONTENTS

Estuarine Fish Farming C. F. HICKLING

1. Introduction . . .. .. .. .. .. A. From Primitive Fish-corral t o Fish Farin . . B. Definition of Estuaries as Sites for Fish Farms

II. Suitability of Estuaries as Sites €or Fish Farms . . A. Their Topography .. .. . . .. B. The Fertility of Estuaries . . .. .. 111. The Species Cultured .. . . .. A. Selection of Organisms by Salinity . . B. The Natural History of the Species..

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IV. Sources of Young Fish and Prawns .. A. Non-breeding and Artificial Breeding B. The Fish Fry and Prawn Fry Industry C. Care and Rearing of Fry .. ..

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V. The Food of Cultivated Fish and Prawns.. .. . . 164 A. Chams, Grey Mullet, Eels, Tilapia and Prawns . . 164 B. Culture of Algal Pasture . . .. .. .. 170 Vl. Management of Brackish-water Fishponds A. Tilapia . . .. .. .. .. B. Russian Limans .. .. .. C. Hawaiian Fishponds . . .. .. D. Brackish-water Ponds at Arcachon . . E. North Italian Lagoons .. .. F. Bheris of West Bengal .. .. G. Prawn Ponds of Singapore . . .. H. Fish and Prawn Culture in Paddy Fields

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178 180 181

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CONTENTS

I. Chanos Culture in Java .. .. J. Chanos Culture in the Philippines . . K. Fishpond Management in Japan . .

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VIII. Profitability . . .. .. .. .. A. Need for a Large Pond Area.. .. B. The State of the Industry-Progress, . . .. . . and Decline . . C. Need for Long-term Credits . , ..

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199

L. Fish Culture in Taiwan

VII. The Rate of Fish Production

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Stagnation

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111. Properties of Petroleum Oils .. .. * . A. Physico-chemical Characteristics . . B. Behaviour of Spilt Oil on Sea and Shore . . C. Detection and Identification.. .. ..

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Effects of Oil Pollution .. .. .. A. Mode of Action and Toxicity of Oils

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IX. Is There a Future for Estuarine Fish Farming! X. References . . .. .. . . 1 .

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The Problem of Oil Pollution of the Sea A. NELSON-SMITH

1. liitroductioii

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11. Sources and Control . . . . .. .. A. Tanker Operation and General-cargo Shipping B. Harbours and Marine Terminals .. .. C. Coastal Industry and Other Sources .. a

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234 236 240 243 243

B. C. D. E. V.

Effects on Marine Communities .. . . .. Carcinogenesis . . .. Rehabilitation of Oiled Birds .. Public Amenity and the Tourist Industry

Removal of Spilt Oil..

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A. Bacterial Degradation and Other Biological Processes 266 B. Dispersal, Sinking and Recovery at Sea . . .. 271 C. Problems in Cleansing Shores .. .. .. 274 D. Mode of Action and Toxicity of Solvent-emulsifiers 280

VI. Conclusions and Prospects . . VII.

References

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288

Scatological Studies of the Bivalvia (Mollusca)

KOHMAN Y. ARAKAWA

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309

111. Morphology . . .. .. .. .. A. Classification of Faecal Pellets .. B. Definition of Pellet-Types . . .. C. Descriptions of Faecal Pellets .. I. Protobranchia (Gastroproteia) 11. Septibranchia (Gastrodeuteia) 111. Gastrotriteia . . .. .. IV. Gastrotetartika .. .. V. Gastropempta . . .. ..

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Introduction

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11. Material and Techniques

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317 320 334

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IV. Biological Significance of the Characteristic Form of .. .. .. .. .. . . 379 Faecal Pellets A. Relation of Faecal Characteristics to the Feeding Habit and Mode of Life of the Animal . . . . 379 B. Relation of Faecal Characteristics to the Structure and Function of the Digestive Organs . . .. 381 V. Use of Faecal Pellets as a Systematic Index

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VII. Biodeposition of Suspension Feeding Bivalves . . .. A. Factors Influencing Biodeposition .. .. B. Daily and Seasonal Aspects of Biodeposition . . C. Effects of Biodeposition upon Various Marine .. .. .. Environmental Conditions

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VI. Evolutionary Trends of Faecal Pellets

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TAXONOMIC INDEX ..

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CUMULATIVEINDEX OF AUTHORS. .

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VIII. Bibliography IX. Plates

AUTHOR INDEX

SUBJECTINDEX

CUMULATIVE I N D E X O F

TITLES

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facing page 436

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Arlu. mar. Biol., Vol. 8, 1970, 1-118

PARTICULATE ORGANIC MATTER IN SEA WATER GORDONA. RILEY Institute of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada

.. .. I. Introduction . . . . .. .. .. .. .. .. .. 11. Distribution of Particulate Organic Matter in tho Sea .. .. .. A. Methods .. .. .. .. .. .. .. .. .. T3. Visual Examination of Non-living Partirulnto Matter .. .. C. Particulate Organic Carbon .. .. .. .. .. .. 111. Chemical Composition . . .. .. .. .. .. .. .. A. Elcrncntary Composition .. .. .. .. B. Uiochemical Composition. . . . .. .. .. .. . . . . .. C. Relationships of Organic and Inorganic Materials .. .. .. IV. Experimontal Studies of Non-living Particulate Matter .. A. Adsorption on Bubbles . . . . . . .. .. .. . . B. In situ Production and Utilization .. .. .. .. .. .. C. Sinking Rates .. .. .. .. .. .. . . . . .. V. General Discussion .. .. .. .. .. .. A. Re-examination of tho Aggregation Process . . .. . . B. Dynamics of Production and Consumption of Non-living Particulate Matter in the Sea .. . . .. .. .. . . .. VT. References .. .. .. . . .. .. .. .. ..

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7 12 29 29 32 36 40 40 49 66

75 75

79 112

I. INTRODUCTION The non-living organic matter in sea water is ultimately derived from a variety of plant sources, including phytoplankton, attached aquatic vegetation in shallow waters, macroscopic pelagic algae, and waterborne and windborne materials of terrestrial origin. The relative importance of these various sources cannot be stated precisely, but there is little doubt that phytoplankton is the most important source. Phytoplankton production, as determined by C14 fixation, falls within the range of 50-15Og of carbon per m2 in a year in the major ocean basins ; however, some regions, notably the high arctic, are less productive than this and higher levels are attained in some coastal and estuarine waters and probably in oceanic areas of intense upwelling. Recent work suggests that present methods may underrate total 1

2

GORDON A. RILEY

production, but the range that has been given establishes a reasonable order of magnitude. Benthic algae and rooted aquatic vegetation of the shore zone can have a level of production that is an order of magnitude higher than the values given for phytoplankton, and the output from rivers and salt marshes can be very important locally. However, the area where such factors are important is less than 1% of the world oceans, and there is little doubt that phytoplankton provides 90% or more of the basic stock of oceanic organic matter. Although much of the organic matter is used quickly, the accumulated remains of non-living organic matter, particulate and dissolved, are larger than the annual production and much larger than the quantity of living organisms that is present a t any one time. Most of the published data for particulate organic carbon, as determined by passing the water through a filter with a pore size of 1 p or less and analysing the collected material for its carbon content, fall in the general range of 5-50 pg C/litre in deepwater samples and are somewhat higher in the surface layer. A reasonable average for total filterable carbon, summed from surface to bottom in an average water column of 4 000 m would be 100 g/m2. Filter-passing organic substances, commonly denoted as the dissolved fraction but obviously including colloids and any particles small enough to escape the filter, commonly have a carbon content of the order of 0.5 mg/litre as determined by wet oxidation, so that the total quantity in a water column of 1 m2 is about 2 kg. This is roughly two orders of magnitude higher than the amount present a t any one time in living organisms, and this may be a minimum estimate, for there are indications (Skopintsev, 1966) that dry combustion methods yield somewhat larger values. The fact that these organic remains have accumulated in such large quantities, even though most of the products of photosynthesis are consumed quickly by living organisms, is sufficient evidence that they have a slow rate of degradation. The reasons for this are not altogether clear, for biochemical characterization of the material has not proceeded far enough t o give us a detailed picture of composition. We know that some of the dissolved substances are highly refractory and have little biological utility. We also know that there are simple sugars, amino acids, and other substances which are potentially usable ; however, except in near surface waters the concentration of such substances appears t o be too low to provide anything more than bare subsistence for a scanty population of heterotrophs. The situation is similar with respect t o non-living particulate matter. The total stock is large, and some of it is assimilable; however, the

PAItTICULATE ORGANIC MATTER I N YEA \\ATER

3

concentration appears to be too low t o fulfill the requirements of pelagic filter feeding organisms in most of the deepwater column. Particles that sink to the bottom, and are thereby concentrated at the water-sediment interface, supply nourishment to a benthic population of bacteria and animals which is considerably more abundant than the bathypelagic assemblage. One can find many references in the early literature to the presence of abundant detritus in the sea and speculation about its possible utility as food. However, little serious attention was given to the nonliving fraction until the 1950s. Nishizawa et al. (1954) published an account of their observations of particulate matter from a submerged diving chamber and aroused considerable interest and curiosity by demonstrating that some of it was aggregated into much larger particles ,than had previously been described. Bathyscaphe divers reported similar flakes of " marine snow " in various oceanic areas. At that time no one had as yet questioned the traditional concept that nonliving particulate matter was derived by decay of dead organisms, but these observations demonstrated that it could clump into masses of considerable size rather than merely disintegrating into smaller fragments, and this stimulated further interest in the possible utility of this material to filter feeding organisms. Krey et ul. (1957) described a newly developed method for protein analysis and measurement of seston weights, and a series of papers followed, describing their results in various oceanic regions. Riley (1959) made a few measurements of particulate matter in Long Island Sound, including total seston weight, difference on ignition, and estimates of phytoplankton biomass. Some of this was fairly closely comparable t o the kind of data being collected by Krey and his associates. Baylor et ul. (1962) and Sutcliffe et al. (1963) discovered that organic particles could be formed experimentally by bubbling air through sea water. Baylor and Sutcliffe (1963) demonstrated that particles so formed would support the growth of Artemia. Riley (1963), in a further study of Long Island Sound, found particles in natural sea water which were similar in appearance t o the experimental product. All this began to cast serious doubt on the idea that the particulate and dissolved organic fractions in sea water are simple intermediary steps in the degradation of living matter into inorganic elements. Reversals in the process seem to be possible, and newly created particles might re-enter the food chain as bacterial substrates or food for filterfeeding animals. New data and new theories as to their meaning promoted further

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CORDON A. RILEY

work on this subject but did not lead to any real solution of the major problems. The validity of the experimental evidence on particle formation has been challenged. If such particles can indeed be formed in the sea, their natural rate of production remains essentially unknown. Opinions differ as to the suitability of non-living particulate matter as food. Menzel and Goering (1966) took the extreme stand that naturally occurring particles below the immediate surface layer are neutrally buoyant and refractory, neither sinking out of the water column nor being utilized. Riley et al. (1965), on the other hand, proposed the hypothesis that the particulate matter sinks slowly, with a mean residence time in the water column of the order of four years, more or less, and is subject during that time to both utilization and accretion, the two processes existing in a state of dynamic balance. One interpretation or the other seems necessary in order t o explain the observed fact that the concentration of particulate matter is relatively uniform with respect to depth and shows no evidence of decrease in deep water due to biological utilization. The paper that follows will attempt t o resolve some of these difficulties. It will survey existing data on the distribution of particulate organic matter in the sea, experimental work that has been done on processes of production and consumption, and the indirect evidence that is necessary in order t o put the subject into proper oceanographic perspective. The discussion will mainly deal with non-living particulate matter but will of necessity also be concerned with its relations with living organisms and filter-passing organic materials.

11. DISTRIBUTION OF PARTICULATE ORGANICMATTERIN

THE

SEA

A. Methods This section will deal with generalities about methods of collection and analysis. Details about the methods used by individual investigators will be taken up in later sections in connection with the discussion of their work. Measurements of particulate organic matter and studies of its chemical composition require the collection of fairly large water samples. Our laboratory uses Niskin bottles of 8 and 28 litre capacity, constructed of polyvinyl chloride. Menzel (1967) has described an all glass sampler which he built t o avoid any possibility of contamination by plastic materials. However, Gordon (1969) has used both kinds of bottles concurrently and has found no significant difference in the results. Collecting bottles need t o be cleaned frequently. There is a tendency for a bacterial slime film to grow on the inner surfaces, and

PARTICULATE ORGANIC MATTER IN SEA WATER

5

this can cause serious errors. Scrubbing and rinsing with isopropyl alcohol are recommended. Various kinds of filters have been used in this work. For purposes of microscopic examination Millipore filters ars commonly used. A sea water sample of 50-250 ml is filtered ; the pad is then dried and cleared and mounted with cedar oil or Permount. Scrupulous cleanliness is required. All glassware should be rinsed with isopropyl alcohol immediately prior t o use, and distilled water or any stains used in the preparation should be pre-filtered. Most distilled water supplies contain considerable quantities of organic matter which can be converted, on standing, into flake-like particles indistinguishable from some of the materials naturally occurring in sea water. The type of work done by Gordon (1970a), using histochemical stains t o characterize the kinds of particles found in the sea, requires fairly extensive treatment with reagents and rinses. He found it necessary to monitor the operation by running frequent reagent blanks and correcting his counts, if necessary, for the presence of any extraneous particles. Millipore filters also have been used frequently for any purpose requiring the water to be freed of filterable particles. Various pore sizes have been used, ranging from 0.22-5 p. If the filtered water is to be used for experimental purposes, the filter should be pre-rinsed. Glass fibre and more recently silver filters have been used to collect material for measurements of total carbon or any other analysis requiring a filter that is relatively free of organic matter. Actually neither type is ideal. They contain carbon that cannot be entirely removed by baking a t the relatively low temperatures that they can tolerate (about 40O0C), and in some cases baking has produced an undesirable increase in the pore size of silver filters. Experience has shown that 0.8 p silver filters contain less carbon than most of the others, and they may be used without baking. However, there is enough variation from one batch to another so that blank determinations on each batch are necessary, and fairly large water samples should be used in order to minimize errors due to variability of the blanks. Silver filters, like the Millipore filters, have the advantage of graded pore size, whereas the glass fibre filters have pores of varying sizes. However, the writer has found no marked difference in the total catch obtained by a fine glass filter as compared with 0.46 p silver filters, nor did Gordon (1969) obtain significant differences when he filtered aliquots of sea water samples through silver filters of different pore sizes. Sheldon and Sutcliffe (1969) examined this situation further, using

a Coulter Counter to determine the spectrum of particle sizes before and after filtration. They found that filters removed almost all of the particles larger than a given pore size, but they retained a considerable fraction of smaller particles that theoretically could have passed through. Thus the filters are not suitable for quantitative size fractionation. This result helps t o explain the observation (Menzel, 1966 ; Batoosingh et al., 1969) that immediate refiltration of a sea water sample will give an additional yield, presumably consisting of particles smaller than the pore size of the filter, which are retained imperfectly but to a measurable degree during subsequent filtrations. Menzel, in the paper cited and in subsequent ones, also suggested the possibility of adsorption of dissolved organic carbon, but this seems unlikely. P. J. Wangersky (personal communication) filtered sea water through a dialysis membrane which removed all organic materials except dissolved substances with a molecular weight of less than 10 000. He then passed the water through silver filters and found no significant increment in organic carbon; however, there was an indication of a slight amount of adsorption on glass fibre filters. After filtration the samples are kept in a dried or frozen state until analysis. Both wet oxidation and dry combustion have been used in the determination of total organic carbon. For a detailed description and discussion of these methods the reader is referred to Wilson (196l), Menzel and Vaccaro (1964) and Strickland and Parsons (1965). Strickland and Parsons also described methods for determining protein, lipid, carbohydrates and crude fibre with sufficient sensitivity to be used in the analysis of naturally occurring particulate matter. Krey et al. (1957) developed a method for determining protein in particulate matter by means of the biuret reaction. They then used a conversion factor t o estimate the quantity of living material. The difference between total dry weight and the calculated biomass was then regarded as detritus. This method, with some later elaboration, has been used widely by Krey and his co-workers and provides useful comparative data in the various regions studied. However, this work cannot be compared directly with the data of other investigators, for there is some protein in non-living particulate matter, and the detritus is by no means exclusively organic material. In addition to these general and widely used methods, there will be references in a later section t o various biochemical studies of a more limited nature, and the reader is referred to the original literature for the methods used.

PARTICULATE ORGANIC MATTER I N SEA WATER

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B. Visual exam,ination of non-living particulate matter 1, General appearance Fig. 1 shows some common types of particles found in sea water. The most obvious ones, particularly in the surface layer, are amorphous masses of material, which will be familiar to everyone who has examined sea water samples. They have commonly been called detritus, although identifiable plant or animal remains are only occasionally evident, and generally as small inclusions rather than constituting the main mass of material. They appear to be flocculated masses of smaller particles rather than decomposing bodies and are similar in general appearance to bacterial slime films that are found growing on submerged surfaces. Riley (1963) called this material organic aggregates, a term that was purposely vague as to mode of origin but served to distinguish them from detritus of identifiable structure. As seen in this figure the aggregates have been flattened on the slide and dried, and they appear more consolidated than they really are. They have been observed in a free floating state in the chamber of an inverted microscope, where they appear as more or less rounded masses. They are delicate and tenuous, with internal spaces that harbour a flora of bacteria, small algae and sometimes protozoa. Pomeroy and Johannes (1968) have provided an excellent account of this assemblage of organisms, which they designated as ultraplankton. One suspects that these microcosms might have quite a different environment from that of the free water, although the quantity of organisms is not large enough t o suggest a rich supply of nutrients. The median diameter of these aggregates in most sea water samples is of the order of 25-50 p. However, there is a wide spectrum of sizes. Particles of one millimetre or more in diameter are not uncommon, and on rare occasions sheets of material and slimey strings of the order of centimetres long have been observed in tide rips. Gordon (1970a) reported that the aggregates stain heavily with Schiff reagent and apparently consist mainly of carbohydrate material. There was virtually no reaction t o lipid stains, and the protein reaction (brom phenol blue reagent) was variable and generally light, except in associated living organisms. Fig. 1 also shows some small, semi-transparent flakes, which are important components of the non-living particulate matter although much less obvious than the aggregates, Indeed the writer has found no reference t o them in the literature prior to his account (Riley, 1963) of their natural occurrence in sea water. They may have escaped the notice of earlier workers or may have been regarded as inorganic

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GORDON A . RILEY

FIG.1. Millipore filtor preparat'ions of part,iculatc organic matter, magnification x 200. (a) Surface sample, Nova Scotia coastal water, with two amorphous aggregates above and flakes below, indicatcd by arrows. (b) A similar surface sample treated with brom phenol blue (protein stain). Flakes show typical deep st,aining reaction. (c) A mid-depth collection (850 m, Slope Water south of Nova Scotia), also treat<ed with protein stain. (d) Particle format,ion in fi1t)eredsea water (carbohydratc stain). The aggregations that can be seen dimly in several parts of the slide are of the type charact,erizecl(section IV, B) as '' pale primordia *' of naturally occurring aggregat,es. There arc numcrons smaller particles which do not show clearly becrtuso of t>hofiltor background.

PARTICULATE ORGANIC MATTER I N SEA WATER

9

inclusions. I n fact the writer did not include them in routine counts until it became apparent that they were visually indistinguishable from experimentally produced organic particles of the type which Baylor and Sutcliffe (1963) demonstrated to be adequate food for the growth of Artemia. Later Gordon (1970a) showed that they react intensely to protein and carbohydrate stains. His counts of stained material were invariably larger than those of unstained aliquots. Some of this material is so nearly transparent that it eludes detection except by staining treatment. The median size of the flakes is about 25 p in their longest dimension. The size range is not large, and few of them are more than 50 p. Gordon categorized two types of larger particles that are found occasionally, which exhibited the same kind of staining reactions as flakes but otherwise were somewhat different in appearance. One type was long and narrow; the other resembled a clump of smaller flakes. It is still an open question as to whether they are different in any really significant way and whether the clumping is a natural phenomenon or an artifact of preparation. Aside from the aggregates and flakes, most samples contain a number of minor organic components such as more or less recognizable remains of phytoplankton and zooplankton, fibres, occasional faecal pellets, and starch grains. Bursa (1968) has described the starch grains that he has found in sea water and plankton samples, sometimes in great abundance a t Arctic stations, with which he was particularly concerned. Similar material was found a t inshore locations in tropical and subtropical waters. The writer has occasionally seen identifiable starch grains in offshore water although never in abundance. However, some of Bursa’s photographs of semi-hydrolyzed grains suggest that there might be difficulty in distinguishing between starch and flakes in some cases unless specific staining procedures are used. The problem of identifying non-living particles is certainly not easy, and further study is in order. Small particles of the order of 5 p down t o the limits of visibility are numerically more abundant than the larger particles. Many of them show organic staining reactions, but visual examination gives little clue as to their origin. Traditionally they have been regarded as breakdown products of larger remains. However, Sheldon et al. (1967) documented spontaneous formation by aggregation of smaller entities, and Chave (1965) has detected coatings of adsorbed organic matter on particles of calcium carbonate. Conceivably other minerals could be similarly coated. Thus the small particles probably are of diverse

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GORDON A . RILEY

origin and composition, but their appearance is not distinctive enough t o encourage attempts t o classify them. 2. Seasonal cycles in inshore waters

Riley (1963) described a two-year study of particulate matter in Long Island Sound based largely on counts and measurements of particles. The results were quite similar to earlier work by Krey (1961) whose examination of the seasonal cycle in the Kiel Bight consisted mostly of protein analyses and seston weights. The following generalizations apply t o both surveys : the level of non-living particulate matter was moderately high in midwinter, when the quantity of phytoplankton was small. It increased further to a peak that more or less coincided with the peak of the spring diatom flowering and thence declined to the lowest point of the year a t about the time of the termination of the flowering. The quantity was relatively small during most of the summer and autumn. The only major difference in the two seasonal cycles was an early summer peak in Long Island Sound, a period of the year when the quantity was low a t Kiel. Within the traditional view that non-living organic matter consists mainly of detritus derived from the death of organisms, certain aspects of these results would be puzzling. Lower values would be expected during the midwinter period of phytoplankton poverty, and increases associated with phytoplankton flowerings should be most pronounced during the terminal stages of the flowering rather than near the peak. However, these new developments cast another light on the problem. An increase coincident with the flowering would be expected if extracellular metabolites could be converted to particles, and mixing processes associated with winter storms might have been responsible for the observed increase in particulate matter during early winter. There was some evidence to support these hypotheses. The number of flakes was larger in winter than in summer, and the yield of particles obtained by bubbling filtered sea water more or less followed the seasonal trend of naturally occurring seston during winter and spring. However, in the absence of any really good methods for determining rates of particle production in nature such hypotheses remain speculatory. 3. Particle counts in offshore waters

The methods developed for counting and measuring particulate matter were subsequently applied t o a series of surface collections obtained on a transect from the Sargasso Sea south of Bermuda to the coast of Africa a t about SON (Riley et al., 1964) and a series of surface

PARTICULATE OKGANlC MATTER I N SEA IVATEIL

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and deepwater samples taken off Bermuda on a more or less seasonal basis (Riley et al., 1965). I n the first paper cited the general range in particle number was 5-40 x 103 per litre. Lowest values were found in the Sargasso Sea and non-upwelling tropical areas ; the highest ones in a region of upwelling in the Guinea Current. There the number of particles intergraded with the general range of 30-100 x lo3 per litre which had been found in Long Island Sound. I n all of these studies the measurements of particles size were used in conjunction with counts to determine their total area as they appeared on the filter in mm2/litre. This was only a crude measurement and could not be converted into an estimate of the volume of material ; however, a measurement of this type was regarded as a necessary precaution against the possibility of variations in mean size of particles from one area to another. I n this set of data the general range of values was 6.3 t o nearly 40 mm2/litre as compared with about 20-160 mm2 in Long Island Sound. From these data on numbers and areas it is apparent that non-living particulate matter varies regionally in much the same way that phytoplankton does, and in fact in the subtropicaltropical transect there was a correlation of 0.95 between area of particulate matter and phytoplankton cell counts. The regression equation was

A

=

0.127C

+ 8.0

where A = organic aggregates in mm2/litre, and C = thousands of phytoplankton cells/litre. The form of the equation is indicative of the fact that even when phytoplankton is scarce there will be a considerable amount of non-living particulates present. The additional amount that is correlated with phytoplankton abundance may be detritus derived from dead cells or it might be particles that have been produced from extracellular metabolites. The latter point was examined experimentally by growing cultures of seven species of diatoms and one dinoflagellate in artificial sea water of minimal organic content and then removing the cells and bubbling the medium. The particulate carbon produced by the bubbling process averaged 20% of the carbon content of the diatoms that had been removed ; however, in the case of the dinoflagellate it was only 2y0. Riley et al. (1965) described six sets of observations obtained on a seasonal basis during the period from April 1963 t o April 1964. Three sets were obtained a t a station about 27 km southeast of Bermuda, a position routinely occupied by various oceanographers working at the Bermuda Biological Station. The other three sets were cruise data taken over a wide area and ranging between 21' and 36"N Lat.

12

G U S D O N -4. RILEY

Mean particle counts in all samples were as follows : surface 11.32 x 103/litre; 100-200 m, 9.20 ; 500-900 m, 7.51 ; 1 000 m 7-86. Measurements of the area of the aggregates and of particulate carbon gave a similar picture of vertical distribution, with a decrease in the upper layers and only slight and random variations in the water below about 500 or 800 m. There was some seasonal variation in particle counts and areal measurements, with largest values in winter and spring, more or less corresponding with seasonal variations in phytoplankton in that area. There was also a seasonal variation in deep water. Much of the discussion in the paper cited was devoted to an attempt to rationalize these deep water seasonal dhanges and the fact that there is not a systematic decrease in particulate matter with depth. Continuation of these discussions is relegated to later sections of this paper. Gordon (1970a) refined the earlier methods for microscopic investigation and expanded the regional coverage of the North Atlantic. He developed methods for cleaning the filtering apparatus and making blank determinations, because it became apparent that contaminants could be involved which were indistinguishable from naturally occurring organic particles. He also used histochemical stains, the mercuric bromphenol blue method of Mazia et al. (1953) for proteins and the Schiff reagent for carbohydrates. Gordon obtained significantly higher counts with stained slides than with unstained material. The flakes, in particular, are often so nearly transparent that they can be missed in routine counts. Therefore his counts are probably more accurate than previous ones but are not directly comparable with those in earlier work. Gordon’s sampling areas ranged widely from the Sargasso Sea to the Irminger Sea. Mean particle numbers were of the general order of 100/ml in near surface waters, and 12-27/ml in deep water. The Nova Scotia and Newfoundland shelf and slope areas were somewhat richer than offshore waters, as might be expected, but regional differences were not pronounced.

+,

C. Particulate organic carbon 1. Regional variations and vertical gradients Wangersky and Gordon (1965) obtained extensive data on particulate organic carbon in subtropical and tropical waters along two transects between the coast of Africa and a point several hundred miles southeast of Bermuda. This was a cruise of the “ Trident ” in 1963, during which the material for particle counts described by Riley et al. (1.964) was also collected.

PARTICULATE ORGdNIC MATTER IN SEA WATER

13

Wangersky and Gordon demonstrated that there was a major concentration of particulate carbon in the immediate surface layer, a t least under the calm conditions prevailing during most of their cruise. Surface bucket samples taken in the Guinea Current had concentrations ranging from 300 t o 800pgC/litre, in contrast t o about 55-2OOpg C/litre in water bottle samples from depths of 13-15 m. I n less productive areas most of the bucket samples had values of 20-120 pg C/litre. Slightly deeper collections, some with the bottles barely submerged or a t most a t a depth of 10 m, were of the order of 14-58 pg. Williams (1967) developed methods for collecting a thin layer of water near the surface and compared organic carbon and various other determinations in these samples with subsurface concentrations. His work confirmed and extended previous conclusions as to the existence of a large vertical gradient in near surface waters. There is of course a tendency for bacteria, nanoplankton and non-living particulate matter to become adsorbed a t the surface film, although pronounced vertical turbulence tends t o disrupt the film and drive some of this material into deeper water. A certain balance between these processes presumably is responsible for the observed vertical gradient. The observations that have been described were obtained mainly in calm weather, because surface film collection becomes increasingly difficult in rough water. Hence such observations represent an extreme development of near surface gradients. However, later evidence will indicate that there is a measurable and biologically important accumulation of particulate matter a t the sea surface a t higher sea states. Most of the investigations of organic carbon in the surface layer of the sea have employed serial casts of various kinds of collecting bottles, with the uppermost bottle a t a depth of a metre or more, so that surface film effects were slight or negligible. These investigations have not been extensive enough yet t o provide a clear picture of either regional or seasonal variations ; however, the available information should be summarized in order to give a general idea of areal coverage and degree of variation. Menzel and Ryther (1964) described some observations obtained in January 1962 between 41" and 37"N Latitude in the western Atlantic, which thus included both temperate and subtropical waters. Another cruise in April of the same year obtained a line of stations between 42'30" and 20"N along the 65"W Meridian. Their Fig. 1, which apparently includes only observations north of the Gulf Straam and includes all data taken in the upper l o o m , shows a total range in January of about 25-200 pg C/litre, with a large majority of observations between 35 and 95 pg. I n April the general range was of the

14

GORDON A. RILEY

order of 53-330 pg C/litre, with about two-thirds of the observations between about 110 and 220 pg. Riley et al. (1965) took about 20 near surface sampIm in the vicinity of Bermuda. The average of March-April samples, which probably represented spring flowering conditions, was 168 pg C/litre, and averages obtained during other seasons ranged from 46 to 111 pg C. Gordon (1970b) compiled data for some 60 surface layer analyses in various parts of the N. Atlantic. I n the Irminger Sea in April-May 1966, the mean of 11 samples was 44 pg C/litre. I n April-May 1967, a line of stations across the temperate N. Atlantic a t approximately 40"N averaged 87 pg. Three cruises were made t o Nova Scotia and Newfoundland shelf and slope waters, and cruise averages ranged from 41 t o 176 pgllitre, the largest values being obtained in April 1968. Five cruises t o the Sargasso Sea yielded averages of 29-142pg, and again the largest values were in April. A few surface water values have been reported for other oceans. I n general they fall within the same range and are similarly variable. One report of considerable interest is that of Szekielda (1967), whose study of the Somali Current off the east coast of Africa revealed a series of bands of high and low carbon content running parallel with the coast. These were believed to be associated with localized upwelling. I n areas where the temperature and salinity structure were indicative of vertical water movement there was an upward thrusting of mid-depth water of relatively low carbon content. I n the immediate surface layer the situation was less clear cut. Theoretically there wilI be a low particle content in newly upwelled water, but this will give way to high values as phytoplankton develops in the new water. Szekielda's data suggest that both phenomena may have been present in different parts of his profiles. This is clearly a case where one needs to know something about relative proportions of living and non-living matter in order t o understand the data on total carbon. Below the immediate surface layer there is almost invariably a decrease in total particulate carbon. The gradient may be quite sharp and of limited vertical extent or it may continue more gradually through most of the main thermocline, but a t some depth of the order of 200-800 m an essentially deep water situation is reached in which carbon concentrations generally are low and relatively uniform, although in some cases pronounced increases have been noted in association with particular water masses. However, there is little or no systematic decrease with depth. I n deep water a large proportion of the total organic carbon is non-living, so that quantitative assessment of that fraction is simpler

PAltTICULATE ORGANIC MATTER 1N S E A WATER

15

than in the sur€ace and transition layers. Also, the quantity found in any one area does not ordinarily vary seasonally, although certain exceptions will be noted, so that regional comparisons are possible, even though the amount of data available for the purpose is still fairly small. Thus the deep water observations deserve a more thorough analysis than was possible in the case of surface layer data. Most of the papers cited above in connection with surface observations also included deepwater data, and a few others are included in

'

+

+ 5 0

FIG.2. Deepwater values for particulate organic carbon (pgilitre) in the North Atlantic Ocean. Each number is an average of all values below 1 000 m at a station or in some cases a group of stations in approximately the area indicated. See text for further details and references to the original published data.

the general compilation of deepwater averages shown in Fig. 2. These numbers were obtained by averaging all values below 200 m for a given station or a group of stations of similar characteristics, so that a single average is derived for a given area. This is justified on the grounds that observed vertical variations in the North Atlantic Ocean tend to be of an essentially random nature. The variations often exceed the analytical error, but present sampling methods, which admittedly have not been as closely spaced as might be desired, have shown little indication of a systematic change with depth or maxima and minima

16

GORDON A. RILEY

that would imply an association with particular water masses. A single exception to this generalization is noted in a station figured by Menzel and Ryther (1964). This station was a t 38"N 65OW. The mean deepwater carbon was about 74 pgllitre ; there was a large variation of about 48-106 pg, with a pronounced maximum a t a depth of 2 000 m. The authors did not comment on possible water mass relations, but waters at this depth commonly lie within the upper limits of the North Atlantic Deep Water. The same investigators also graphed a station a t 36"N 6 5 " w , which is included in Fig. 2, and Menzel and Goering (1966) presented average data for two lines of stations in the western tropical and subtropical North Atlantic and a station in the Caribbean in which the concentrations were essentially constant with respect t o depth below 200 m and averaged about 18 pg C/litre. Wangersky and Gordon (1965) obtained observations in waters underlying the Guinea Current in a general depth range of 400-1 200 m, where large values were obtained. They also made deeper stations farther offshore in non-upwelling areas of the tropical Atlantic and in the southeastern Sargasso Sea. These declined toward levels of the sort observed by Menzel and Goering (1966) in the western Atlantic. Riley et al. (1965) published data obtained from waters near Bermuda on a more or less seasonal basis. Observations obtained between November and January were markedly higher than other times of the year in all deep water samples, and two values are given for this area, representing winter and summer averages. This rather remarkable seasonal change, later confirmed in other years by Gordon (1970b), will be discussed later. The latter author is credited with most of the remainder of the observations in the North Atlantic, but Duursma (1960), who did classical work on dissolved organic matter in the North Atlantic, provided data on particulate carbon at one station which is included in the figure. Observations in other oceans are inadequate for regional charting, but a brief review of the literature is in order. Parsons and Strickland (1962a) analyzed eight samples taken in depths of 500-3 000 m in the temperate North Pacific and obtained a mean value of about 5 0 p g C/litre. They also made a biochemical study of this material which will be discussed in a later section. Dal Pont and Newell (1963) reported carbon and nitrogen values for a station in the South Pacific a t approximately 34'5 153"E. Thirteen samples taken between 237 and 4 210 m averaged 96 pg C/litre. The total range was 36-207pg, and vertical variations in carbon content appeared t o be associated with particular water masses.

PARTICULATE ORGANIC MATTER I N SEA WATER

17

There was a sharp increase in carbon t o a maximum of 150 pg/litre at the level of t he intermediate salinity minimum, which presumably represents the core of the Antarctic Intermediate Water. Lower values of 36-82 pg/litre were found between about 1 400 and 3 300 in. The deep salinity maximum lies within this zone. This salinity maximum is believed t o be due t o the influence of North Atlantic Deep Water which leaves the Atlantic Ocean around the southern tip of Africa and traverses th e southern part of th e Indian and Pacific Oceans as a northern component of the general Antarctic circumpolar drift. Values at 3 763 an d 4 210 m were respectively 207 a n d 106 pg C/litre. Probably this is primarily Antarctic Circumpolar water. Holm-Hansen et nl. (1966) described a station off California, which also showed considerable vertical variation. There was a general increase at t he depth of the oxygen minimum layer with a maximum concentration of 100 pg C/litre. Most of the deepwater values were between 20 a nd 6 0 p g C/litre, b u t here too there was evidence of distinct layering. This station was taken in one of the deep basins off the California coast, and in several respects it was not quite typical of open oceanic conditions. There is a possibility t h a t the unusual amount of variability observed here was associated with peculiarities of basin circulation. However, in the present state of our ignorance an equal possibility exists t h a t closer sampling intervals at oceanic stations would reveal a similar degree of layering. Hobson ( 1 967) reported th e results of six cruises t o a n area in the North Pacific between 45" and 48"N and 135-140"W. There was a large range in deep water values, from 8-110 pg C/litre, with considerable layering and with systematic seasonal variations, which appeared to represent a movement through th e profile of discrete parcels of water with relatively high concentrations of 30-1 10 pg C/litre. Computed currents were more or less in accord with the concept of seasonal transport of water masses through th e area. Menzel (1964) published figures for two stations in the Arabian Sea. Individual analyses in waters of 200 m or more ranged from 17-80 pg C/litre, and the average values for the two stations were 32 and 35 pg. Menzel's work was primarily concerned with dissolved organic carbon in the western Indian Ocean, and his data on particulate matter were incidentaI t o the main purpose. Later work by Szekielda (1967) provided more extensive coverage of this region, including transects off the Somali coast an d one across the Gulf of Aden. I n general his values were higher t ha n those reported by Menzel. The lowest ones obtained were of the order of 60 pg C/litre, an d they ranged upward t o about 140pg in the core of the Red Sea outflow an d in near-bottom waters

18

GOItDON A. RILEY

of Antarctic origin. Thus there were definite indications of water mass effects in his profiles. 2 . Discussion of results

(a) Methods. The various investigators who contributed to present knowledge about the distribution of particulate organic carbon have used a variety of methods in almost every phase of the work. Thus the question naturally arises as t o whether the results really are comparable. This problem has largely been dealt with in the methods section in connection with the intercomparisons made by Gordon (1969) of various pore sizes of filters and of plastic versus glass Sampling bottles. I n no cases were there significant differences. His carbon analyses were carried out in part on an F. & M. Model 185 CHN analyser and partly in an apparatus assembled by P. J. Wangersky, which involved dry combustion and determination of the CO, in a Beckman infra-red analyser. He also arranged with D. IV. Menzel for an intercomparison of determinations by the method of Menzel and Vaccaro (1964). Again no significant differences resulted from differences in methods. This removes much of the uncertainty from regional comparisons of the type that is intended here. To be sure, great care must be exercised in maintaining cleanliness in sampling bottles and filtration techniques, and corrections for the carbon content of filters are always troublesome. Some investigations, and particularly the early ones, may be in error due to failure t o recognize all of these problems. However, there are no indications of gross error that warrant discarding any of the early data. But there is no simple solution to the problem of analysing particulate organic matter. Ranging in size from macroscopic particles t o colloids, it cannot be sampled quantitatively by any filter now in use, nor is there any present filter that effects a precise size fractionation. Further study of size spectra is needed, but for routine examination of regional variations the prime requirement is for general agreement on a standard method, and as is usual in such cases, some compromise is necessary. A fairly large quantity of water must be filtered in order to minimize errors in blank determinations, and this is inconsistent with the use of filters with very fine pores, which would be desirable in order t o retain small particles. I n practice, silver filters of 0.8 or 1.2 p pore size appear to be a good compromise. Menzel (1967) assumed that dissolved organic matter is adsorbed on filters and attempted t o correct this. Subsequent papers by Menzel and Ryther (1968a and b) did not provide details as t o whether similar

PARTICULATE ORGANIC MATTER IN SEA WATER

19

corrections were made. However, in all three papers the values reported for particulate organic carbon were considerably lower than most analyses previously reported. As indicated earlier, preliminary work by P. J. Wangersky (personal communication) has shown no evidence that dissolved organic materials with molecular weights of less than 10 000 are adsorbed t o an appreciable extent on silver filters. Thus a correction factor probably is improper, and in either case these analyses are not comparable with older data and cannot be included in the general regional analysis. (b) Regional variations in organic carbon. As far as the surface layer is concerned, present data are insufficient to draw conclusions about seasonal cycles or regional variations. I n areas where data are available for more than one season, large variations have been observed, and in most cases the highest values have been obtained in spring. This is to be expected. A spring diatom flowering in the open sea commonly has a chlorophyll concentration of the order of 2-10 pgllitre, and the carbon content of a phytoplankton crop of this size probably is about 60-300 pgllitre. Thus a large proportion of the total carbon reported during the spring season may be derived from living cells. However, the particle counts described earlier suggest that non-living organic matter also may increase a t this season. At other seasons the quantity of phytoplankton carbon is likely to be at least an order of magnitude lower than a t the height of the spring flowering. Referring again to particle counts, the non-living fraction may then be somewhat larger than the quantity of phytoplankton carbon. No great improvement in our knowledge of particulate organic matter in the surface layer will be obtained until more information is available on seasonal cycles of organic carbon and on fractionation into its different components. A semiquantitative estimate of phytoplankton carbon can be made on the basis of chlorophyll analyses; however, chlorophyll : carbon ratios are variable, and there is lack of agreement as to limits of variability. ATP analyses (Holm-Hansen and Booth, 1966) and DNA analyses (Holm-Hansen et al., 1968) possibly will supply a more precise separation of living and non-living fractions. Modern techniques seem more or less adequate for the task, but accumulation of data will be slow. After many years of phytoplankton investigation there are still only a few open ocean areas where we have a satisfactory knowledge of seasonal cycles and average abundance. The larger task of determining total organic matter and its components will also require years t o accomplish.

20

GORDON A . EILEY

While regional coverage of deep water is still fragmentary, there are enough observations for some generalities t o emerge. The total variation is large, but a major proportion of the observations fall between 15 and 100 pg C/litre. I n general there appears to be about a three-fold t o five-fold variation between poor areas and rich ones. Markedly high values have been found underlying the upwelling zone off the west coast of Africa, and the averages decrease to low and relatively constant values through most of the tropical and subtropical regions of the western North Atlantic. Dal Pont and Newel1 (1963) obtained fairly large values in the South Pacific, particularly in waters of Antarctic origin, which presumably are productive. Thus t o some extent, a t least, the magnitude of deep water carbon concentrations is associated with high levels of surface productivity. Temperate waters of the North Atlantic and North Pacific presumably have a level of primary productivity that is above average, and in those areas carbon values tend to be moderately large, although some low ones have also been obtained. Low values were obtained by Gordon (1970b) in the Irminger Sea, which probably is not highly productive on an annual basis, despite good phytoplankton flowerings during the short subarctic summer. More recently he has obtained extremely low values in tropical Pacific waters off Hawaii (personal communication). Menzel (1964) obtained intermediate values in the Arabian Sea, an area that is subject t o seasonal upwelling but probably is not as productive on an annual basis as the west African coast. The west coasts of the Americas should be productive, and moderately high values were recorded by Holm-Hansen et al. (1966) in the California offing. I n summary, there are substantial regional variations in deep water particulate organic matter, which appear t o be related a t least in part to primary productivity in the surface waters above. This presupposes that vertical transport of materials down through the water column is more rapid than horizontal displacement by deep water currents and diffusion. McGill ( 1964) obtained essentially similar results in studies of deep water distribution of organic phosphate, despite the fact that the latter was primarily in dissolved rather than particulate form. However, this is a broad generality, which is subject to exceptions. Deep water transport probably is not negligible, for variations have been reported in carbon concentrations which hardly can be explained in any way other than water mass transport. This will be discussed in the next section.

21

PARTICULATE ORGANIC MATTER I N SEA W A T E R

(c) Temporal variations in deep water. The work of Hobson (1967) in the North Pacific has been mentioned, in which seasonal changes in deep water particulate carbon were explained in terms of seasonal variations in computed currents. I n certain other areas that have been examined on more than one occasion, systematic differences have been noted, for example, in Slope Water south of Nova Scotia (Gordon, 1970b). These have not been examined thoroughly enough to determine whether they are of a seasonal nature or are associated with variations in transport systems. The best documented case of a consistent seasonal variation is in the north-western Sargasso Sea, in the vicinity of Bermuda and northward toward the Gulf Stream. There Riley et al. (1965) obtained a three-fold variation between winter and summer values with highest values in winter. These values, averaged and plotted in Fig. 2, showed similar variation throughout the whole deep water column. Vertical variations in T-S relations suggest that there are several fairly distinct water masses in this area, and the kind of generalized changes that were found obviously could not have been the result of intrusion of a particular water mass. These samples were taken in 1963-64. Subsequently Gordon (1970b) obtained another series of samples in 1966-67, with the results shown in Table I. Results were essentially identical with the earlier ones. TABLEI. NUMBERS OF DEEPWATER SAMPLES AND CRUISE AVERAGES GIVEN BY GORDON ( 1 9 7 0 ~ FOR ) THE NORTHWESTERN SARGASSO SEA No. January 1967 April-May June-July Jan.-Feb. 1968 April

11 61 24 17 20

pg

Cllitre 47.3 19.7 12.3 47-1 8.8

Other variation& of a somewhat similar sort have been noted. McGill et al. (1964) reported a seasonal variation in organic phosphorus in deep water off Bermuda, and to a lesser degree in other stations to the northwest. At Bermuda the highest values were found in winter and were accompanied by a slight increase in salinity. Subsequently Bricker (personal communication) has found seasonal variations in the ratios of Mg and Sr to total salinity, which again extend through the whole water column. .4.WB.-8

2

22

CORDON A. RILEY

The possibility that the observed seasonal change in organic carbon resulted from in situ biological events can be ruled out fairly quickly. The total seasonal increase is of the order of 100 g C/m2 of sea surface, which is equal t o or greater than net phytoplankton production a t the surface. The observed increase therefore cannot be accounted for in terms of sinking of organic matter from the surface, and even if i t could be, the seasonal cycle of phytoplankton production is not of a sort that would lead to a short winter maximum. The other possibility of in situ processes is conversion of part of the dissolved organic matter t o particulate matter. Later information will suggest that such transformations might be possible, but for them to occur on a seasonal basis in an essentially seasonless deep ocean seems unlikely. Moreover, this would not supply a reasonable explanation for the increase in organic phosphorus noted by McGill et al., which included dissolved and particulate fractions. The alternative to in situ changes is a seasonal movement of water of different composition through the area, and this must be a generalized movement from surface to bottom rather than an intrusion of particular water masses. The only presently known source of water with a sufficiently high content of organic carbon to account for the observed winter maximum is the area t o the north, where Menzel and Ryther (1964)reported fairly large concentrations in winter and spring just north and south of the Gulf Stream. The concomitant winter increase in salinity in Bermuda waters is in accordance with such movement, for there is a slight but significant north-south gradient in salinity in the great water mass lying between the Gulf Stream and the tropics. Direct evidence on current movements is fragmentary. This is not an area where estimates of deep currents by geostrophic rrethods are simple and reliable, and no direct current measurements have been made on a year around basis. Even information on surface currents is scanty. Day and Webster (1965) described a series of current observations obtained with a buoy moored a t about 28"N 65"W in which currents were recorded continuously from October 10, 1962 t o January 27, 1963. The long term drift a t this station, located some 250 miles south of Bermuda, was toward the north during the first part of this period, averaging about 8 km/day. I n early January the current veered t o the east and then to the south. During the last two weeks of the observation period the water was flowing south at an average speed of 14 km/day. If currents of this magnitude prevailed over a considerable area, they could move surface water from the vicinity of the Gulf Stream to the latitude of Bermuda in about a month.

PARTICULATE ORGANIC MATTER IN S E A WATER

23

The measurements ended too soon t o determine the length of the period when the water was drifting south. Nor is there any assurance that this kind of event occurs every year or in deep water as well as the surface layer, However, there is evidence of an indirect sort that a flow from the south is re-established by early spring. Some years ago the writer attempted t o calculate coefficients of vertical eddy conductivity based upon temperature data taken in the vicinity of Bermuda. These were to be used for computations of biological rates of change of essential micronutrients. The attempt failed because temperature increases in waters underlying the surface layer were Iarger than the solar input during the spring and early summer and were patently the result of advection of warmer water from the south, so that the problem could not be treated in simple terms of vertical transfer. A hypothesis can be proposed that there is a net northward transport of water past Bermuda during most of the year, with a short period of reversal in winter, and that this is a generalized phenomenon involving the whole water column. This hypothesis does not violate any of the information available, although admittedly the evidence is tenuous. Furthermore, a movement of this sort, which is distributed through the water column rather than involving intrusions of particular water masses, probably is indicative of a change in major circulation patterns in that part of the Atlantic Ocean. Iselin (1936) and Sverdrup et al. (1942) noted that Gulf Stream transport increases northward toward the New England offing and decreases thereafter. Principles of mass continuity are best satisfied by a theory that all of the Sargasso Sea is in a state of slow, clockwise motion around a central high that is located far toward the northwestern edge of the area. Sverdrup’s transport diagram shows about a tenth of the Gulf Stream water peeling off in a tight little gyre in the vicinity of Bermuda. This was only a diagrammatic sketch, which was drawn without any real knowledge of the trajectory of the water past Bermuda, and it merely serves as a reminder that the Sargasso Sea is an integral part of the eddy rather than dead water within the encircling currents of the North Atlantic gyre. The actual geometry of the system can be expected t o change with variations in Gulf Stream transport. Iselin (1940) analyscd all of the information available a t that time which had a bearing on transport variations in the Gulf Stream. There was clear evidence of systematic seasonal variations. His compilation of geostrophic computations and tide gauge records from Miami and Charleston showed that transport was minimal in mid-

24

GORDON A . KILEY

autumn and rapidly rose to a peak in mid-winter. This was followed by a gradual and irregular decline, with a tendency toward a secondary maximum in late summer, and then a rapid drop to mid-autumn. Iselin proposed that an increase in transport would tighten and deepen the central eddy, and Stommel’s (1948) theory of westward intensification suggests that an acceleration of the system could move the position of the centre closer to the Stream. A relatively small shift in the centre could place Bermuda either in the northward drift between the centre and the Stream or in the part of the gyre that moves toward the southwest. Furthermore, expansion and contraction of the central eddy could involve displacements of deep water throughout the whole eddy system as well as in the Gulf Stream itself. This hypothesis provides a framework that can logically contain all of the fragmentary evidence on seasonal variations of conservative and non-conservative properties in the Bermuda area. It also provides a possible reason for a systematic seasonal change which involves only a slight geographical displacement rather than a profound change in circulation patterns. However, this is a limited view of a very complex problem, as will be apparent to anyone who reads Iselin (1940) and other works involving the broad implications of changes in Gulf Stream transport. More thorough study is needed, over a broader area, and until that is done any conclusions are speculatory. If there is any validity in the hypothesis that has been proposed, one would suppose that oceanic areas subject to that kind of seasonal variation would be quite limited. Variations due t o intrusions of particular water masses are likely to be more common. I n the long range view the Bermuda case may seem trivial, hardly warranting the present long and diffuse discussion. However, in the present state of our knowledge a difficult problem such as this one cannot be neglected. Finally, it should be pointed out that although only a few cases have thus far been documented of either seasonal cycles in deep water or vertical variations which can be correlated with specific water masses, other instances are likely to arise. Many oceanic areas are subject t o random or seasonal transgressions of water masses. These movements are not well understood by physical oceanographers, and there are indications here that properties such as organic carbori and phosphorus and ratios of major ions may be more sensitive tools for tracing water mass movement than ordinary T-S relations. Clearly we should move toward a more inter-disciplinary approach to these problems. (d) Small-scale variations in distribution. The earlier account of organic carbon concentrations indicates that there is a large concentration in the surface film and a rapid decrease in the waters immedi-

PARTlClLlLATE ORGANIC MATTER 1N SEA WATER

25

ately underneath. In deeper water there have been occasional indications of microstratification, as in the work of Holm-Hansen et al. ( I 966). Most studies of particulate organic carbon have been based on collections in which the bottles were spaced much too far apart for microstratification t o be detectable as such. However, concentrations frequently vary with depth in an irregular manner. Gordon (1970b) found that most of the observations a t his stations in the North Atlantic were significantly non-random. I n other words, present data indicate that vertical variations exist at all levels from surface to bottom, even though the form of the variation remains obscure. I n Gordon’s comparison of PVC sampling bottles with the Menzel glass-teflon sampler, the bottles were spaced 15 m apart. Here, too, there were significant differences in some of the pairs, although the overall averages were not significantly different, and this suggests that some of the vertical variations may be quite abrupt. Thus the available data on organic carbon provide only hints about small-scale variations in particulate organic matter. However, a t the present time there is increasing interest in non-random distributions of filter feeding organisms and their particulate food, so that a discussion of the problem and an account of indirect evidence on the subject are in order. Baylor et al. (1962) and Sutcliffe et al. (1963) brought forward evidence that organic particles could be produced by adsorption of organic materials on near-surface bubbles. There were indications that phytoplankton and bacteria might also be trapped on bubbles. The net effect would be a concentration of organic materials in the surface, and this could be a t least part of the reason for the near-surface gradients in organic carbon which were discussed above. This local concentration a t the surface is subject to gradual removal by vertical turbulence and convection cells. These are long cells oriented more or less downwind, in which the surface layer moves laterally and downwind into narrow, parallel lines of convergence. Sutcliffe et al. (1963) measured the sinking speed of the water in these convergences and obtained values of the order of 3-6 cm/sec. Between the narrow curtains of descending water, principles of mass continuity require a corresponding upward movement. This is presumably slow and diffuse, for the area between the convergences is much broader. The trajectory of any particular parcel of water in such a cell is probably a spiral motion downwind, although little is actually known about these cells except the surface water movements (Woodcock, 1944) and the downdrafts. In an area such as the Sargasso Sea, where convergences are marked

26

GORDON A. RILEY

by accumulations of Xargassum and floating debris, the configuration of the cells can be determined visually over a considerable distance, and the situation is seldom as diagramatically simple as might be implied from the foregoing account. Broad and well-established weed lines are likely t o be interspersed with small and discontinuous ones, and a t wind speeds of about 15 mjsec or more these are in turn interlarded with foam lines and irregular spume patterns which might be the result either of small-scale convective sinking or turbulent exchange. Indeed McLeish ( 1968) has presented theoretical and experimental evidence in support of the hypothesis that turbulence can be responsible for all of the observed surface phenomena and has expressed doubt as to the reality of the classical picture of the Langmuir spiral. For present purposes, the exact form of the circulation pattern is not particularly important. The existence of downdrafts is well substantiated. Commonly, although not always, the water in the downdrafts is slightly cooler than the surrounding water, and when these movements occur in an essentially homogeneous mixed layer, they probably can extend down to the top of the thermocline and can create a vertical transfer of plankton and particulate matter that is biologically significant and is more important than the question of what the remainder of the circulation looks like. For convenience the rest of the discussion will assume the validity of Langmuir circulation, but the conclusions would not be seriously altered by qualifications about the character of the motion. Sutcliffe et ul. (1963) believed that organic particles accumulating in the surface layer would drift into the convergences, providing curtains of particle rich water which might serve as sources of abundant food for zooplankton. They found an increase in turbidity and phosphate in the convergence lines. There was evidence that zooplankton did in fact congregate in such areas, and a paper by Baylor and Sutcliffe (1963) suggested that particles formed a t the surface might provide useful food, for particles produced experimentally in the laboratory supported the growth of Artemia. These results were exciting t o biologists on two counts. I n the first place, although our knowledge of the biochemistry of sea water was limited, and still is, it was apparent that many substances known t o exist in soluble or colloidal form in sea water would be suitable food for filter feeding animals if the material could be aggregated into sufficiently large particles t o be retained by their filtering apparatus. This kind of non-living material appears more promising as food than semi-decayed detritus. Secondly, since the evidence is fairly good that there is a physical

PARTICULATE ORGANIC MATTER I N SEA WATER

27

mechanism which concentrates phytoplankton as well as non-living particles into convective downdrafts, and moreover since zooplankton appears to congregate in these areas, the real environment of zooplankton with respect to its food supply is very different from the impression that we get with our ordinary crude sampling devices. We can expect that the usual zooplankton tow will cut across a number of lines of convergence and will provide a realistic estimate of average zooplankton abundance, although it tells us nothing about microdistribution. Phytoplankton samples are commonly taken from a string of water bottles, They are almost never taken in sufficient abundance to give a true picture of micro-distribution, and by the law of averages most of the samples will be taken in the broad areas between convergences rather than in the convergences themselves. Neither type of observation is realistic with respect t o phytoplankton-zooplankton relations as they exist in nature, but one suspects that the true abundance of zooplankton food materials has been seriously underrated. This should give some comfort to the investigators, too numerous to cite by reference, who have examined zooplankton grazing rates and food requirements in the laboratory and found that the estimated requirements often exceeded the amount of food that appeared to be available in nature. One can hardly deny nature, and there seemed to be some flaw in the experiments. Now it seems likely that the flaw may be in our observations of nature-that food may be more abundant than we realized in the micro-habitats where it is being consumed. What has been said about horizontal micro-distribution in the surface layer applies equally to horizontal layering in underlying thermoclines. There is increasing evidence (Stommel and Federov, 1967 ; Lovett, 1968; Tait and Howe, 1968) that thermoclines are not smooth gradients but rather are laminar structures in which thin, almost homogeneous layers alternate with transitional zones of steep temperature gradient. Theoretically, sinking particles will accumulate in any gradient strong enough t o cause a significant reduction in sinking speed. Many writers have described accumulations of phytoplankton in thermoclines, although seldom in sufficient detail to reveal fine structure. Biggs and Wetzel (1968) found concentrations of particulate carbohydrate, presumably mostly non-living particles, in discontinuity layers. Limbaugh and Rechnitzer (1955) reported that this layering can be sufficiently dense t o be detected visually when diving. Moreover, Harder (1968) demonstrated experimentally that copepods and some other marine plankton organisms tend to congregate a t discontinuity layers. Thus there is abundant qualitative evidence that physical processes

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concentrate small particles in shear zones-vertical convective movements in the surface layer and horizontal shear zones in the thermocline-and that zooplankton tends to congregate in gradients. Interest in this problem is giving rise t o incraasingly sophisticated devices for examining micro-distribution of plankton. However, a t the present time there is still little quantitative information on the physical dimensions of these loci of abundance or the absolute magnitude of the concentrations. The temperature-depth curves given by Lovett (1968) indicated that layering decreased with depth in the main thermocline. Deviations from the mean gradient were small, below 1 000 m, but were detectable t o a depth of about 2 000 m. Tait and Howe (1968) found quite distinct layering to a depth of 1500 m in the vicinity of the Mediterranean outflow. The writer is indebted to Henry Stommel and W. F. Simmons for permission t o examine unpublished records of a similar sort which were taken with a Salinity-Temperature-Depth recorder in the Sargasso Sea. On two occasions the instrument was raised and lowered continuously between 1 000 and 1 200 m, more or less, for a period of about half an hour, while the ship drifted freely. Marked layering was observed, including temperature inversions. The same pattern was repeated with only minor differences throughout each half-hour period, indicating that the discontinuities extended horizontally for some distance, although their true size remains an open question. A single lowering a t another station to a depth of about 1 900 m showed irregularities in temperature structure throughout the column, although the amount of variation in the gradient decreased with depth. There was virtually no variation in the salinity gradient below 1 000 m. Paramonov et al. (1966) measured optical characteristics of the water column as determined by lowering an instrument which bore a fixed light source and a photocell and hence was able to determine vertical variation in optical attenuation as the instrument was lowered through water. Theoretically the attenuation would include both absorption by dissolved materials and absorption and scattering by particles ; however, the effect of particles is likely to be the dominant feature in deep water and should provide a semi-quantitative indication of variations in particulate organic matter. These authors found that there were pronounced variations in attenuation in the surface layer and in the main thermocline. I n the deep ocean there were no sharp peaks. Minor variations were found, but they were gradual changes in the level of the attenuation coefficient rather than sudden shifts. Thus there is considerable likelihood that the thermal structure of

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the main thermocline concentrates particulate food into micro-habitats which are favourable for filter feeding animals. The evidence is less clear for the deep ocean. Some thermal structure certainly exists. In boundaries between water masses of markedly different T-S characteristics, as in the Mediterranean water mass described by Tait and Howe (1968), considerable vertical variation may occur in the deep ocean. This is a matter that deserves further attention, for, as will be seen later, the concentrations of particulate organic matter that have been observed in routine sampling programmes are minimal for the support of bathypelagic organisms.

111. CHEMICAL COMPOSITION There have been excellent and well documented reviews (Provasoli, 1963; Parsons, 1963; Degens, 1968) of the chemical composition of particulate and dissolved organic matter. The present account will not repeat the whole story but will merely summarize the salient facts. Collecting methods have involved centrifugation or accumulation 011 filters of moderate pore size, and the discussion centres on particles of the order of 0-5 to 1 p or larger. The so-called dissolved fraction obviously contains smaller particles, including colloids. Little is known of their composition or even of the quantity of such substances, as compared with materials in true solution. Fox et al. (1952) examined the properties of various filters and effected a reasonable separation of colloids and dissolved materials. The quantity of organic carbon retained on their ultra-fine filters ranged from 0.16-0.47 mg C/litre in a series of samples taken a t depths of 100-600m off the California coast. By way of comparison, Holm-Hansen et al. (1966) found that the particulate carbon removed by ordinary glass fibre filters was 30100 pg C/litre a t comparable depths at their station in the California offing, and the dissolved organic carbon was about 0.45-0.60 mg/litre. This type of comparison is crude, but the results suggest that a considerable fraction of the so-called dissolved component may actually be filter-passing particles and colloids. Thus the section that follows deals with only a fraction of the total particulate matter, and possibly a minor part of it. However, it is the part that is most immediately concerned with animals and bacteria.

A. Elementary composition Parsons and Strickland (1962b) measured carbon and nitrogen in samples obtained from surface and deep waters a t several stations in the temperate North Pacific (about 19-50°N, 132-145OW). The mean

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C : N ratio by weight was 5-7 :1 in the surface layer and 3.6 :1 in waters of 300-3 000 m. Menzel and Ryther (1964) obtained essentially similar results in surface waters of the temperate North Atlantic. I n some of their deep water analyses the ratios were as little as 2.7 : l . Values reported by Duursma (1960) in the North Atlantic and Dal Pont and Newel1 (1963) in the South Pacific fell within this same general range. Duursma (1960) and others had found that C : N ratios in dissolved organic matter in sea water also were of a similar order. Provasoli (1963) called attention to the fact that these ratios were much lower than in fresh wat,er or soils, and the results suggested that humic materials, which ordinarily have a C : N ratio of 8 :1 or more, may be scanty in the sea. However, recent work indicates that the situation is more complicated and variable than it appeared to be in the early 1960's. HolmHansen et al. (1966) obtained a ratio of 5.5 : 1 in the surface layer of their station off the California coast, but the ratio increased below the surface, and the average for the whole water column was 1 2 . 5 : l . Subsequently Gordon (1970b) analyzed three sets of cruise data obtained in temperate and subtropical waters of the North Atlantic. Altogether about 120 samples were analyzed, and the means were 7-8 :1 and 8.3 :1 in the first two cruises and 15-5 :1 in the third. The possibility that the apparent variability was due to differences in analytical technique cannot be entirely ruled out, although this seems unlikely. All except the last author cited used wet oxidation methods, which are possibly suspect on the grounds that oxidation of carbon may not have been complete. Gordon used dry combustion, and his higher ratios could have been due t o differences in method. However, Holm-Hansen et al. also obtained markedly high values by wet oxidation methods. Here one might argue further that conditions might have been atypical in the isolated basin that these investigators were studying. However, the argument is not very convincing, because high ratios were obtained well above the level of the sill depth of the basin. Handa (1968) also obtained moderately high C :N ratios a t a station in subtropical waters off Japan (35"N, 139"36'E), and he found that the ratio increased with depth. He regarded this as indicating that protein is utilized more readily than carbohydrate as the material sinks into deep water. His biochemical analyses, which will be summarized later, indicated that a large proportion of the carbohydrate existed as polysaccharides which one would expect to be refractory. There is no longer much doubt that observed variations in C : N

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ratios are real, but more investigation will be needed in order to determine limits of variation and regional aspects if any. Handa’s evidence of a systematic increase in C : N ratios with depth cannot be accepted as a general phenomenon without more thorough examination in other regions. Data from the North Pacific by Parsons and Strickland (1962a) and from the south-eastern Indian Ocean (Newell, 1966) show no such evidence. There is some possibility that these variations can be ascribed to differences in the relative proportions of amorphous aggregates and flakes, the former consisting mostly of carbohydrate, while the latter are also rich in protein. Counts now available do not reveal marked differences in the proportion of these components ; however, counts are somewhat deficient in their assessment of large aggregates, which are few in number but might have sufficient mass to influence bulk chemical analyses. Holm-Hansen et al. (1966) found that the ratio of particulate carbon to phosphorus in thc surface layer was about 33 :1 by weight. A considerable fraction of this particulate matter presumably consisted of living phytoplankton, and they regarded the observed ratio as being fairly typical of phytoplankton growing in a nitrogen deficient medium. The C : P ratio declined t o a minimum a t mid-depths and increased in near-bottom waters. The average C : P ratio for the vertical column was about 50 : l . The average ratio for the dissolved fraction was about 90:l.

Menzel and Ryther (1964) found essentially no phosphorus in deep water particulate matter. McGill ( 1964) was particularly concerned with total organic phosphorus, particulate and dissolved, in his monographic treatment of phosphorus distribution in the Atlantic Ocean. The concentrations were of the order of 0-0.15 pg a t P/litre and averaging about 0.05 pg, suggesting that the C : P ratio by weight might be several times higher than that obtained by Holm-Hansen et al. (1966) for the dissolved fraction. These observations, admittedly scanty, suggest that deep water particulate matter is poor in phosphorus as compared with plankton and the general assemblage of particulate matter in the surface layer. Much of the phosphorus in living organisms is regenerated quickly after their death, and apparently only a few components are resistant t o biological utilization. Holm-Hansen et al. (1966) suggested that most of this deep water phosphorus is in nucleic acids, and thought that the increase in C : P ratios which they observed in near-bottom waters was due to the fact that nucleic acids were more resistant to decomposition than protein.

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B. Biochemical composition The work of Parsons and Strickland (1962a) is the most complete of the early accounts of the biochemical composition of oceanic particulate matter. Their analyses of carbon and nitrogen were discussed in the previous section, and the remainder of the work will be outlined now. Measurements of carbon, nitrogen and carbohydrate were obtained a t three stations. These comprised two samples from the surface layer and nine samples a t various depths from 300-3 000 in. I n addition, a composite sample from 400 m was used for more detailed biochemical study. Seven amino acids were identified in the protein hydrolysate. Glycine and alanine predominated, with lesser amounts of aspartic acid, lysine, arginine, serine and proline. There were indications that a considerable fraction of the total nitrogenous components was not recovered. I n the carbohydrate fraction, 70% was crude fibre. Glucose constituted 50% of the carbohydrate hydrolysate. Other sugars included galactose, mannose, arabinose, and xylose. No glucosamine was detected, indicating that chitin is not an important constituent, nor hexuronic acids. Fat content was less than 1%. The two surface samples averaged 202 pg C/litre, equivalent t o about 400 pg of organic matter. Carbohydrate, calculated as glucose, was 115 pg. Protein, estimated as N X 6.25, was 219 pg, leaving an unidentified remainder of roughly 65 pg of organic matter per litre. The deep water samples averaged 40.7 pg/litre of glucose equivalent and 87.3 pg of protein, calculated in the same way as indicated above. These two fractions added together would account for more than the organic matter that could reasonably be expected to be present since the total carbon averaged 50 pg/litre. Attempting to resolve this dilemma in another way, the carbohydrate is subtracted out, leaving an average of 34 pg of non-carbohydrate carbon. The C : N ratio of this material would be 2.4 :1, which seems anomalously low for proteins but is not entirely out of reason in view of the predominance of nitrogen rich amino acids such as glycine and alanine. This helps t o resolve the dilemma but does not imply that it is the whole story. Some of the " crude fibre )' may be complex polysaccharides containing nitrogen. The paper by Holm-Hansen et al. (1966), which was discussed a t some length in the previous section, contained notes on biochemical composition which will be summarized here. They assumed that organic phosphorus was present as a nucleic acid component because

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Strickland and Solozano (1966) had failed to find carbohydrates containing phosphorus. I n the case of deep water particulate matter, they reported that large quantities of carbon remained unaccounted for after correcting for carbohydrate determined by direct analysis and subtracting likely values for protein carbon. They suggested that there must be “ either a high proportion of non-nitrogenous material or an inert organic residuum with a very high carbon to nitrogen ratio.” This of course is very different from the situation that Parsons and Strickland (1962) had found earlier in offshore waters of the North Pacific. Hobson (1967) determined carbohydrates in particulate matter collected a t his three stations in the North Pacific. Deep water values for carbohydrate-carbon in his various cruises averaged 13-28% of total carbon. These are slightly lower than the average obtained by Parsons and Strickland. Handa (1968), whose studies of C :N ratios in the waters off Japan have already been reported, was particularly concerned with the biochemical characterization of the carbohydrate fraction and has provided the most detailed information on that subject which is thus far available. His paper also contains incidental information on the ratio of amino N to total organic N, which is summarized briefly in passing by saying that amino N was more than 85% of the total and tended t o increase slightly and irragularly with depth. The total particulate carbohydrate carbon was of the order of 20 pg C/litre in surface waters and 4-6 pg between 100 and 1 000 m. The most important monosaccharide components were glucose, galactose, mannose, xylose, and glucuronic acid. D-glucose was the most important one in the immediate surface layer, decreasing gradually in the upper 200 m. Otherwise there was little change in relative proportions with depth. One hundred litre samples were taken a t depths of 20, 500 and 700 m for detailed analysis. Soluble carbohydrate was removed from the particulate matter with a hot aqueous solution followed by ethanol and acetone treatment for further fractionation. The remaining particulate matter was treated with Schweizer’s reagent t o extract polysaccharides. The material so obtained was examined by standard methods t o determine the monosaccharide composition and the degree of polymerization that was involved. Two of the soluble fractions gave only D-glucose upon acid hydrolysis. One of these was specified as containing D-glucose and two oligosaccharides ; the other was D-glucan. A third fraction contained galactose, glucose, mannose and xylose. The water insoluble fraction

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yielded these four plus glucuronic acid. Glucan and the water insoluble polysaccharides were the most important constituents a t all depths and were virtually the only significant ones in deep water samples. Handa used periodate oxidation to determine types of linkage. His results will not be discussed in detail, but some generalities emerge. He identified a 1,3-glucan which is similar t o laminarin, and this general class of compounds probably is derived from algal storage carbohydrates. The water insoluble carbohydrate is a mixture of polysaccharides having 1,2- or 1,4-linkages. The latter is a common constituent of algal cell walls, and Handa believed that these polysaccharides are a mixture of algal remains. Such material is more resistant to bacterial attack than storage polysaccharides and is the chief component of deep sea particulate carbohydrate. Degens (1968) has presented a detailed examination of nitrogenous compounds in particulate and dissolved matter and sediments a t several collection points in both the Atlantic and Pacific oceans. Total amino acids in deep water particulate matter ranged from 5.3-1 8.6 pg/litre and constituted more than 50% of total particulate matter. Major amino acids included glycine, serine, glutamic acid, alanine, arginine, lysine, and aspartic acid. The amino acid composition of particulate matter was not markedly different from that of the so-called dissolved fraction, and indeed the distinction between them is somewhat arbitrary. With regard to this subject and also the structural relations of the nitrogenous molecules, a quotation from Degens is pertinent : “Much confusion has also been generated due to the arbitrary separation of particulate and dissolved matter into distinct classes of compounds. Chemically speaking, such a classification has little information content with regard to the elucidation of the molecular nature of the organic compounds in the sea. The so-called dissolved organic matter is composed of more than 90% of material that has a molecular weight (MW) greater than 400. The bulk of the organic compounds with MW > 400 falls in the 3 000-5 000 MW-range as ascertained by molecular sieve techniques. Hydrolysation of this material will release substantial amounts of monomers ; yet, some high molecular weight products are still intact after this treatment. “ The generally low C : N ratios of dissolved organic matter, the presence of urea even after hydrolysis, the high abundance of amino acids and aromatic compounds in connection with the high yields of oxygen, suggest that oxygen and nitrogen are used in the structural stabilization of the high molecular weight fraction. Peptides do account for some of the materials. Urea may easily react with aldehydes and produce long-chain polymers ; alternatively, its oxy-

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gen may be used for co-ordinative purposes. The molecular structure of urea . . . easily renders itself for such molecular work assignments. . . . “ Aside of arrangements involving peptides or urea condensates, oxygen may also be organized in the form of chlathrates. Thus phenols, quinones, amino acids, amines, sugars, fatty acids, alcohols, and their respective polymers may coexist within the same polymer by virtue of their oxygen functions. Further molecular stabilisation can be achieved by metal ions involving ion co-ordination polyhedra.” Degens’ paper also contains information on CI3/Cl2 ratios in particulate and dissolved organic matter and in sediments. Evidence is presented of isotopic fractionation associated with metabolic use of the organic matter, and these data will be discussed in a later section which deals with biological transformations. The use of histochemical stains by Gordon (1970a) has beel mentioned earlier. The mercuric bromphenol blue method (Mazia et al., 1953) combines directly with free amino groups and by coupling with mercury to sulphydryl, aromatic and free carboxyl groups. Thus all classes of proteins are stained, and staining of non-proteins is reported to be virtually negligible. The Schiff reagent is specific for 1,2-glycol groups and therefore is a generalized carbohydrate stain. These stsins have been useful in determining general classes of components in different kinds of particles. As indicated earlier, flakes and many smaller particles stsin heavily with the protein stsin as well as with the Schiff reagent. Amorphous aggregates chiefly take the carbohydrate stain. Any protein staining in the latter, aside from bacteria and other small inclusions, tends to be a light greenish blue rather than the intense reds and blues that characterize the staining reactions of flakes. Thus the protein reaction of aggregates is either quantitatively small, or, as suggested by Gordon, it may indicate a qualitative difference in the type of protein. The further question of availability to biological attack was studied by Gordon (1970a) under simplified laboratory conditions by treating collections of naturally occurring particulate matter with digestive enzymes. Duplicate samples were prepared by simultaneous filtration of measured amounts of a water sample through paired silver filters. One filter was used for determination of organic carbon content before treatment. The second filter was treated with a mixture of trypsin, chymotrypsin and a-amylase. The filters were then washed t o remove hydrolyzed materials and analyzed for carbon, and the difference between controls and the residue after hydrolysis provided an indication of the amount of digestible organic matter.

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Commonly 30-60y0 or more of the total material in surface collections was hydrolyzed, although in one cruise the mean was only 10%. In deep samples, comprising a total of about one hundred analyses, cruise averages varied from 19-26y0. Hydrolyzed material was examined microscopically after treatment with the usual histochemical stains. Protein was largely removed, but the carbohydrate was not noticeably altered. As Gordon pointed out, the digestive processes of filter feeders may be more effective in the hydrolysis of this material than the simple in vitro treatment. How much of the organic matter can really be used remains obscure. Earlier work by Parsons and Strickland ( 1 962a) suggested that the percentage might be very much higher. Their values for crude fibre averaged about 25% of the total, and the composition of the remainder suggested that most of it might be assimilable. The later work by Holm-Hansen et al. (1966),as described above, was more nearly in line with the concept that a large fraction of the total might be biologically inert. This is another case in which it is impossible, in the present state of our knowledge, to distinguish clearly between differences in methodology and real differences in composition of the organic matter. It is tempting at this point t o suggest that the different kinds of substances described by Handa and by Degens may be largely segregated morphologically into the two major types of non-living particles that have been observed in the sea. There are indications that amorphous aggregates may represent a conglomeration of refractory remains of phytoplankton and other detrital material and that bacteria living on these particles may be dependent primarily upon dissolved organic matter rather than the aggregates themselves for their sustenance. On the other hand, the proteinaceous substances described by Degens and ranging in size from macromolecules t o flakes would appear to be not only more usable metabolically but also chemically more susceptible to polymerization and cementing into flake-sized particles. The proposition probably is stated more categorically than is fully deserved but present information and experimental evidence t o be introduced later suggest that there is some validity in the generalization.

C. Relationships of organic and inorganic materials Wangersky (1965) remarked. that " aggregates are called organic only by courtesy and precedent, since their organic fraction seldom exceeds 30% ". This is quite true. Biologists have recognized for some time that organic matter is a minor fraction of the total particulate matter in sea water, but they took little interest in the inorganic fraction.

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Chemical oceanographers have paid some attention to a few of the inorganic constituents, but as yet there is no clear picture of the overall composition of the inorganic component. Riley (1959) used the relatively crude technique of measuring dry weight and difference on ignition of samples of particulate matter filtered from Long Island Sound waters. The difference on ignition averaged less than half of the dry weight. Riley et al. (1965) measured the dry weight of samples of surface and deep water from the Sargasso Sea and then proceeded to carbon analysis. Deep water carbon averaged only about 2% of total seston dry weight. These collections from relatively unproductive waters probably represent minimum limits for organic content. Gordon (1970b) used similar methods on a broader regional scale and concluded that the organic content, estimated as twice the carbon, averages about onethird of the total dry weight. Hobson (1967) used a Coulter counter to measure the volume of particulate matter a t his North Pacific stations and then estimated the weight of particulate matter by assuming a specific gravity of unity. His results indicated that organic carbon averaged 20% of the total particulate matter or more in most of his groups of deep water samples. The method of Krey et al. (1957) and a subsequent series of papers by Krey and his associates had been mentioned earlier. A good general summary of their work is given by Krey (1967). Tabular data are given for a series of stations in the North Atlantic in the general latitudinal range of 45-55"N. Groups of stations were averaged, and deep water values for total particulate matter ranged from 25-141 pgllitre. The protein fraction was 15-45 pgllitre. Krey regarded this latter fraction as living substance, but biochemical studies cited above would suggest that some non-living protein also is involved. The results do not give a clear separation between organic and inorganic materials, but they suggest that the total organic matter would be a t least 30% of the total and probably more. Wangersky and Gordon (1965) found that calcium carbonate in their open ocean collections was about equal to the organic carbon, with the majority of deep water samples falling between 30 and 80 pg/litre. Chave (1965, 1968) has found that naturally occurring flakes commonly have crystals of calcite embedded in them. Moreover, particles of calcium carbonate adsorbed a coating of organic matter which was sufficiently impermeable t o protect them from re-solution in undersaturated water. Suess (1968) has presented further studies of the physical chemistry of this process.

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Most of the naturally occurring aggregates also contain particles which appear to be clay. This is particularly true of inshore collections but generally can be seen in oceanic material as well. X-ray diffraction analysis of Long Island Sound material showed clear evidence of clays. More remarkable, sodium chloride was present, although the material had been well washed before analysis. Moreover, one occasionally sees crystals in organic aggregates which appear to be sodium chloride. Wangersky (1965) described experiments which may help to explain these strange results. He prepared solutions of each of the major sea water salts in distilled water and bubbled them. The inorganic material was carried into the air above the water surface as an aerosol and fell back as a collection of fine crystals which floated on the surface until they gradually redissolved. I n another series he found a sudden increase in the yield of inorganic material together with appreciable yields of organic carbon. Further investigation showed that the distilled water contained organic material which apparently was the result of a spring diatom flowering in a local reservoir and had survived treatment with alkaline permanganate and triple distillation. These and other experiments suggested that crystals of any of the sea salts might be stabilized by the adsorption of organic skins, possibly of denatured protein. Indeed it would be difficult to account for the large inorganic content of the particles in any other way. This is particularly true in the case of flakes produced by experimental bubbling of filtered water, although even there the possibility exists of a contribution by inorganic solids that are small enough to pass the filters which are ordinarily used. The truth of the matter awaits further analysis. Wangersky’s theory about the process of particle formation is a little different from the one postulated by Sutcliffe et al. (1963) which will be described later. Experimental evidence will be introduced which will require further modification of present viewpoints. However, these theories are not mutually exclusive, and the later discussion will attempt t o pull the evidence together into a generalized picture which fits all of the available facts. Organic particulate matter is of considerable potential interest to geochemists. Sebba (1962) described a process which he called ion flotation, whereby bubbling air through a solution contsining surfaceactive organic materials resulted in the concentration of ions in particulate matter formed in the foam. He demonstrated that aluminium, uranium, alkaline earths, iron, nickel, and traces of other metals could be concentrated in this way.

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Wangersky (1965) and Wangersky and Gordon (1966) were particularly conceriied with the geochemistry of calcium carbonate and manganese. Their calculations indicated that the carbonate fraction of organic particles could contribute significantly t o the fine fraction of carbonate in deep water sediments. Manganese was particularly interesting. The presence of abundant deposits of manganese dioxide as nodules, crusts, and layers in sediments has been a subject of debate for many years. No known mechanism could account for the existence of soluble manganous ion which could be oxidized in bottom waters to produce these deposits. However, Wangersky and Gordon found that organic aggregates in deep water contain manganous ion in concentrations of 0.0 1-0.06 pgllitre, and decomposition of this material could liberate Mil++, which would then be available for oxidation and precipitation. The ability of particulate matter and associated clay particles to adsorb trace elements is well known, yet little of the geochemical literature shows much awareness of this problem. Some geochemists use whole water samples for analysis ; others filter their samples. In some cases various kinds of collections are indiscriminantly mixed, and often the exact procedures are not stated. This lack of realization that many of the problems are biogeochemical rather than merely geochemical has slowed the development of the field. The occurrence of inorganic substances in oceanic aggregates obviously can result from either adsorption of ions or scavenging of solid materials. The presence of manganous ion presumably is the result of direct adsorption. Calcium carbonate could be scavenged from the water column or formed as crystals on bubble surfaces or possibly adsorbed. Wangeraky and Gordon (1966) reported that experimentally produced aggregates had a calcium carbonate content that averaged about 2.5% of the dry weight. However, Siege1 and Burke (1965), using tracer methods t o study the uptake of various cations on particulate matter formed by bubbling, found essentially no calcium uptake. They reported that zinc, strontium, and manganese were adsorbed t o a slight degree on particles that were formed both by bubbling and on standing, but the amount was invariably less than 1yoof the amount added. The amount added was not stated specifically but was said to be negligible compared with the natural concentration of the cation in sea water. This raises questions about the interpretation of their results. I n the case of calcium carbonate, for example, the amount that Wangersky and Gordon found in their aggregates was a significant component of the aggregates but was generally less than 0.2% of the amount normally found in sea water. I n this case one

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would expect a tracer experiment to give a low yield, and a high yield could not be expected unless both naturally occurring and added cation are present in such low concentxation that a significant fraction of the total would be adsorbed during the bubbling period.

IV. EXPERIMENTAL STUDIES OF NON-LIVING PARTICULATE MATTER A. Adsorption on bubbles Earlier sections have mentioned the experiment31 finding of Baylor et al. (1962) and Sutcliffe et al. (1963) that particles can be produced by adsorption on bubble surfaces, and this process will now be considered in some detail. Their experimental work was not the first indication that this process might occur, but for the first time attention was focused on the possibility that such substances might be biologically important. Their work had been preceded by about forty years of study of atmospheric sea salt nuclei, a subject that has been ably reviewed by Blanchard (1963) and is mentioned here only as background information. These nuclei are formed and flung into the air by bubbles breaking at the surface, and there have been indications that they contain adsorbed organic materials and organisms as well as sea salts (Woodcock, 1948 ; Wilson, 1959, and others). Fox and Herzfeld (1955) suggested that bubbles in the water column could be stabilized by the presence of a skin of adsorbed organic matter. Moore (1953) found that sea foam was stabilized by an organic skin of protein. Baylor et al. (1962) called attention to a peculiar distribution of phosphate which has often been noted in near surface waters, in which the concentration in the immediate surface layer is slightly higher than in the underlying mixed layer. A biological explanation for the observed negative gradient seems unlikely although not impossible. Baylor et al. (1962) developed a more sophisticated hypothesis which probably was suggested by the work on sea salt nuclei and certainly drew supporting evidence from those studies. They made an experimental examination of the proposition that phosphate is transported toward the surface by adsorption on rising bubbles. Their evidence supported the hypothesis and they further found that organic matter was associated with the adsorption process. The experimental apparatus consisted of a sea water column with a stream of air bubbles entering a t the bottom and a sloping glass surface above the liquid which collected droplets of aerosol produced by bursting bubbles and allowed them to accumulate and flow off by gravity into a catchment flask. With this

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apparatus phosphate was removed exponentially. They furthermore found that preliminary extraction of the sea water with xylol reduced the effectiveness of the bubbling process and suggested that organic substances adsorbed on the bubbles served as anion binders. Their conclusion was that phosphate could be transported toward the surface by rising bubbles. Part of it could be ejected with the aerosol droplets, but materials remaining in the surface film or released by dissolution of bubbles before they reached the surface could produce the observed vertical gradient in phosphate. Sutcliffe et al. (1963) pursued this subject further, and their attention was now focused primarily on the organic component and its possible biological significance in the sea. Experiments testing the surface activity of the adsorbed material and partitioning of the phosphate by dialysis led to the conclusion that some of the phosphate is bound to organic substances with a molecular weight in excess of 300. Their hypothesis regarding the mode of formation of organic particles was as follows : " Such large organic-active molecules adsorb t o bubbles and produce monomolecular films which may be aggregated into insoluble organic particles in the following way: in the foam produced by aeration the ever increasing area of the adsorbed monomolecular films causes them to fold into polymolecular layers and to form colloidal micellae or to collapse into fibers (Goldacre, 1958). The agitation of foaming produces repeated collisions and results in multiple-layer coalescence of colloidal particles t o produce a semi-stable suspension of organic material. Many of the larger particles undergo further aggregation and settle out." Subsequent studies have brought to light certain aspects of the aggregation process which are not fully in agreement with this early hypothesis. The evidence will be examined in a later discussion; however, the experimental work and theoretical implications developed by these investigators were directly responsible for a rapid expansion of programmes of enquiry into various aspects of this subject. Because of the fact that these experimental studies stemmed from earlier meteorological work, it was natural that the problem was approached from an aerosol point of view. Experimental devices were designed to catch droplets above the water surface or t o drain off surface foam. We now know that methods of this type give the largest continuing yield of any method thus far discovered. As primary interest shifted from studies of the immediate surface film t o problems involving a larger portion of the water column, experimental methods changed. Most investigators bubbled a volume of water for a given length of time, allowing the particles to accumulate

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in the water. Invariably the rate of production was smaller, and under some experimental conditions the yield was negligible. Hence some doubt began to arise as to the validity of the earlier work. The matter became further confused by the fact that a variety of techniques was used, with little knowledge as to the effects that they might have on experimental results. Various kinds and pore sizes of filters were employed. Plastic containers have been used because of ease in fabrication, but they have been criticized on the grounds of possible introduction of contaminants. Air supplies are always suspect. Furthermore the quantity of carbon obtained is small, so that analytical errors are troublesome. The above section on " Methods )'has outlined precautions that are necessary for good analytical results ; this kind of information has accumulated gradually over the years, and early investigations did not include all of the precautions that are now regarded as necessary, nor did they always include enough replications for good statistical tests of the results. However, the results of earlier work should be summarized, and insofar as possible they will be reexamined in the light of more recent information about possible errors. Riley et al. (1965, Table 5) described the results of bubbling experiments on surface and deep water from the Sargasso Sea. The water was first filtered through 0.45 p Millipore filters in 15-20 litre lots into polyethylene carboys, bubbled for 3-6 hours with filtered ambient air supplied through an aquarium pump, and then re-filtered through glass fibre filters for measurements of particulate carbon. The conclusion a t that time was that all of these waters, surface and deep, contained filter passing material which could be converted into particulate matter by the bubbling process. The use of polyethylene and ambient air and the lack of unbubbled controls have been criticized and raise some doubt as to absolute values ; however, the whole series was done under essentially the same kind of conditions, permitting internal comparison. There were 44 experiments in all, about equally divided between samples taken in the upper 200 m and those obtained a t various depths from 500 m t o 3 000 m. The mean yields in these two series were 70 and 39 pg C/litre, respectively, and the difference was significant to the 1% level. Any errors due t o artifacts or lack of control should be about equal in the two sets. Thus, while one cannot say with certainty that bubbling produced particles in the deep water samples, the higher values obtained in the surface layer were a definite indication that this process was significant, even though the region under investigation is relatively poor in organic productivity. The other question, whether organic matter in deep water also can be converted into particles by the bubbling process,

PARTICtTLATEORGANIC MATTER I N SEA WATER

43

has no practical significance. It would be more interesting to find out whether other adsorption processes which could operate in situ in the deep ocean could increase the quantity of particulate matter. The paper in question had examined a good deal of oceanographic evidence, direct and indirect, as to probable causes of the lack of a systematic decrease in particulate matter with depth in the deep sea. The most reasonable explanation seemed to be a dynamic balance between utilization by bacteria and other biological processes and renewal by adsorption from the dissolved fraction. Preliminary experiments were undertaken t o explore the adsorption problem, and results looked promising. Subsequent work has enlarged this subject to the point where it deserves a separate subsection later. Riley et al. (1964) explored the question of whether extracellular substances produced during phytoplankton growth could be converted t o particulate matter by experimental bubbling. Seven species of diatoms and one dinoflagellate were grown in pure culture in an artificial medium in which the only organic additives were minimal quantities of vitamins which are necessary for growth of most of these forms. They were grown under varying conditions of light and temperature which were believed to be optimal for the species in question ; in general the light intensity was 500 ft. c. or less. At approximately the end of the log phase of growth the cultures were filtered through glass fibre filters. The medium, generally one litre, was bubbled for 24 hours in a pyrex flask using the laboratory air supply, and then was filtered again. The yield was highly variable, ranging from 2-75% of cellular carbon. The dinoflagellate was at the lower end of the range ; the diatoms averaged 20%. The mean carbon yield in all experiments was 1.16 mg C/litre. These results are open to criticism on the grounds of possible contamination by the laboratory air supply. However, during the same period another series of experiments was carried out with natural sea water and using the same air supply and other experimental equipment, so that any errors due to artifacts should be about the same in both series. Results obtained with natural sea water were an order of magnitude lower than the yield from culture experiments. Thus there is no evidence that the latter were seriously vitiated by contamination. Hellebust (1965) described the kinds and amounts of organic excretion obtained from 22 species of algae. His results also were highly variable, although his maximum yield of 25% was exceeded by about a third of the experiments listed by Riley et al. (1964). However, Hellebust’s yields represented only a 48-hour experiment rather than the week or more required for growth to the end of the log phase. Only two species

were duplicated in the two lists. Hellebust obtained a total carbon excretion of 9% from the diatom Skeletonema costatum and 4.4% from the dinoflagellate Peridinium trochoideum in the series grown a t 3 000 Lux. Figures obtained by Riley et al. (1964) for particle production in these species were respectively 31 and 2%. Possibly not all of the particulate matter was formed from dissolved extra-cellular metabolites. The coarse filtration used in these experiments would perhaps permit fragments of broken or dead cells to pass through and subsequently become incorporated in the particles. However, the procedure more or less simulated natural conditions in that surface waters of the sea can be expected to contain an array of dissolved metabolites, cell fragments, and possibly cell sap exuded during zooplankton grazing. All such phytoplankton products conceivably can be aggregated and were believed to contribute toward the observed relationship between phytoplankton concentrations and non-living particulate matter observed by Riley et al. (1964). About this time the writer began to find evidence of peculiarities in yield, depending upon the type of bubbling process used. Several experiments were designed to explore this situation. The series was interrupted before sufficient data had accumulated for publication, but some of the results warrant a brief account here. One experiment, for example, involved filtration of three lO-lit,re samples of essentially identical Long Island Sound water through 0.45 p Millipore filters into glass carboys. The first one was bubbled for 3 hours with filtered air from the laboratory compressed air supply and filtered through a glass fibre filter. The yield was 50 pg C/litre. The second was passed continuously through a 250 ml bubbling chamber, entering the bottom and draining off a t the surface into a filter. The yield was 61 pg C/litre, a t least equal to that of the first experiment, although the mean time in the bubbling chamber was only 12 minutes. The third one was bubbled for 24 hours, and the yield was 110 pg C/litre. The first carboy was rebubbled and filtered twice more during the first 24 hours, with additional yields each time, and the total yield was 184 pg, considerably more than that attained by continuous bubbling. Six more periods of rebubbling and refiltration for a total of 140 hours increased the total yield to 474 pg. Three additional experiments involving alternate bubbling and filtering were carried out with surface water taken near Bermuda. These were done in glass carboys, using ambient air introduced with aquarium pumps. One relatively short experiment of 21 hours duration gave a total yield of 157 pg C/litre. Two longer ones of 40 and 80 hours, each involving six periods of bubbling and subsequent filtration yielded

PARTICULATE ORGANIC MATTER IN SEA WATER

45

417 and 438 pg. No measurements of dissolved organic carbon were made, but these amounts were not markedly less than some of the figures for total dissolved organic carbon in oceanic waters as determined by wet oxidation methods (for example, see Duursnia, 1960). These results suggested (a) that the rate of production of particles by bubbling is very high initially and rapidly decreases with time, and

/

0

20

40 TIME

60 IN

00

I00

120

HOURS

Fm. 3. Comparison of total carbon yields in different kinds of bubbling experiments performed with aliquots of the same sample of Nova Scotia coastal water. A. Water was continuously circulated through EL bubbling chamber, through a filter, and back into the chamber. Filters were harvested a t intervals indicated by dots on the curve, and the amounts of carbon obtained were summed to give the integrated curve shown in the figure. B. Sample was alternately bubbled, filtered and rebubbled, and yields were integrated as in A. C. A sample was bubbled continuously for the entire period. If the yield during the first six hours was similar t n that of the other two aliquots, the subsequent yield was as indicated by the dotted line.

(b) that the reduction in rate is due to some process of inhibition by the particles produced, for their periodic removal permitted extensive further formation. There were indications in experiments described by Wangersky (1965) that excessive collection of particles at the surface inhibited further formation. If this is correct, there should be some advantage not only in draining away the surface layer continuously but also in having a large surface to volume ratio in the bubbling chamber. An

46

QORDON A. RILEY

apparatus was constructed consisting of a flat polyethylene pan with inlet and bubbling stone a t one end and an outlet a t the other a t a level permitting a 2-litre capacity with a surface area of about 550 om2. Water from the outlet passed through a glass fibre filter into a small collecting bottle and thence was pumped a t a rate of 2 litre/hr back through the bubbling chamber. Yields obtained with this device were compared with the results of alternate bubbling and filtering of an aliquot of the same sample. I n all cases continuous harvesting increased the yield. The results of one such experiment, conducted with Nova Scotia coastal water, are shown in Fig. 3. Each point represents the time when a filter was removed and the total yield up to that time; the latter obtained by adding all of the carbon analyses together. The form of the curve is typical. The quantities obtained were higher than average, and the sample presumably had an unusually rich supply of organic matter. Unfortunately, however, no data on dissolved organic matter are available for this series of experiments. The work was not completed and published a t that time, because there was evidence that organic contaminants in air supplies could be serious enough to produce spurious results, and further examination of possible errors was necessary. Menzel (1966) found that bubbling with his laboratory air supply for three hours increased the dissolved organic carbon by a factor of 2-4. He therefore developed a method for combusting his air supply in order to get rid of airborne organic material. He also developed an all glass apparatus. With this he conducted some twenty-five experiments using local coastal water (Woods Hole), surface and deep water from the Sargasso Sea, and culture media in which algae had been grown. The water was triple-filterzd through 0-8p silver filters prior t o the experiments, and he then compared the results of 3-6 hours of bubbling with unbubbled controls. I n all cases small quantities of carbon (1836 pg C/litre) werz retained 011 the filters, both experiments and controls, but the difference between them was statistically insignificant. Menzel’s work was done with considerable car3 with r3spect t o cleanliness in all materials used arid statistical design. His bubbling chamber was a glass cylinder, which is relatively inefficient for particle production, and a longer bubbling time might have been used ; however, these factors in themselves should not have been sufficient to prevent significant particle production. The discrepancy between his results and earlier ones that have been described required re-examination of various factors which might affect the bubbling process. I n addition t o questions of technique and artifact, the role of bacteria also needed t o be examined, for Barber (1966) had reported that he was unable t o

PARTICULATE ORGANIC MATTER I N SEA WATER

47

get significant formation of organic carbon in experiments that were effectively sterilized by filtration through 0.22 p filters. An evaluation of some of the major problems was published by Batoosingh et al. (1969). A prerequisite for such work was of course the development of an air supply that was reasonably free from contamination and, incidentally, a comparison of this air with the ordinary supply of laboratory compressed air as an aid in evaluating previous results. Combustion of the air seemed an unnecessary refinement. Instead, a small volume of air (about 0.5 litre) was recirculated. One method was simply to carry out the experiment in a tightly stoppered pyrex bottle with magnetic stirring sufficiently vigorous to introduce subsurface bubbles. The other was t o introduce a bubbling stone and circulate the air via silicone tubing and an AC vibratory pump. The latter provided more efficient mixing with little likelihood of serious contamination. Results showed that the laboratory air supply in current use was not objectionable, so that previous experiments such as the one described above on yields obtained with Nova Scotia coastal water were not invalidated. The use of a polyethylene bottle instead of a glass one did not introduce a significant error. A major factor in determining the yield was the pore size of the filters. Highly significant yields were obtained by bubbling sea water that had been prefiltered through silver or Millipore filters of 1.2 p pore size or glass fibre. The yield declined t o a very slightly significant level when 0.22 p filters were used for pre-filtration. The results implied that particles in the size range of 0.22-1 - 2 p, and possibly larger, are important as nuclei in the formation of larger particles. Further tests compared triple filtration through 0.8 p filters, as practised by Menzel (1966), with single filtration, and the latter gave higher yields, indicating that part of his difficulty in producing particles was due simply t o the fact that triple filtration, even through filters of medium pore size, removes a considerable fraction of the small particles that facilitate aggregation. However, Batoosingh et al. (1969) found that even after triple filtration the yield was not entirely insignificant. They used a 24-hour bubbling period, as contrasted with 3-6 hours in Menzel’s experiments, and the bottle used as a bubbling chamber had a larger surface : volume ratio than his cylinder. These factors may have been responsible for the difference. Barber’s (1966) conclusion that bacteria are necessary for the aggregation process was not confirmed, but it was not categorically denied. Batoosingh et al. (1969) found that samples filtred through 0.22 p filters were sterile, as judged by plating test, yet there was a slight

48

GORDON A. RILEY

amount of particle formation. However, if samples treated in this manner proved not t o be entirely sterile, growth of bacteria might provide nuclei for further aggregation and therefore might lead t o larger yield of particles. There is no doubt that bacteria become incorporated in particles, and indeed prolonged bubbling is an effective means for sterilizing sea water, but there is no evidence that bacteria play a physiological role in the process. Thus it was suspected that bacteria or any organic particles of a similar size might be equally effective as nuclei for aggregation. This matter can be tested further by coarse filtration and some kind of sterilization of the filtrate. However, most kinds of sterilization are suspect on the grounds that they might introduce artifacts into the experiment. Treatment with cyanide seemed to be one of the least objectionable methods. This was tried in a series of replicates using 1.2 p pre-filters in which one set was bubbled for 24 hours in the usual way, and replicate pairs were given identical treatment except for an addition of 50 mg KCN/litre. There was no significant difference in the results. The experiments did not fully prove the point because the KCN treatment did not establish complete sterility. Plate counts indicated that there could be as many as 100-200 bacterial cells/ml a t the end of the experiment. However, this was one to two orders of magnitude less than the quantity in untreated controls, and if living bacteria were very important one would expect t o find some difference in experimental results. Having demonstrated that air supplies did not contribute significantly t o particle formation, Batoosingh et al. (1969) carried out a few experiments of longer duration in further examination of time relations involved in particle formation. One experiment involved continuous bubbling for 15 days, with daily removal of an aliquot for carbon analysis. A concentration of 227 pg C/litre was attained on the second day, and no further increase was observed thereafter. Earlier experiments on alternate bubbling and filtering were repeated with better controls and more thorough replication and with essentially the same kind of results that had been obtained earlier. I n a series of nine experiments the mean total yield for four successive oneday bubbling periods was 492 pg C/litre and this was 3.3 times the mean yield for the first day, indicating that particle formation can continue for some time with only slight diminution if the particles are removed a t intervals. Another experiment was devised to provide continuous yield, and the amount obtained during one day was larger than in four days of alternate bubbling and filtering. I n short, the results reaffirmed

PARTICULATE ORGANIC MATTER I N S E A WATER

4!)

previous conclusions that the rate of particle formation is rapid a t first but cannot be maintained unless the particles are removed. Certain aspects of experimental particle formation in sea water have not yet been thoroughly studied. For example, quantitative studies of the effects of variations in air flow and bubble size are still lacking. More important, the physical chemistry of the aggregation process remains obscure, and information about the kinds of compounds involved is little more than anecdotal. These basic problems probably could be studied more effectively with artificial sea water of known composition, both organic and inorganic, than with natural sea water. Wangersky (1965) began a programme of this sort but was unable to continue it because of difficulties in removing organic contaminants from his distilled water. The problem still awaits study, and little further progress can be made until the basic facts become clearer. A second kind of problem, of a different and perhaps even more difficult sort, is the assessment of natural rates of particle production at the sea surface and attempts to synthesize laboratory results and field observations into a coherent picture of what is actually happening a t the sea surface. These problems will be considered in a later discussion, but the discussion will raise more questions than it answers.

B. I n situ production and utilization 1. Organic carbon

Riley et al. (1965) proposed the hypothesis that the lack of a systematic vertical gradient in particulate matter in the deep sea could best be explained as an expression of a dynamic balance between utilization and further adsorption from the filterable fraction. To test whether such adsorption might occur, particles were produced artificially by bubbling, and small quantities of this water were seeded into carboys of filtered deep sea water and were allowed to stand without disturbance for a day. The carboys were then filtered for analyses of particulate carbon, and there was in fact more carbon present than had been added. Further experiments involved additions of experimentally produced particles and natural particulate matter to filtered deep sea water, and the water was allowed to stand for several days. The same kind of results was obtained, but there was also a significant accumulation of carbon in controls that were simply filtered, with no addition of substrate for adsorption. This suggested the existence of some sort of process whereby particles could be produced by aggregation of filter

50

OOKUON A. RILEY

passing materials. The experiments were not published a t that time, pending further study of the problem. Sheldon et al. (1967) were the first to publish an account of the early events in this aggregation process. They followed the course of particle formation in filtered sea water with repeated Coulter Counter measurements of the total volume of particles in a graded size series between 1-58 and 5 p. Initial particle formation was rapid, leveling off after a few days. The smallest sizes were most abundant a t first, and there was a gradual shift toward a maximum concentration a t a particle size of about 4 p. The process could occur in the light or in the dark ; it was stimulated by addition of glycine and inhibited, a t least temporarily, by mercuric chloride. They suggested that bacterial action probably was implicated in the formation, but this could not be established beyond question. When the sample was filtered after equilibrium concentration had been raached, another cycle of production occurred, and as many as four cycles were induced in one sample by repeated filtration. They estimated that the concentration of particulate organic carbon might be of the order of 25 pg/litre a t the time of equilibrium. The writer has followed the course of particle formation in filtered sea water by means of carbon determinations and to a lesser extent by visual examination. The initial increase described by Sheldon et al. (1967) is near the limits of sensitivity of the carbon method, and during the first 2-3 days the results generally are not significantly higher than those obtained by immediate refiltration. However, after about five days a difference can generally be notc3d, and the quantity is more or less in agreement with the estimate made by Sheldon et al. (1967). The quantity of particulate organic carbon continues to increase in these experiments, commonly reaching a peak in 10-20 days, and a t that time the carbon concentration is likely to be a t least an order of magnitude higher than the equilibrium value estimated by Sheldon et al. (1967). Visual examination shows that the most important constituents are hazy, amorphous masses of material in the general size range of about 25 p , which stain heavily with Schiff reagent. The protein staining reaction is generally less marked and may be absent. Bacteria are associated with this material ; their protein staining reaction makes them clearly visible. Presumably these are two chapters of the same story. The so-called equilibrium stage described by Sheldon et al. ( 1 967) probably is merely an equilibrium of small particles within the size range that he was particularly concerned with, but these particles gradually coalesce into larger masses, are worked over by bacteria, and are gradually

PARTICULATE ORQANIC MATTER I N SEA WATER

51

transformed into materials that look like pale primordia of the aggregates found in nature. I n short, the writer suspects that the equilibrium condition is a dynamic steady state rather than a static situation. Otsuki (1968) has described the formation of aggregates in fresh water during the course of bacterial utilization of dissolved organic matter produced by addition of dried algae of the genus Scenedesmus. There are photographs of these aggregates in the paper, and they are indistinguishable from the marine material that has been described. I n one experiment of 185 days’ duration the dissolved organic carbon decreased from 213 to 87.1 mg/litre, due t o the combined effects of bacterial utilization and conversion of dissolved t o particulate carbon, but no estimate was made of the amount of particulate carbon formed. There was a concomitant decrease in the carbon : nitrogen ratio from 11.1to 5.9 (atoms). Similar decreases in C : N ratios were noted in many other decomposition experiments. Parsons and Seki (1968) have described aggsegation phenomena in both filtered and unfiltered sea water. They mentioned having seen several different kinds of particles, but special attention was focused on clumping of bacteria. The size of the clumps was of the order of 4 p , more or less, and these authors apparently felt that the clumps consisted primarily of bacterial biomass although there was a suggestion of cementing action by excreted polymers together with inclusions of other materials. The writer is aware of this clumping phenomenon, but again this appears to be a stage in the production of larger aggregates in which the major part of the organic matter is non-living. Admittedly the early stages in the development of aggregates are not very noticeable without the aid of staining procedures, and they are so delicate that they may be broken up during Coulter Counter measurements. Two special cases of i n situ aggregate formation are mentioned in passing. Sieburth and Jensen (1968) described extracellular production of organic matter by Pucus and other littoral brown algae, and they found that some of the end products of decomposition of the exudates condensed into organic aggregates. The authors described this material as “ polyphenol tanned carbohydrate and proteinaceous material ”. A second example was reported by Johannes (1967), who found a marked increase in organic aggregates in shallow water passing over an atoll reef. The aggregates apparently were formed from mucus secreted by the corals; however, this probably was a result of fragmentation of pre-existing materials rather than in s i t u condensation from smaller particles or dissolved substance. Hence the example is not quite comparable with other studies that are being reviewed in this section.

52

ClOCDOX A . EILEP

The rather surprising finding that particles form spontaneously in filtered sea water deserves study in an attempt to determine the processes that are involved and to try to find out whether this is a phenomenon that is likely t o occur in nature. The writer began such a study several years ago. At the same time other experiments were underway in an attempt t o find out something about rates of utilization of particles produced i n situ or by experimental bubbling. As it turned out, all of these converged into a single problem. One of the first experiments involved bubbling two 10-litre samples of water, addition of a small amount of an actively growing culture of

01 0

10 TIME

IN

I

I

20

30

DAYS

FIG. 4. I n situ formation and utilization of particulate organic matter. Duplicate samples were bubbled to produce particulate matter and then were inoculated with Pseudomonas sp. Curves show ensuing concentrations of organic carbon with respect to time.

a marine bacterium, Pseudomonas sp., and removing one-litre samples initially and a t intervals thereafter for measurements of total particulate carbon. The results are shown in Fig. 4. The initial values were 0.13 and 0-16mg C/litre. There was a three- t o five-fold increase in the first two days. Estimates of the increase in bacterial cell substance, based on plate counts and known carbon content of the original inoculum, indicated that 20% or less of the observed increase was due to bacteria per se. If these estimates were realistic the results could only mean that additional organic carbon from the filter passing fraction was being converted into particulate form. Subsequently the quantity decreased and more or less levelled off for a considerable period. After the 24th day (the last analysis shown in Fig. 4) there was only a little

53

PARTICULATE ORGANIC MATTER I N SEA WATER

water left. It was held for a final analysis twenty days later, and a t that time the level of particulate carbon had gone up again and was 0.61 mg C/litre in one bottle and 1.08 mg in the other. Some of the water was plated, and it was found that the original inoculum had been contaminated by growth of about five other species. It remains uncertain as t o whether the increase a t the end of the experiment represented a second cycle of growth by these other species or was simply the result of suspension of materials which previously had been adhering too tightly t o the carboy t o be included in the analysis. Visual examination from time to time had revealed the presence of bacteria, flakes, and amorphous aggregates. The form of the curve for

0

5 TIME

IN

10 DAYS

15

FIG. 5. I n situ formation and utilization of particulate organic matter. This experiment is similar to the ones shown in Fig. 4 except that Pseudomonas was added directly to filtered water without prior bubbling.

total carbon suggested a typical laboratory cycle of bacterial growth, with an initial phase of logarithmic increase followed by decline and senescence, yet all evidence pointed toward the conclusion that living bacteria were a minor part of the total organic matter. Thus it seemed likely that bacterial growth was basically responsible for initial aggregation of organic matter and for utilization of the portion that could be metabolized readily, so that the living and non-living fractions varied together. Fig. 5 shows the results of an experiment in which Pseudomonas was added to filtered water which had not been bubbled. A similar curve A.M.B.-8

3

54

UORDON A . RILEY

was obtained, but the peak was reached more slowly. I n this experiment the dissolved organic carbon was initially 2.7 mg C/litre. At the time of the peak it had declined to 1.95 mgllitre. Thus about half of the observed decrease represented conversion to particulate matter, living or dead, and the remainder can be ascribed to metabolic utilization. During the remainder of the experiment the dissolved organic carbon remained constant within the limits of error of the analytical method. Thus the decrease in particulate carbon after its peak is presumed to be due to bacterial utilization, and this is followed by a steady state situation in which the metabolism of the bacteria proceeds at a rate that is too low to be detected by the methods employed. Studies of in situ accumulation of particulate matter have been carried out by filtering sea water through Millipore or silver filters with a pore size of 0-45 p or larger, which allow part of the bacterial flora t o pass through and repopulate the water. Filtration has been carried out by gravity, or sometimes using pressure generated by a peristaltic pump. I n an attempt to reduce air-water interfaces as much as possible, most of the experiments have utilized polyethylene bags as experimental containers. These bags were of about 4-litre capacity and were sealed at the top, with a polyethylene tube sealed into each corner. One tube extended to the bottom and served as an inlet. A short tube at the other corner was used to evacuate the bag slightly. Thus the inflowing water gradually filled and expanded the bags and had no contact with air except for occasional accidental bubbles. Glass bottles were used in some experiments for comparison with results obtained in bags and also for experiments involving long period storage. The bottles were filled by gravity filtration with as little disturbance as possible and were allowed to overflow before sealing, in the same way that bottles are filled for oxygen analysis. Fig. 6 shows two examples of the kind of results obtained in bag experiments, using water that was filtered but with no bacteria added. Each point represents the mean carbon content of a bag that was refiltered at the time indicated on the graph. The particulate carbon during the first few days was indistinguishable from the amount obtained when water is refiltered immediately. Subsequently the carbon content increased to roughly the same value that was found when samples were inoculated artificially, the only difference being in the length of time required to reach the peak. Other experiments that lasted longer have shown that in this type of experiment, as in others described above, the particulate carbon decreased later and tended to level off. However, in both types there have been a few experiments which did not follow the usual pattern in that the carbon increased and

55

PARTICULATE ORGANIC MATTER I N SEA WATER

levelled off without showing a definite peak. Hence the experiments as a whole show a large variation in maximum concentration. I n an attempt t o find out whether bacteria are essential to this process, some of the experiments have been poisoned with potassium dichromate (1 mg/litre) or potassium cyanide (50 mg/litre). No peaks were observed in these experiments, but there was some slight increase

P

“Oi

b

10 TIME

20

30

IN DAYS

FIG. 6 . In situ formation of particulate organic matter. allowed t o stand, with no further treatment.

Samples were filtered and

to a “ steady state ” a t a lower level than would be expected when bacteria are allowed to grow freely. However, the results were equivocal, for bacteria were not completely eliminated, particularly in the case of dichromate treatment. TABLE11. AVERAGECONCENTRATIONOF PARTICULATE ORGANICCARBON (MG C/LITRE) OBTAINEDIN STUDIESOF BACTERIAL EFFECTS ON BUBBLED WATERAND in situ PRODUCTION AND UTILIZATION

Bubbled ; bacteria added In aitu, with or without bacterial inoculation In situ, poisoned _____

~_____

Maximum

Steady State

0.44 & 0.25

0.20 f 0.06 0.15 f 0.08 0.10 0.02

0.41 f 0.30 -

*

~ ~ _ _ _ _

Table I1 summarizes the results of fifteen experiments with inshore waters taken near the Nova Scotia coast, showing maximum observed concentrations averaged for each type of experiment and the “ steady state ” concentration attained after the peak.

56

GORDON A. RILEY

The question of bacterial effects was reopened a t a later time with somewhat more conclusive results. The work by Batoosingh et al. (1969), involving comparison of bubbling tests with water that was refiltered immediately, provided the necessary material, for it was found that two filtrations in rapid succession were a fairly effective means of sterilizing the water. Two sets of experiments were performed in which this was held for 5 days and for 12 days and then was filtered a third time, and the 12-day set was plated t o determine the degree of sterility. A duplicate set was given an immediate third filtration to serve as a control to gauge the amount of additional formation during the 5-12 day periods, and a fourth set had 1 ml of raw sea water added to the doubly filtered water to examine the question of whether double filtration would in itself affect the normal process of aggregation. Plate counts indicated that the filtration procedure was reasonably effective. Some plates were completely negative and none had more than a few bacteria per ml. The observed increase in organic carbon averaged 19 pg C/litre in five-day experiments and 6 pgllitre in the twelve-day set. The general range in values was such that these averages were not significantly different from each other nor from the controls. If one chooses to believe that the increase is real, it is no more than might be expected from incipient aggregation of the type described by Sheldon et al. (1967), which may be entirely a physical phenomenon, although this is not as yet definitely established. I n contrast, inoculated 12-day experiments showed a mean increase of 155 pg C/litre, which was significantly higher than controls or the accompanying experiments, although it is somewhat lower than is commonly obtained in water that has been filtered only once. A cruise on the Canadian oceanographic ship “ Hudson ” in JuneJuly 1967 provided an opportunity for experiments with samples taken from a variety of water masses off the eastern Canadian seaboard. Complete time series were difficult under shipboard conditions, for it was desirable to process as many different samples as possible within the limited time available. Most of the experiments were of 6-8 days’ duration, which should be long enough to determine whether particle formation was occurring but probably not long enough to establish a maximum concentration. Samples were filtered by gravity through silver filters (0.45 or 1.2 p ) into four-litre polyethylene bags. Approximately one week later the water was removed through filters of the same pore size used initially, and these were frozen for later analysis ashore. Results are summarized by area and depth range in Table 111. The area listed as coastal and bank waters includes the Scotian Shelf, one station each on the Grand Banks and Flemish Cap, and one

57

PARTICULATE ORGANIC MATTER I N SEA WATER

TABLE111. MEAN PARTICULATE CARBON ( pC/LITRE) IN 6-8 DAY dn S i t U AGGREGATION EXPERIMENTS WITH WATER SAMPLES TAKEN FROM THE AREASAND DEPTH RANGES LISTED. NUMBER OF EXPERIMENTS IS SHOWN I N PARENTHESES

Depth Range 1-150 500-1 200 2 000-4 000

Coastal and Bank Waters

Slope Water

168 ( 6 )

67 (2) 159 (6)

~

68 (4)

Sargasso Sea 46 (1) -

102 ( 1 )

station at the eastern edge of the Grand Banks in the core of the Labrador Current. The Slope Water stations were made in various locations between the Scotian Shelf and the edge of the Gulf Stream. The station labelled Sargasso Sea was in the northern fringe of that area or possibly in the southern edge of the Gulf Stream. For purposes of statistical analysis the data were lumped into two groups of approximately equal size, comprising all samples taken between the surface and 600 m and all those in the 1 000-4 000 m depth range. No samples were immediately refiltered on this cruise to provide paired samples for comparison. Alternatively they were compared with a set of refiltrations of inshore water (Batoosingh et al., 1969) which yielded values of 46 f 20 pg C/litre when 1.2 p filters were used. The means for shallow and deep samples were respectively 127 f 87 and 115 65 pg C/litre. Using the " t " test for unpaired samples, the differences between these means and the ones obtained upon immediate filtration were significant to the 2% level of probability. This is regarded as a reasonably acceptable test in view of the fact that the method is not as critical as comparisons of paired samples. One five gallon carboy of water from this cruise and two from a previous one have been used t o examine the so-called steady state which develops after the initial burst of bacterial growth. The first two were obtained from depths of 2 000-3 000 m in the Sargasso Sea, the third from 900 m in Slope Water south of the Grand Banks. All were filtered immediately on collection. The first carboy was seeded with 1 ml of an actively growing culture of Pseudomona,s. A week later a sample of 1 litre was withdrawn for carbon analysis, and this was followed by successive one-week samplings, 17 in all, until the supply was exhausted. The mean particulate carbon during this period was 63 f 30 pg/litre. There was no obvious upward or downward trend. The variations possibly were real or may

58

GORDON A. RILEY

have been due to sampling errors, for the particulate matter sinks to the bottom of the carboy and may not be thoroughly detached during the mixing process prior t o sampling. Nevertheless, there was an indication of the maintenance of general level, if not a true steady state, for a considerable period. The second carboy was subjected to overnight bubbling to create an initial substrate. Pseudomonas was then added as before, and carbon was again determined a t weekly intervals. During the first 1 2 weeks the average was 98 f 37 pg C/litre. At the end of the 13th week there was an anomalously high value of 1.29 mg C/litre. Probably the filter was grossly contaminated. The possibility of a second cycle of bacterial growth seemed unlikely; there was no evidence of it in plate counts made concurrently. During the remaining four weeks the organic carbon was somewhat higher than it had been in the early part of the experiment (140-170 pg C/litre). The third experiment was inoculated with 10ml of unfiltered surface water. The first sample taken a week later had a concentration of 191 pg Cjl. Subsequently the level declined, and there was a more or less steady state of 106 34 pg C/litre until the end of the 15th week. During the last two weeks there was an increase in organic matter (270-300 pg C/litre). The experiments described thus far indicated that newly formed aggregates in filtered sea water are subject to a rapid increase initially, followed by rapid bacterial utilization, so that by the time the stable phase has been reached the concentration of particulate organic carbon will be roughly half the earlier maximum. At this point the remaining organic matter would appear to be resistant to further attack, although persistence of living bacteria implies either further utilization a t a very slow rate or some slight degree of further aggregation of filterable materials or direct utilization of dissolved organic matter by bacteria. Some variability has been noted in the so-called stable phase in some experiments, and the level varies somewhat from one experiment to another. However, the variation is not large, and the general level of the experimental product is more or less the same as that of naturally occurring particulate matter in the sea and is also similar to that attained in bubbling experiments. The bubbling experiments and in situ production appear to be somewhat analogous in that the rapid rate of initial particle production quickly gives way to a situation in which further formation appears t o be inhibited. This might be a physical inhibition of the sort postulated for bubbling experiments, or it might be caused by exhaustion of substances suitable for aggregation. If this is simply a matter of physical inhibition by the standing

PARTICULATE ORGANIC MATTER I N SEA WATER

59

stock of particles, partial removal of the stock should lead to further formation, as it did in the ca.se of continuous harvesting in bubbling experiments. However, bacterial aggregation is a slower process than adsorption on bubbles, so that a relatively slow rate of harvesting would be necessary in order to test this point. A bottle of freshly filtered inshore water was inoculated with 5 ml of unfiltered water. The bottle was wrapped with dark cloth to eliminate any possibility of autotrophic growth. Water was pumped from the bottle at a rate of 0-6 ml/minute via silicone tubing to a glass fibre filter and thence was returned to the bottle. Every four days a filter was harvested and replaced by a fresh one. During this period the calculated flow through the filter was 3.45 litres. The total volume in the experiment was 4.3 litres, so that the rate of harvesting was 20% of the total volume in a day. I n calculating results the measured amount of organic carbon on the filter could then be used to determine the total amount harvested and also the mean concentration in the bottle during each four-day period. Table I V shows the mean concentrations and the total yield, the latter obtained by summing successive four-day yields. AND TOTAL YIELDIN TABLEIV. ORGANICCARBONCONCENTRATION MG C/LITRE UNDER CONDITIONSOF CONTINUOUS,SLOW HARVESTING

T i m e in Days

Mean Concentration

Total Yield

0-4 4-8 8-12 12-16 16-20 20-24 24-28 28-32 32-36 36-40

0.31 0.12 0.12 0.13 0.10 0.14 0.11 0.08 0.09 0.09

0.25 0.34 0.44 0.55 0.63 0.75 0.83 0.89 0.96 1.03

The mean concentration during the first four days was indicative of an initial burst of bacterial growth. Subsequently there was a more or less uniform concentration until the end of the fourth week and a slight decline thereafter. The concentrations were only slightly smaller and probably were not significantly different from the value of 0.15 f 0.08 mg C/litre reported in Table I1 (p. 5 5 ) for steady state situations in

60

GORDON A . RILEY

experiments using essentially the same kind of water but with no harvesting procedure. This experiment was repeated, using a surface sample taken from the Slope Water area well offshore. Other samples from the same collection were used for additional experiments of a comparative nature. One of these was a 14-litre sample which was stored in a polyethylene carboy. On alternate days 1 litre was removed and filtered for carbon analysis, and the filtered water was then returned to the carboy. Practically speaking, this approximated t o a continuous yield experiment but the daily rate of removal averaged only about 3.5% instead of 20%. The third was a control carboy in which there was no cropping, but 1 litre was removed each week for carbon analysis, and the filtered water was then discarded. Preliminary filtration was carried out a t sea, and the water stood for five days before the experiment began. Initial experiment readings were about 0-40 mg C/litre, and the concentrations declined as the experiments proceeded. Table V shows the mean concentration of carbon during the first four weeks and the total yield.

v. MEAN CONCENTRATION AND TOTALYIELD O F PARTICULATE ORGANICCARRON(MG C/LITRE)WITH THREEDIFFERENT LEVELSOF CROPPING

TABLE

No removal 776 removal, alternate days 20% daily removal

Concentration in Suspension

Total Yield

0.23 0.20 0.13

0 0.24 0.74

Results show that the lower rate of cropping can lead to a significant yield over a period of time with little reduction in the amount of particulate matter in suspension. The higher rate of 20% cropping reduced the concentration by an amount that appeared to be slightly significant (P falls between 0.05 and 0.02). However, a t the expense of only a slight decrease in concentration the yield was substantially increased and was only slightly less than was obtained during the first four weeks of the experiment with inshore waters. During the fourth week there was a general decline in the amount of particulate carbon in all bottles. The concentration in the control bottle was 0.17 mg C/litre at the end of this period, and the averages for the fourth week in the other two bottles were respectively 0.23 and 0.06 mg.

PARTICULATE ORGANIC MATTER I N SEA WATER

61

At the end of the fourth week 1 ml of raw sea water was added to each bottle to find out if a fresh and active supply of bacteria could alter the trend. It did not do so. When the experiment was terminated at the end of the fifth week the concentrations in the three bottles were 0.14, 0.1 I , and 0.05 mg C/litre, respectively. I n summary, the maximum sustained concentration of particulate organic carbon, either in long term bubbling experiments or in situ aggregation, is seldom much more than 0.2 mg C/litre. This concentration tends to be maintained for a considerable period even when part of the product is removed. However, a downward trend has been noted, particularly in cases of heavy cropping, and deep sea samples frequently have a lower steady state concentration than surface samples. A hypothesis has been proposed that particle formation inhibits further formation and that this is responsible for the upper limit of concentration, but there are further indications that the supply of filterable organic matter can be reduced to a point where a maximum concentration of particles cannot be attained or maintained. 2. Bacteria

Most of the experiments were accompanied by plate counts of bacteria. Aliquots of water from experimental bottles were diluted serially in autoclaved water and the series was plated on enriched sea water agar. The plates were then incubated for 3-5 days at 1 5 O C . The dilutions ranged through several orders of magnitude. Ideally the investigator can discard plates which have too few colonies for a good count, or are so heavily overgrown that there is danger of fusion of colonies, and can base his judgement on one or two plates in the middle of the range. There are subjective elements in this procedure, and replication is not as precise as might be desired. Moreover, as everyone who has dealt with this problem is well aware, plate counts commonly give much lower values than direct visual examination. These and certain other difficulties will have to be taken into account in evaluating the results that are to be described. The early series of experiments, for which carbon values are given in Table 11, were mostly of short duration. Some involved an initial inoculation with Pseudomonas sp., and in other cases a natural flora was allowed to develop. I n the former case the method was simply to introduce a small quantity of an actively growing culture of bacteria in the proportion of 0.1 ml of culture per litre of filtered sea water. The actual number of bacteria was not determined in each case, but on two occasions when samples of sea water were plated immediately after inoculation, the count was of the order of lo3 cells/ml.

62

GORDON A . RILEY

I n all cases examined, higher values were obtained subsequently. The maximum count recorded was 87 x l o 3 cells/ml. Most experiments had maximum readings of 30 x l o 3 or higher, but a few were lower. The sampling interval may not have been frequent enough t o record true maxima, and this may have accounted for some of the variability. However, there was an extreme case in which the maximum bacterial count was only 8 x 103 cells/ml, and this was one of the experiments in which organic carbon did not exhibit a n early peak either. The maximum observed concentration during the 10-day experiment was 140 pg C/litre. The earlier discussion indicated that addition of an inoculum of bacteria is not necessary for formation of particulate carbon. Aggregation occurs more slowly when bacteria are not added, but there is little difference in the peak concentration that is eventually achieved. Plate counts showed that there was in fact an abundant increase of bacteria in filtered sea water, but maximum concentrations were seldom as large as those obtained when an inoculum had been introduced. This is best demonstrated in six pairs of experiments which were replicates with respect to the original source of sea water and mode of handling and differed only in that one set was inoculated with Pseudomonas and the other was not. Maximum observed concentrations averaged 39 x lo3 cellslml in the first set and 16 x lo3 in the second. I n experiments inoculated with Pseudomonas, plate counts declined rapidly after the early maximum, and by the end of the first week most counts were of the order of I2 x lo3 cells/ml or less. The total range was of the order of 1-12 x 103, and a reasonable average for the series is about 8 x 103. There was considerable variation from one plate count to another in the same experiment, but comparisons of replicate plates and successive dilutions suggest that the variation can be ascribed largely t o sampling error rather than to real variation in the size of the population. I n cases where the bacterial population was derived from natural flora which escaped the filter, the experiments generally did not last long enough to permit a good estimate of the population level subsequent t o the maximum. This matter will be taken u p in connection with long term experiments to be described later. The relationship of plate counts t o the real population is always problematical. Direct counts in natural sea water commonly range from 1t o 4 orders of magnitude larger than plate counts (Jannasch and Jones, 1959). The culture medium and often the physical conditions of the laboratory environment are not suitable for the growth of a large majority of naturally occurring species. However, the Pseudo-

PARTICULATE ORGANIC MATTER IN SEA WATER

63

monas that was used as an artificial inoculum had been grown in the laboratory for some months and had demonstrated that it was one of the species that thrives under such conditions. Hence there is little reason t o doubt that it could be monitored by plate counts with a fair degree of accuracy. If these counts are accepted as realistic they can be used to evaluate the biomass of bacteria in relation to total particulate carbon. HolmHansen and Booth (1966) estimated that average carbon content of bacteria is about 4 x 1 0 - 7 pglcell, and rough computations based on the measured volume of Pseudomonas suggest that this is a reasonable value t o use for present purposes. Thus the largest observed population of 87 x 103 cells/ml should contain about 35 pg C/litre, and in the later period after the maximum, an average figure would be about 3pg. Thus the living fraction would appear t o be less than one-tenth of total particulate carbon a t all times, and to anyone who has examined the material visually, this is not an unreasonable conclusion. Earlier estimates of 20%, based on direct carbon analysis, possibly are too large, for the inoculum contained some non-living flocculated material. There is a possibility that these figures represent a slight underestimate on the grounds that the Pseudomonas inoculum probably was contaminated by the growth of adventitious bacteria introduced by the filtering process, and some of these may not have been susceptible to detection by plating methods. However, the experiments themselves impose a selective process for bacteria which can grow at room temperature, and in the absence of an artificial inoculum the plate counts for such bacteria were half as large a t the time of the maximum as in the Pseudomonas experiments. If the difference had been an order of magnitude or more, the possibility of the existence of a large undetected population would seem more likely. The paper by Holm-Hansen and Booth (1966) which was cited above was particularly concerned with the development of a method for measuring adenosine triphosphate (ATP) and using it to determine relative proportions of living and non-living particulate organic matter. They established the fact that ATP disintegrates almost instantly upon death of the organisms, and the concentration varied within reasonably narrow limits in various organisms examined, so that the method appeared to be thoroughly suitable for its intended purpose. I n deep water, where bacteria are believed to constitute the major living component, the method was used to estimate the number of viable bacteria. The average for the depth range of 183-1 025 m a t a station off the California coast was 6 x 103 cells/ml, and the authors pointed out that these results were in good agreement with direct counts reported

64

GORDON A.:RILEY

by Kriss (1963) in various deep water situations. The values obtained by these authors are similar to the estimated population of Pseudomonas during the stable phase after the early maximum, suggesting that the balance between living and non-living fractions in these experiments is essentially similar to that which occurs in the sea. Plate counts were made during the experiments involving long term storage of deep water and in two of the three experiments summarized in Table V. The third experiment in Table V, involving continuous circulation and a 20% cropping rate, was not monitored. These experiments included inoculation with Pseudomonas, inoculation with raw sea water, and simple filtration with no additions, but in all cases results were essentially the same, differing only in minor details of timing and relatively insignificant variations in population level. The early maximum was observed in some experiments and not in others, as indicated in the earlier account of carbon concentrations. There was a stable phase with populations of the order of lo4 cells/ml, more or less, which was approximately the same as in short term Pseudomonas experiments described earlier. This persisted for 2-4 weeks, and then the plate counts began to increase steadily. The number of dilutions required for suitable counts had t o be increased. Errors were suspected, and the water used for dilution was examined frequently and found to be negative. The increase continued to the end of experiments lasting 17 weeks, and the concentration at that time was of the order of 10" cells/ml or more. No good reason has been found to reject the plate counts. On the other hand, the quantity of bacteria found, if they had a " typical " carbon content of 4 x 10-7 pglcell, would total 0.4-1 mg C/litre. This is about an order of magnitude higher than the total carbon that was demonstrated t o be present. Moreover, visual examination gave no indication of a marked increase in bacteria. Careful microscopic counts were not attempted but any such increase would be clearly evident. If the plate counts are valid, the bacteria would have to be small enough t o be unrecognized as such in these examinations and of a size to permit their total carbon content t o fall within reasonable limits. This probably means a size range of 0.2-0.4 p for much of the p o p lation. There is a common impression that most marine bacteria are of the order of 1-3 p (ZoBell, 1946). However, water that has passed through 0-45 p filters is almost never sterile, and the most rigorous precautions do not always prevent growth in water that has gone through 0.22 p filters. Later examination of such filtrates commonly shows the presence of bacteria that are too large to pass through the filters.

PARTICULATE ORG-4NIC MATTER I N SEA WATER

65

Hence, while there may be marine bacteria that fall within this size range in their normal vegetative state, some of them must have nonactive stages of a smaller size. Senescent stages are commonly reduced in size. The writer can find no information as to whether any marine bacteria are known to produce filterable " L phases ", although certainly many of them are highly pleomorphic. Thus the existence of viable, filterable bacteria in sea water is established beyond reasonable doubt, but the form in which they occur is open to question, and their quantitative abundance is not known. They are possibly a small percentage of the total biomass. The ATP analyses described by HolmHansen and Booth (1966), indicated that total biomass was essentially in agreement with observed numbers of cells of normal size. The situation described in long term experiments is clearly different, but experimental procedures induce an artificial selection of filterable forms and eliminate any bacteria that require special conditions such as low temperature or high pressure. The ones that survive and multiply in the experiments have a good chance of being suitable for detection in plate counts. Although these considerations supply a possible explanation for astronomical numbers of bacteria despite the relatively low value for particulate carbon, they do not provide much insight into the dynamics of the situation. The early part of the experiments suggested a stable state in both bacteria and particulate matter. The latter remained more or less constant thereafter, although increasing slightly during the final few weeks of some experiments, while the bacterial population was anything but stable. However, there is a question as to how many of these bacteria were in an active physiological state. There was a small, continuing p o p lation of bacteria of normal size, which was readily detectable by visual examination. One can postulate that these were existing on the verge of senescence, some being able to divide normally and others being transformed into smaller forms which gradually accumulated in a viable but inactive state. I n summary, the early stages of the so-called stable state seem realistic in that carbon levels and bacterial counts approximate those found in the sea. The later sequence of events remains realistic only with respect to maintenance of a reasonably stable carbon level, and problems involving dynamic relations with bacteria require further study by more exacting methods. Here again, as in bubbling experimenfs, there is a need for an examination of basic problems, preferably with defined artificial media. Moreover, the experiments are deficient in that data are largely

66

GORDON A. RILEY

lacking on dissolved organic carbon. The ordinary method for determining organic carbon by wet oxidation is not precise enough to provide useful information on some of the small changes that are involved here, and as indicated earlier there is considerable doubt as to whether all of the organic carbon is oxidized. Methods for carbon determination by dry combustion are under development in this laboratory by P. J. Wangersky, but data are not yet available for incorporation in the present account. C. Sinking rates 1. Introduction to the problem Various problems involving the biological and geochemical significance of non-living particulate matter require some knowledge of its residence time in the ocean and its sinking rate. Thus far no direct measurements of sinking rates have been published. Riley et al. (1965) made an indirect calculation based upon estimated food requirements and availability of assimilable organic matter in deep water. The results implied an average residence time of four years and an average sinking rate of 2 miday, with a general range in likely estimates of 1-7 m/day. This estimate was of course dependent on the validity of several assumptions about biological relationships. In particular, recent work by Gordon (1970b) on enzymatic hydrolysis of particulate organic matter suggests that the assimilable fraction may be less than was assumed by Riley et al. (1965), and if this is so, the sinking rate was over estimated. I n contrast with these estimates Menzel and Goering (1966) postulated that the particulate matter below a depth of about 200m is unassimilable and neutrally buoyant. Some experiments were described in which water samples were filtered, and the filter pads were inoculated with surface water. No significant decrease in carbon was found after 90 days’ storage. The further postulate that the particles are neutrally buoyant was not supported by data but was regarded by these authors as the simplest explanation for observed patterns of distribution. Hobson (1967) used a Coulter Counter to measure the size-frequency distribution of deep water particulate matter, and with certain assumptions about the specific gravity of the particles he was able to calculate sinking rates, using a modified version of the Stokes equation. He concluded that most of the rates should be within the general range of 0-1-10 m/day. This large range resulted from a variety of assumptions about specific gravity ; there was no certainty about the correct value to be used, and the range was too large to be very useful.

PARTICULATE ORGANIC MATTER I N SEA WATER

67

Direct measurements clearly are needed. The present section will deal with a few measurements that have been made on naturally occurring surface and deep water particulate matter and experimental preparations. The quantity of data is not large, but it is enough for present purposes. 2. Methods

This work was done with an inverted microscope, using a counting cell 1-5cm in diameter and 4.5 cm high, which was merely a lucite tube with a cover slip cemented to the bottom. Lucite was used because it has a lower rate of thermal conductivity than glass, and it is important in experiments of this sort to reduce convective mixing as much as possible. A cover slip was laid on top to inhibit evaporative cooling. Preparation of the material presented some problems. I n the beginning an attempt was made to follow the method used by Smayda and Boleyn (1965, 1966a and b) to measure the sinking rate of diatoms. These investigators filled their tubes with sea water of a salinity of 32%, and introduced a diatom population at the surface in a medium of 30%,. Initially all of the diatoms were within the upper boundary layer, and the bottom of the cell was then scanned at intervals thereafter to determine the rate of sinking from surface to bottom. There were practica1 difficulties in applying this method in the present case. I n order to get enough material for a good count, the particulate matter had to be concentrated into a small volume. Under these conditions the aggregates tended to coalesce, and the sizefrequency distribution was unrealistic. Thus it was necessary to use whole water samples or slightly concentrated ones and to fill the entire cell with uniformly mixed material. The method sacrificed any possibility of determining sinking rates of individual particles but permitted a statistical estimate of the average rate of passage of the various kinds of particles through a mean vertical path. Repeated examinations were made of the same area on the bottom of the cell, consisting of a strip 0-3 mm wide and 10 mm long, under a magnification of x 300. The ocular grid that was used as a guide in counting was subdivided into smaller squares equivalent to 30 p and 6 p, which served as convenient references for estimating particle size. The categories that were recorded were flakes, four size ranges of amorphous aggregates, and miscellaneous particles in the size range of 2-6 p. The character of the latter could not be clearly distinguished in all cases. Phytoplankton and bacteria were excluded, of course, but the less structured material which was counted probably included

68

GORDON A. RILEY

particles that were primarily silt or minerals rather than organic matter. Initially the cell was filled to a height of 4 cm with a well mixed suspension, and the chosen area was scanned rapidly at intervals of 5-15 minutes in order to get as much information as possible on the arrival time of large and rapidly falling particles. After the first hour or so the counting interval was gradually increased. The time required for scanning also was longer with the arrival of increasing numbers of small particles on the bottom. I n each case the time was recorded as the mid point of the scanning period, but the actual time involved varied from 2-20 minutes. Counts ordinarily continued for a total elapsed time of 6-8 hours, with a final check at the end of 24 hours. Further increases between 8 and 24 hours were slight and were only in the smallest size category. 3. Results

Figs. 7 and 8 show examples of the kinds of results obtained. The counts were made on a sample of surface water taken just offshore in the area near Halifax. Arrival times of four size ranges of amorphous

0

100 TIME

FIG.

200 IN MINUTES

300

400

7. Sinking speeds of organic aggregates. The number of particles of the size range indicated is plotted against the times when they appeared on the bottom of the counting chamber. These data were used to compute generalized sinking rates shown in Exp. 1, Table VI. Short vertical lines indicate points arbitrarily chosen for determining variations in sinking speed. See text for further details.

PARTICnLATE ORGANIC M-4TTER I N SEA WATER

69

5OOr

8 12 16 20 24 TIME IN HOURS FIG.8. Sinking speeds of small particles and flakes. Format is similar to Fig. 7 and again refers t o Exp. 1. Table I7I. 0

4

aggregates are illustrated in Fig. 7 , and Fig. 8 shows the results for flakes and miscellaneous small particles. The interpretation of experimental results for flakes and small particles is fairly simple. The curves rise in a linear fashion and tend to level off abruptly. This is what one would expect if the particles have a uniform sinking rate, and the latter is calculated simply as the total length of the column of water divided by the time required to reach the break in the curve. The curves in Fig. 7 are more complex, indicating a spectrum of sinking rates. Maximum and minimum values can be determined roughly from the initial slope and the final point of leveling. In these experiments and others there have tended t o be sharp inflection points in the curves, so that with only slight over-simplification they can be broken into two or more linear segments, implying that the curve is compounded of two or more kinds of particles, each with uniform sinking rates. I n such cases we can postulate a simple solution which is illustrated diagrammatically in Fig. 9. Here there are two kinds of particles, X and Y , each with a constant sinking rate, so that all of X reaches the bottom of the cell a t time tx and all of Y at time t , as indicated by dotted lines in the figure. Then X+Y=B where B is the total number at time t,, and the number A a t time t, is

70

GORDON A. RILEY

From these equations individual values for X and Y can be computed. The inflection points serve to establish maximum and minimum values for the sinking rate, and the two rates can be combined into a single mean value by appropriate weighting of the relative numbers of X and Y . A more sophisticated analysis would be possible, but elaborate treatment is hardly warranted. The size-frequency distribution of particles varies somewhat from one sample to another, and other variables and experimental errors are such that precise evaluation of any one experiment is not very meaningful.

0

I00

ZOO

TIME

11.1

300

400

MINUTES

FIG.9. Diagrammatic exposition of methods used for estimating ranges and mean values for sinking rates. For further details see text.

Two other experiments were performed with inshore water, all collected during early summer, and the results are given in Table VI. The methods that have been described served to establish a general range of sinking rates and a mean value for each category. Four samples were taken at a station in Slope Water at various depths from the surface to 3 000 m. Some difficulties were encountered in sinking rate experiments with this material because the number of particles was minimal for good counts, and flakes were not easy to distinguish in the natural, unstained condition. Counts of flakes are omitted, for successive counts were inconsistent. The remainder of the results are shown in Table VII. This clearly is not as consistent a

71

PARTICULATE ORGANIC MATTER I N S E A WATER

TABLEVI. SINKING RATEI N MIDAY OF VARIOUS SIZES AND KINDS OF PARTICLES FOUND IN INSHORE SURFACE WATERS Range ~~

~~~

~

0.78-1'4 0.31-0.93 0.31-0'57

Exp. 1. Aggregates >60 p 30-60 15-30 6-15 Mist. particles 2-6 p

-

2.9 1.1 0.70 0.50 0.10 0.17

0.24-2.6 0.24-1 '6 0.10-0.97 -

1-0 0.44 0.19 0.15 0.18

0.97-2.1

1.a 0.54 0.45 0.31

-

Flakes

Exp. 2. Aggregates >30

Mean

~

p,

15-30 6-15 Misc. particles 2-6 p

Flakes Exp. 3. Aggregates >60 p

-

30-60 15-30 6-15

0.32-0.54 0.22-0.54

set of data as the other set, but it shows that there is a general similarity between the sinking rates of surface and deep water particles. A few comparative studies were made of experimental material. Flakes produced artificially by bubbling had a mean settling rate of 0.30 m/day. The fact that they had a slightly faster sinking rate than natural flakes is not surprising, for many of the natural ones are filmy in appearance and presumably have been partly degraded by bacteria. Measurements were also made of aggregates produced experiment ally by bacterial activity. The material was taken from an experiment described earlier in which the particulate matter was routinely cropped TABLEVII. MEAN SINKING RATESOF AGGREGATES AND SMALL PARTICLES IN SURFACE AND DEEPWATERS OFF THE SCOTIAN SHELF. ALL VALUES ARE LISTED

Size range >60p 30-60 15-30 6-15 2-6

0

1.1 0.83 0.32 0.26 0.27

500

0.48 0.29 0.19 0.12

AS METRESIDAY

1000

3 000

0.58

0.43 0.34 0.29 0.38 0.22

0.38 0.26 0.25

72

GORDON A. RILE1

a t a rate of 7 % on alternate days, and the test was done a t the time when the experiment was terminated after a period of about five weeks. The following rates were obtained : 15-30 p, 0.82 m/day ; 6-15 p , 0.50 ; 2-6 p, 0-20. These are in general higher than the rates for natural particles of comparable size, but the difference is not marked.

4. Discussion Observed sinking rates of different kinds of particles vary by more than an order of magnitude, and within any particular size category there is some variation from one experiment to another. Further variation is introduced by size-frequency differences between one sample and another, so that there is no simple answer to the question of what the sinking rate of non-living particulate matter in the sea may be. The largest particles measured here could sink from the surface t o the deep ocean bottom in a little more than two years if they maintained their integrity that long. The smallest ones would require fifty years or more. I n general the largest particles in these counts are volumetrically the most important constituents. The large volume of individual particles in the upper size ranges more than compensates for the fact that they are minor elements numerically. This is illustrated by a volume computation based upon data obtained in Exp. 1, Table VI. Particles larger than 60 p are assumed t o have a mean volume equivalent to a spherical particle with a diameter of 80 p. The other aggregates are calculated as spheres with mean volumes equivalent to the size ranges listed. Most of the aggregates appear to be more or less spherical in the free floating condition, so that this is not a serious over-simplification. Flakes are calculated as disks with a diameter of 25 p and a thickness of 5 p. The latter is a guess, for they are seldom oriented in a position that permits measurement of their thickness with any degree of accuracy. TABLEVIII. VOLUME COMPUTATIONS Amorphous aggregates

Size range >60 p 30-60 15-30 6-15 Misc. particles 2-6

Flakes

Number

6 16 38 58 420 75

BASEDUPON DATAFROM Vol./particle

EXP. 1

1 0 3 p3

Total Volume (X 103 p 3 )

270 63 7.9 0.96 0.058 2.4

I620 1008 300 56 24 180

PARTICULATE ORGANIC MATTER I N SEA WATER

73

The results are shown in Table VIII. In this particular set of data particles with a diameter of more than 30 p constituted only 3.6% of the total number but represented over 80% of the total volume. One can go a step further, multiplying the total volume in each size range by its mean sinking rate, summing, and dividing by the total volume of all particles, and thus arriving a t a mean sinking rate for the assemblage as a whole. I n the present case it is 1.4 miday. A similar computation for Exp. 2 gives a value of 0.8 m/day. Mean sinking rates for the three deep water samples in Table VII ranged from 0.41-0.50 m/day. These results suggest that most of the organic matter is concentrated in larger particles and sinks at a rate that is near the upper limit of experimentally determined values. However, this conclusion must be qualified by adding that the organic content of the particles probably is not precisely proportional to volume. Most of the large masses are visibly less compacted than the small ones. I n the free floating state they are loose conglomerates of material, with open spaces in which bacteria and microflagellates can be seen circulating freely. Thus the overall volume is not a good index of the actual volume of solid material. Moreover, there are large numbers of particles that are smaller than 2 p, ranging down toward the limits of visibility and presumably beyond, into the colloidal range. As indicated earlier, the quantitative aspects of this size spectrum have not been described in a satisfactory way. Thus a significant but essentially unknown fraction of the total particulate carbon occurs in small particles with a sinking rate that is probably less than 0.1 m/day. Experimentally determined sinking rates were used to estimate the specific gravity of the various size ranges of particles. This calculation was based on Stokes’ Law which may be stated as

W

=

219 g

Pz

--

P1 r z

~

P

where W is the sinking speed, g the acceleration of gravity, p1 and pz the density of the water and of the particle, respectively, and p is the dynamic viscosity of the water. All values are in the c.g.s. system. For application to present experimental conditions we can assign values of p1 = 1.02 and p = 0.01. W has been measured experimentally, and r is designated as the median value for each of the size ranges that has been examined. Table I X shows results obtained by application of the formula to data from Exp. 1. The increase in specific gravity with decreasing size is consistent

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GORDON A. RILEY

but slight down to the 6-15 p size range, and that particular result may be atypical, for the observed sinking rate was unusually high. A similar computation based on the observed sinking rate of 0.19 m/day in Exp. 2 gives a value of 1.070 for particles in this size range. The slight variation in specific gravity with size is in accord with visual indications that the larger particles are less compacted, tending to have larger water-filled interstices. There is no reason to suppose that there is any real difference in the organic matter itself. TABLEIX. COMPUTATIONOF SPECIFIC GRAVITY AS DETERMINED FROM MEASUREDSINKINGSPEEDSIN EXP. 1 Aggregates >60 p 30-60 15-30 6-15 Misc. particles 2-6

Assumed Radius

Sinking Speed

Specific Gravity

40 22 11 5

2.9 1.1 0.70 0.50

1.033 1.034 1.057 1.146

2

0.10

1.152

Observed sinking speeds of 2-61” particles have varied from 0.100.25 m/day in the various experiments, and the estimate of specific gravity in Table I X is a minimum figure. Some of the estimates run as high as 1.40. These small particles cannot be seen easily at the magnification that was used, and their nature is open to question. Some of them undoubtedly are minerals and clay particles with an organic coating. Flakes are omitted from the present discussion. Stokes’ Law is of course inapplicable. There are methods for dealing with disks and plates (Munk and Riley, 1952), but they require better estimates than are now available of the thickness of the flakes. Experimental material appears to be more variable than naturally occurring particles. Some sinking rates were mentioned above which were obtained from an experiment that had been underway for some weeks, and calculated specific gravities of amorphous aggregates were 1.05-1.13. These aggregates probably were lying on the bottom of the container much of the time and may have been more compact than free floating material. I n contrast, one measurement was made during the early stages of an experiment in which the aggregates were diffuse and only barely visible. The sinking rate of 25 1“ particles was 0.19 m/ day, and the calculated specific gravity was 1.027.

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I n looking over the results as a whole, a good many ambiguities remain. The complete spectrum of sizes and sinking speeds has not been examined. Most of the particle counts described in earlier parts of the paper were based on samples of not more than 250 ml of sea water. These commonly contain a few particles of the order of 100200 p in diameter, which could have sinking rates of as much as 5-10 m/ day, The " marine snow ') which has been reported by direct observation probably is still larger and can have a correspondingly faster sinking rate, but the concentration is too small for them to appear in small samples except on rare occasions. However, in certain circumstances they may be important. Aggregates predominate in the surface layer, and the larger sizes are volumetrically the most important. They are presumably delivered into deep water in the same proportions as they exist in the surface layer, and the apparent decrease in abundance is due in large part to the fact that they have a rapid sinking rate and a relatively short residence time in the deep water column. The rate of delivery t o animals in deep water or t o the deep ocean bottom will be proportional to the product of the sinking rate and the concentration in the water. Dynamic considerations of this sort will seriously underrate the importance of large particles if one considers them only in terms of observed concentrations in the water. The relationship between temperature and sinking rate has not been examined experimentally. This would be desirable, but temperature effects are minor compared with some of the other variables that have been discussed above. I n the case of small particles with a specific gravity of 1.1 or more, the effect of increased density of the water would be less than the differences recorded in individual experiments. The most important temperature dependent variable would be the increase in the viscosity of the water, which could reduce sinking rates nearly 50% in deep ocean waters. Since the reduction in specific gravity of large particles is believed to be due mainly t o water inclusions, temperature equilibration as particles sink would tend to maintain a constant difference between the specific gravity of particles and water except in strong temperature gradients. Again the main variable is expected to be increased viscosity.

V. GENERALDISCUSSION A. Re-examination of the aggregation process The original aggregation process as described by Baylor et al. (1962) and Sutcliffe et al. (1963) involved migration of surface active substance toward the surface via bubbles and the formation of particles containing

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organic compounds with molecular weights in excess of 300. This much was well documented. Their hypothesis that particle formation involves the compression of monomolecular surface film into a polymolecular complex seemed at that time to be the simplest explanation of further events. However, Garrett (1968) has questioned whether film compression occurs to a significant degree, and other findings suggest that processes acting a t the sea surface are more complicated than was originally supposed. Wangersky ( 1965) believed that adsorption of organic matter on crystals of sea salts was an important feature of the process, and apparent stabilization of the crystals indicated that the organic " skin " was relatively impermeable. Further evidence supporting these results was the demonstration by Chave ( 1 965, 1968) and Suess (1968) that calcium carbonate can be protected from solution by absorbed organic coatings and that naturally occurring particles frequently contain calcite crystals. Papers by Garrett (1964), Jarvis (1965), Jarvis et al. (1967) and several others by this group of investigators have examined the physical and chemical characteristics of surface films in the sea, using a collecting device which picks up the upper 0.15 mm of water and associated surface film materials. The materials were used for chemical analysis and experiments on physical properties. Garrett (1964) reported the presence of free fatty acids, fatty acid esters and alcohols, and probably nonpolar hydrocarbons. Small quantities of carbohydrates and polypeptides or amino acids (as determined by anthrone and ninhydrin reagents, respectively) were reported. These latter materials were passed through a Sephadex column which had an exclusion limit of about 5 000 molecular weight units. The majority of the carbohydrate material was in the high molecular weight fraction, and most of the ninhydrin reactive substances had molecular weights of less than 5 000. Jarvis (1965) reported that surface active material can be transported to the surface and collected there by either bubbling or stirring. Jarvis et al. (1967) found in experimental studies of film pressure that the calculated average film thickness of some of their samples was a s much as 300& more than an order of magnitude larger than the calculated thickness of a monomolecular film. They concluded that there is a selective process involved in which initially all of the organic materials arriving a t a clean water surface would be adsorbed a t the interface, but with further addition only the more surface-active molecules such as long-chain fatty acids would remain strongly adsorbed, and more weakly adsorbed and water soluble molecules would be displaced.

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This interesting series of papers helps to broaden our knowledge of surface film processes. Some of the facts they report may have a direct bearing on problems of particle formation. There is a possible mechanism here for fractionation of materials and for concentration of the carbohydrate and protein components, which appear to be particularly important in flake formation. These authors presumably were thinking of downward displacement of the less surface-active materials. However, it is equally possible that water droplets derived from bursting bubbles and carrying the usual array of surface film organics and inorganic ions could, when they fall back into the sea, release the most surface-active molecules to the interface and leave the other substances floating on a ‘‘ dry ” surface. Such phenomena could be involved in the kind of flake formation described by Wangeraky (1965). Other investigators, cited in an early section of the paper, thought that particulate matter formed as little skins of organic matter on submerged bubbles. This is a distinct possibility ; flakes can form very quickly. The writer has collected flakes by holding a microscope slide over filtered sea water, turning on an air supply, and removing the slide as soon as it collected a few droplets. Typical flakes could be seen, sometimes still adhering to intact bubbles. Several investigators have mentioned the fact that bubbles can scavenge bacteria and nanoplankton as well as non-living particles. These materials have been shown to accelerate flake formation and presumably are incorporated within them. However, there is little visible evidence of this. The flakes are not a mere conglomeration of small particles. Enough other materials have been added to give them a emooth surface and essentially homogeneous appearance. This implies additional adsorption of colloids and/or polar molecules. The impermeable nature of the material formed and the presence of molecules of high molecular weight suggests that synthesis is occurring. Electrical charges a t air-water interfaces are an obvious source of energy for organic synthesis. If these are the main factors involved, then the various sites and modes of formation mentioned above are not mutually exclusive. I n contrast with the flakes, most of the particles that form spontaneously in filtered water have a granular appearance, suggesting that they are merely an aggregation of discrete smaller particles, and as indicated earlier their reactions to histochemical stains are also different. However, some flakes are invariably found in these experiments on i n situ aggregation. The most careful precautions to eliminate airwater interfaces during and after filtration do not entirely prevent their formation. Moreover, there is circumstantial evidence that they are

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formed in deep ocean water, although it is not obvious how they could be provided with the necessary energy for organic synthesis or moulded into the characteristic leaf-like shape under these circumstances. However, within the general size range of 15-35 p, flakes are the most abundant particles in deep water (Gordon, 1970a). Gordon also showed that most of their protein can be removed by enzymatic hydrolysis, and present work indicates that they are readily attacked by bacteria in experiments of some weeks’ duration. They are found in deep water in various stages of decay, and some look as fresh as newly produced particles. If they were produced only in the surface layer and sank through a typical vertical column of 4000m, the residence time would be more than 50 years. Since some of them are being degraded by bacteria, a t least t o the point of virtual invisibility if not complete destruction, one would expect a decrease with depth, but this is not the case. There seems to be no reasonable alternative to some mode of formation in deep water. The other type of in situ aggregation, by simple amalgamation of smaller particles, is not so mysterious, although certain aspects have not been fully worked out. The study of early stages of formation by Sheldon et al. (1967) leaves some doubt as to whether the initiation of the process requires bacteria or is entirely a physical process. The later production of large quantities of aggregated material requires a bacterial flora in the experiments that have been described. The general principles have been known for years and have been used to practical advantage by sanitary engineers in the treatment of sewage. The problem appears to be simply one of reducing the electrical charges which ordinarily keep particles dispersed, and sewage effluents are aggregated and precipitated by chemical or bacterial treatments which accomplish this purpose. I n summary, observations and experiments with natural sea water indicate that particulate matter can be formed on air-water interfaces and within the water column by aggregation of smaller particles, together with adsorption of colloidal and probably dissolved organic material. Adsorption of organic matter on inorganic particles has been demonstrated. Biochemical synthesis and incorporation of inorganic ions appear to be important. Formation of particles on air-water interfaces by compression of monomolecular films remains as a possible mode of formation but has not been undeniably established. The complexities of the natural sea water system have placed several constraints on attempts t o sort out the various processes that are involved. Attempts to examine the physical chemistry and biochemistry of simplified artificial systems have been abortive because of difficulties

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of preparing artificial sea water that is sufficiently pure for the purpose ; however, basic studies of this type are essential for further progress. B. Dynamics of production and consumption of non-living particulate matter in the sea 1. Physical process of movement and dispersal (a) The surface layer. The fact that particulate matter is concentrated in the immediate surface layer of the sea and in convective downdrafts is sufficient evidence that production of particles a t the surface is effective ; but we know very little about actual rates of production. Batoosingh et al. (1969) found that when unfiltered sea water was bubbled, the yield was not significant. This was presumably due to the same phenomenon that is observed in any other bubbling experiment ; the accumulation of particles inhibits further formation, and natural sea water contains a large enough concentration of particles to be inhibiting. One would suppose then tha,t the rate of formation is just sufficient under ordinary circumstances to replace losses, although seasonal variation in the non-living fraction indicates some degree of imbalance between production and removal. The sea surface presents some of the features of a continuous yield experiment, and this is primarily responsible for continued production of flakes. Vertical turbulence in the surface layer tends t o counteract upward flux on bubbles: and convective processes drain off the surface water and transfer it to a deeper level, in some cases all the way to the bottom of the mixed layer. The discussion would profit from a better knowledge of the rate of convective overturn. As indicated earlier there is some doubt as t o whether Langmuir convection cells are as simple and well organized as has been indicated in some of the literature, and there is little real knowledge of either volume transport or depth of penetration of the downdrafts. However, the mere fact that downdrafts exist, removing surface materials t o a deeper level, is biologically significant, and with some possible over-simplifications an order of magnitude estimate should be attempted. Sutcliffe et al. (1963) measured the sinking speed in convective downdrafts associated with Langmuir cells by measuring the vertical displacement of a slightly buoyant disk 20 cm in diameter. They obtained values of the order of 3-6 cmisec. The sinking speed was correlated with wind speed, the higher values being obtained with winds of about 6 m/sec. The width of the downdrafts is somewhat variable but commonly

80

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is not much more than the diameter of the disk that was used for measurement. I n order to make a rough estimate of the rate of coilvective circulation, the downdraft is assumed to be exactly 20 cm wide with a vertical velocity of 6 cmisec. The wind speed is assumed to be 6 misec, and according to Faller and Woodcock (1964) the distance between convergences would be about 30 m, and the depth of the mixed layer would be about 24m in temperate latitudes. If the downdraft went all the way to the bottom of the mixed layer, the time required would be 400 seconds. Between the convergences there would be a return movement toward the surface of a more diffuse nature, and if it were uniform throughout the 30 m distance between convergences the speed would be 1.4 rn/hour. The time required for complete overturn would be about three-quarters of a day. This rate of upward movement is many times faster than observed sinking rates, and particles caught in the upward movement would be carried back toward the surface. Stommel (1949) developed a mathematical model of the behaviour of sinking particles in these convection cells. The model possibly is somewhat idealized in that he postulated a symmetrical rotation, and the actual cells appear to be very assymmetrical, with strong downdrafts and much more diffuse upward motion, but the general principles should hold. Stommel showed that when the velocity of upward motion exceeds the sinking rate there is an " area of retention " in the ceiitre of the cell, in which the upward motion of water is sufficient to overcome natural sinking tendencies, and particles within this area will be trapped. Theoretically they would remain there permanently. I n actual fact, as Stommel pointed out, there would be turbulent exchanges between this area and surrounding regions. However, the important point is that the rate of loss by sinking from the surface layer would not depend simply on the sinking rate of the particles but would be determined by all the variables involved in the convective system. The smaller the sinking rate is in relation t o the velocity of the water, the larger the area of retention will be, and in the present case it would be quite large. The net effect is presumed to be that only a small proportion of the particle content of the mixed layer will be subject to loss a t any one time, and this loss will occur as water reaches the bottom of the convective layer and spreads laterally and before it begins to rise again. The form of the convective movement can be traced more easily in the columnar cells that occur in calm weather than in Langmuir cells. These are often seen in the Sargasso Sea, where the Sargasso weed

PARTICULATE ORGANIC MATTER I N SEA WATER

81

drifts into more or less circular patches five or ten metres across, which readily identify these areas as points of convergence and confirm the fact that convection cells are indeed columnar. The temperature is commonly 0-1-0.3°C cooler in the convergences than in the surrounding water, and the temperature is essentially constant with respect to depth. I n the water between the convergences a slight negative temperature gradient is likely to develop during these calm periods. Often, particularly during the night and early morning hours, a temperature inversion can be found just a t the top of the thermocline. This is a thin layer, generally less than half a metre thick, in which the temperature is slightly warmer than that immediately above it but generally cooler than the surface water. I n order to be stable in the position in which it is found, this water also must be slightly more saline than the water just above it. On one occasion the writer took a series of closely spaced bathytherms as the ship drifted slowly across and beyond one of these convergences. The temperature in the downdraft was the same as in the inversion beyond, and the profile gave the impression that the latter was derived by spreading from the base of the convergence. There seems to be little doubt that this water is the product of surface cooling and evaporation. Particulate matter in a thin layer of this sort will not have to sink very far in order to be trapped in the thermocline, and it will be apparent that the rate of loss from the mixed layer will depend more on the rate of convective overturn than on the actual sinking rate of the particles. A direct estimate of rates of loss is impossible in the present state of our knowledge. However, the problem is essentially the same as that of removal of phytoplankton from the mixed layer, a subject that has been given some attention over the years. Riley et al. (1949) developed some mathematical models of the vertical distribution of phytoplankton, and in order to obtain a realistic vertical distribution it was necessary to assume a sinking rate of about 3 m/day in temperate waters and 6 m/day in the subtropics. Steele (1958) and Riley (1965) used similar assumptions in further models that have been developed. The assumption of a regional variation in mean sinking rates was adopted on the grounds that variations in viscosity associated with regional temperature differences could have this effect. However, measured sinking rates of phytoplankton vary enormously, and most of them are less than the values postulated here. The idea that average sinking rates of natural populations should be so nearly constant, or

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should vary so precisely with changes in viscosity, is questionable. Riley (1965) suggested alternatively that this apparent sinking rate might actually be due to convective overturn, a concept which has been developed more fully in the present discussion. If this concept is correct, regional variations in apparent sinking rate would be related to thermal structure of the water rather than to differences in viscosity. I n general, the seasonal thermocline tends to be somewhat deeper in tropical and subtropical waters than in higher latitudes. Many local exceptions can be found, such as the shallow thermoclines associated with upwelling. However, when the thermal structure develops as it simple function of wind stress, the depth of the mixed layer for any given wind speed theoretically should be inversely proportional to the sine of the latitude (Rossby and Montgomery, 1935). This kind of variation was implicit, though not explicitly stated, in the model of regional variations of phytoplankton (Riley, 1965). The assumption of a regional variation in sinking speed could have been stated alternatively as an advective removal of a certain fixed proportion of the population in the mixed layer each day, and the results would have been essentially the same. I n order t o get numerically realistic results one would have to assume that about 5-7% of the mixed layer loses its population each day. Convective motion obviously deserves more thorough study, both as a physical process and in respect t o its biological implications. The hypotheses proposed here remain tentative, but they provide possible explanations for problems that eluded solution for a long time. There are further implications with regard to non-living particulate matter. Particles which have the same spectrum of sinking rates as phytoplankton should have approximately the same area of retention and the same rate of loss from the mixed layer, and this rate of loss could be several times larger than observed sinking rates. Of course this must be qualified by saying that there are times when negative gradients in the surface layer are too strong to permit convective overturn, and at such times both phytoplankton and non-living matter will be removed at their natural sinking rate. A correlation has been noted between the quantity of phytoplankton and non-living matter. Moreover, flakes and aggregates are of a size that should make them susceptible t o removal by indiscriminate filter feeders to about the same degree as phytoplankton. These similarities in processes of removal suggest that rates of production also are probably more or less similar. I n general this means that the daily increase in particles of 5 p or larger could be about 5-15% of the amount present.

PARTICULATE ORGANIC MATTER I N SEA WATER

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Particles smaller than 5 p in diameter will be taken less efficiently or not at all by indiscriminate grazers, and they are not likely to be transferred downward very effectively either by ordinary sinking or by the mediation of convective downdrafts. A thin layer of water spreading laterally from these downdrafts will not remain in contact with the thermocline very long and is unlikely to lose an appreciable quantity of particles that have sinking rate of 0.1 m/day or less. The analogy with phytoplankton dynamics is inapplicable, and small particles may be subject to other processes, such as aggregation into larger particles, which cannot be estimated quantitatively. As indicated earlier, the amount of organic carbon in the various size categories cannot be stated precisely, and this creates a major difficulty in arriving at a quantitative treatment of this subject. Some approximations will be presented in a later section as to likely rates of transfer into deep water, and these will be compared with independent estimates of rates of utilization. (b) T h e transition zone. I n certain respects deep ocean conditions begin at the top of the thermocline. Here convective flux ceases to be a factor. Further transport into deep water will be essentially a sinking phenomenon, somewhat augmented by vertical eddy diffusivity, for there is usually a negative gradient in particle concentration. Bubbles at this depth are most unlikely, and any formation of new particles should proceed according to processes described in experiments on unbubbled samples. However, there is a zone underlying the mixed layer which differs from the deep sea in several important respects. This transition zone is best defined as that part of the vertical column in which there is a marked decrease with depth in particle number and total organic carbon. The maximum limit of this zone has been found at depths of as little as 200 m (Menzel and Goering, 1966) or as much as 900 m in some other sets of observations (Riley et aZ., 1965). Below this depth variations tend to be smaller and of a random nature, or occasionally there may be larger variations associated with particular water masses, but there are no systematic negative gradients. Particles enter the top of the transition zone a t a rate that is at least equivalent to observed sinking rates of near-surface material and may be somewhat larger, if there is any validity in the hypothesis presented above with regard to speeds of convective transfer. If this rate of transfer between the mixed layer and the transition zone is greater than the natural sinking speed, there will be an increase in the concentration of particulate matter in the upper part of the thermocline. This kind of distribution is well known in the case of phytoplank-

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GORDON A. RILEY

ton distribution, but sampling for non-living organic matter has not been conducted a t sufficiently close intervals to reveal the fine detail of vertical distribution. An earlier discussion of micro-distribution cited the work of Paramonov et al. (1966) which demonstrated marked vertical variations in transparency. These probably were associated with thermal discontinuities. The decrease in organic carbon that has been noted in the transition zone presumably is merely an indication of a general trend, and the gradient is by no means a smooth one. I n the surface layer aggregates constitute the bulk of the material. Below the transition zone small particles and flakes are predominant. This may be due to selective consumption of large particles in the transition zone by animals, or it may be simply a matter of physical sorting. Large particles with a rapid sinking rate will spread downward through the layer more quickly than small ones, and their concentration per unit volume will be proportionately reduced. The rate of decrease of organic matter in the transition zone cannot be determined with any degree of generality, for few areas have been sampled in sufficient detail to provide a representative spectrum of vertical profiles. A sample analysis will be presented here, based on data obtained by Riley et al. (1965) in the Sargasso Sea. The results will be useful in a later discussion of biological problems in this area, but it serves no other function except as an exposition of method. I n this area the mean concentration of particulate organic carbon was about 100pg/litre a t 100 m and 30pgllitre a t a depth of 900m. These figures can be regarded as representing the upper and lower limits of the transition zone. A first approximation t o consumption is simply the decrease in organic matter between the top and bottom of the transition zone. This can be converted to a rate of consumption if the residence time is known. Experiments suggest that the mean sinking rate is of the order of 0.25 m/day, allowing for a decrease in sinking rate with increasing viscosity, and this leads to an estimate of 0-022 pg C.litre - 1.day- 1. This kind of computation is applicable t o a static situation ; however, in the present case we can assume that organic matter is imported into the top of the transition zone faster than it is exported from the bottom, so that total consumption is greater than is indicated by mere decrease in concentration. If the concentration exists in a steady state, the mean rate of decrease can be determined simply as the difference between the two vertical fluxes. The flux F , directed positively downward, is given as AC F = d C - A - AX

85

PARTICULATE ORGANIC MATTER IN SEA WATEK

where S is the mean sinking rate of organic carbon C, A is the coefficient of vertical eddy diffusivity, and AC/Az is the mean vertical gradient in organic carbon in the depth range in question. The mean sinking rate S is assumed to be 1 m/day at the top of the layer. This is approximately the observed value in experiments with inshore water and surface Slope Water. A sinking rate of 0.24 is postulated for the bottom of the layer. This again is obtained from experiments on Slope Water, using mid-depth values and correcting for a change in viscosity. These obviously are crude estimates. No experimental observations are available for the area in question, and there is no good way to make allowance for the possibility that the rate of delivery of large particles may be enhanced by convective movement or that there may be a significant quantity of small particles with a negligible sinking rate. Eddy coefficients are generally small in the thermocline. A value of 1 cm2/sec is postulated for the upper part of the zone and 0.5 for the lower boundary. The mean decrease in carbon content between 100 and 200 m is about 25 pg C/litre, and between 500 and 900 m it is about 20 pg C/litre. These values are used to compute eddy flux. The calculation is carried out in the c.g.s. system, but results are translated into terms of daily flux in Table X. TABLEX. COMPUTATIONOF VERTICAL FLUXOF CARBONIN THE TRANSITION ZONE (WG C . C M - ~ . D A -l) Y AND CARBONLoss WITHIN THE LAYER( P G C.LITRE DAY - l ) . FORFURTHER DETAILS SEE TEXT

S( W a y ) A(cm2/sec)

cs

Eddy Flux Total Flux (F) Carbon loss

100 rn

900 rn

1 1 10 0.9 10.9

0.25 0.5 0.75 0.2 0.95 0.12

The total flux F in the table is in terms of pg C passing downward through a horizontal area of 1 cm2 in a day. It will be noted here that the eddy flux is relatively insignificant and might have been omitted ; however, it could be important in areas where gradients in organic carbon are steeper. The difference in flux between the upper and lower limits of the zone represents total carbon loss. This difference, divided by the volume of water in which it occurs, gives an estimate of carbon loss of 0.12 pg C.litre-l.day-l, which is shown as the last entry in the A.H.B.-S

4

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GORDON A . RILEY

table. This is about five times the earlier estimate, which considered only the observed decrease with depth. These results will be discussed later in relation t o biological problems in the transition zone. The values that have been obtained suffer from all of the qualifications that have been made about the difficulties of relating experimental determinations of sinking rates to the mean rate of sinking of total particulate carbon. Examination of biological processes within this layer provides an independent method of calculating carbon consumption which will be useful for comparative purposes. (c) The deep ocean. Data presented in earlier sections of this report indicate that particulate matter in the deep ocean varies regionally in a manner that is more or less in accord with the productivity of the surface layer. Thus it would appear that the residence time of particulate organic matter in the deep ocean is short enough for the influence of surface conditions at the point of formation to be more important than later horizontal movements of deep water masses. However, the latter can be important occasionally, as indicated by the fact that Dal Pont and Newel1 (1963) found maxima and minima associated with particular water mass characteristics of the south Pacific Ocean. More commonly vertical variations appear to be of an essentially random nature, although admittedly the sampling intervals have seldom been close enough to delineate vertical gradients realistically. The residence time of much of this material is fairly long, but McGill (1964) has reported a similar pattern of distribution of organic phosphorus, most of which was present in the dissolved fraction and probably had an even longer residence time. Menzel and Ryther (1968b) have reported similar regional variations in dissolved organic carbon. The reason for the slow rate of horizontal dispersal in deep water lies in the fact that the major transport systems in the deep ocean are localized, to a degree that was not realized until recent years, and further mixture of the main mass of deep water is a slow diffusion process. This was clearly demonstrated in an analysis by Wiist (1955) of currents in the South Atlantic Ocean based on Meteor data, in which major north-south currents were concentrated along the eastern slope of South America. Stommel and Arons (1960) applied the theory of westward intensification of ocean currents to the deep ocean system, and their conclusions indicate that this kind of localization is to be expected in all of the major ocean basins. Even non-conservative properties of an inorganic nature are somewhat localized in their distribution despite the fact that this distribution is developed during the entire residence time of the water itself. Riley (1951) examined oxygen distribution in the Atlantic Ocean, based

PARTICULATE ORUANIC MATTER IN SEA WATER

87

mainly on expedition data of the " Discovery '), " Dana )', " Meteor ", and " Atlantis ", and found that the oxygen minimum layer is quite complicated in its structure, being compounded of several loci of marked oxygen depletion which are imperfectly diffused and show evidence of regional variation of the same kind that has been noted in connection with particulate organic matter and organic phosphorus. Oxygen minima are most strongly developed off the West African coast a t the roots of the North and South Equatorial Currents. They are centred at a depth of about 400 m on ut surfaces 26.9-27.0. There is also a secondary minimum a t a depth of only 100m in the Guinea Current according to " Dana " data. Another locus of low oxygen concentration is found in the Sub-Antarctic a t about ut 27.5. This tends t o spread northward but remains separate from the tropical oxygen minimum above, so that stations between 25" and 40'5 Lat. sometimes show double minima a t about 400 and 1 800 m. I n the tropical North Atlantic the oxygen minima spread westward across the ocean and are centred a t a ut surface of 27-0-27.1, indicating that some local consumption occurs along the way a t a slightly deeper level. The great mass of northern water that drifts down to the central North Atlantic has a weak and diffuse oxygen minimum layer, and individual stations have minimum oxygen values at various ut surfaces from 27.3-27.7. I n the vicinity of the Mediterranean outflow the minimum is also weak and is centred around the 27.5 surface. These waters of various origins meet and mix in the western North Atlantic, where current systems are relatively strong. Because two of the sources have oxygen minima that are vertically diffuse, no secondary minima are formed. Instead there is coalescence into a single well defined minimum a t ut surface 27.2-27.3. To the southward it grades upward toward the tropical minimum, while to the east and north it grades toward denser water. The fact that the oxygen minimum layers occur on different density surfaces in different parts of the ocean is indicative of a more or less local origin. More recently Menzel and Ryther (1968a) have suggested that this layer is derived from the upwelling area of the tropical eastern Atlantic and spreads from there by isentropic mixing. However, this is clearly not the case except in a very limited area, and even in their short north-south profile (ca 8-36"s) in the South Atlantic, which was the main subject of their analysis, the northward intrusion of the deep Antarctic oxygen minimum layer can be seen a t the southern end. In deep ocean waters vertical turbulence is believed to be slightly stronger than in the mid-depth region of maximum stability. This

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tends t o facilitate the maintenance of a fairly uniform vertical distribution and to accentuate regional differences in particulate organic matter and other properties. However, evidence has been presented of the existence of thermal discontinuities in the deep ocean. These could lead to the development of layers of particle rich water, although the somewhat inadequate data now available suggest that the distribution is only slightly non-random (Gordon, 1970b). Vertical turbulence can have some effect in dispersing any local concentrations that develop, but it cannot produce a systematic downward flux, for the mean vertical gradient of particulate carbon is essentially nil in most areas that have been examined. Thus particulate matter sinks passively from the main thermocline to the ocean floor and seems to be influenced very little by any of the physical oceanographic processes of the deep ocean except in a few local situations that have been described. 2. Biological relationships Having brought together available information on physical movements of particulate organic matter in the sea, we come now to questions of the character and magnitude of the biological processes which can be expected to affect the concentration and distribution of the non-living matter. This involves a consideration of the degree of balance between production and consumption at various depths in the sea. Theoretically, of course, total carbon fixation by photosynthetic plants is expected t.0 equal total respiratory utilization of ca,rbon by animals and heterotrophic plants, neglecting the residue that remains unoxidized in sediments or in the water column. This residue is large in its total accumulation, but it has accrued during a long period of time and is believed to be a small percentage of total carbon production. For practical purposes we can assume a balance over a period of a year or several years. During shorter periods there are seasonal imbalances in the surface layer, so that the discussion must be limited to areas where the information is available on mean annual production and consumption. This balance between total production and consumption is of course realized only by considering the total vertical column from surface to bottom. At any particular depth there is an imbalance. All photosynthesis occurs in a thin surface stratum which seldom exceeds 125 m in depth and generally is less. Much of the consumption also occurs here, and the remainder of the surface production, once it has been delivered to deep water by one means or another, provides minimal sustenance for the sparse deep water fauna and flora.

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Various estimates have been made of production and consumption in near-surface waters, and by difference we can estimate the amount of food available for deep water organisms, allowing the assumption of total balance. Some attempts have also been made to evaluate deep water consumption rates. These are of a lower order of validity, but they deserve comparison with other items in the overall balance sheet. Riley et al. (1965) developed this kind of balance sheet in connection with studies of non-living particulate matter in the Sargasso Sea, with particular emphasis on respiratory rates and food requirements of the deepwater population. Four essentially independent analyses were presented : ( 1 ) Menzel and Ryther (1960) measured C14 uptake by phytoplankton in the surface layer off Bermuda and found that the mean daily production, averaged over a period of a year, was 200 mg C.m-2.dayThe same authors (1961) made quantitative collections of zooplankton to a depth of 500 m, obtaining an average value of 1.08 g dry weight/m2. The mean respiratory requirement was determined experimentally and averaged 0.12 g C/g dry wt of zooplankton. Thus the total requirement of zooplankton in the upper 500 m was estimated t o be 130 mg C.m-2. day-l. This leaves an excess of 70 mg C/day which might be available to deep water organisms ; however, there are two ambiguities : (a) no allowance is made for bacterial metabolism in the upper water, and (b) part of the photosynthetic product may be liberated to the water in dissolved form and hence is not measured as production by the C14 method. Thus there is some doubt as to whether the amount of food materials available to populations in deep water might be more or less than 70 mg. (2) Riley (1957) used observed vertical gradients of oxygen and computed eddy coefficients t o estimate net organic production in the upper 300 m of the Sargasso Sea. This method theoretically measures the difference between oxygen production by phytoplankton and oxygen consumption by all components of the population. Hence it is free of some of the difficulties mentioned in item ( l ) , but the method of computation is indirect and is subject t o considerable error. The results indicated that net oxygen production between the surface and the compensation depth was equivalent, on a mean annual basis, to 134 mg C.m - 2.day- l. The net decrease from the compensation depth to 300 m was 81 mg. The difference of 53 mg C.m-2.day-1 provides an estimate of potential deep water consumption. (3) Riley (1951) used similar but somewhat more complicated methods t o compute deep water oxygen consumption. This was a generalized calculation for the central Atlantic basic and might be an

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over-estimate for the relatively poor waters of the Sargasso Sea, but in view of other possible errors this is a quibble. Results indicated that total oxygen consumption between 300 m and the bottom averaged 48 mg C.m-2.day-1, a figure in good agreement with the one obtained in item (2). Most of this utilization occurred in the mid-depth region. The estimated consumption between 1 000 m and the bottom was only about 8 mg C/day. (4) Riley (1951) compiled data on deep water zooplankton from Leavitt (1938) and estimated what the respiratory requirements might be if they were similar to experimental results obtained with surface water forms a t low temperatures and surface pressure. I n the absence of any solid information about respiratory rates of deep water animals under normal environmental conditions, this is the best that can be done, and the results seemed more or less realistic. The total for the depth range between 300 m and the bottom was 38 mg C.m-2.day-1. An additional 4 mg C/day were estimated as the requirements for benthic bacteria and animals. Riley et al. (1965) pointed out that although each calculation has obvious deficiencies or possibilities of error, the four independent methods yield a range of only 42-70 mg C/day, suggesting that this is considerably better than an order of magnitude estimate for deep water consumption. An obvious deficiency in these estimates is the failure t o specify the respiratory needs of bacteria and other small heterotrophs. Their inclusion would tend t o lower the estimate in item ( I ) , which is the highest one, and increase item (4),the lowest one. There was no attempt to do this in the paper cited, because the results appeared t o be adequate for the purpose that was in mind at that time, namely to attempt to evaluate the residence time of particulate organic matter in deep water. However, further examination of the biological system seems desirable a t the present time. Some of the published views represent a piecemeal approach to the problem. Menzel and Ryther (1961) concluded that zooplankton in the Sargasso Sea were capable of consuming virtually all of the phytoplankton production, whereas Parsons and Strickland (1962) suggested equal capabilities for small heterotrophs. A compromise must be chosen between these extremes, for total phytoplankton production must be shared between the two kinds of populations. Moreover, in the period since Riley et a,Z. (1965) published their analysis several papers have appeared containing data which are in marked disagreement with values used in the earlier analysis. The whole subject needs to be reopened t o further discussion. Thus the

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problem of heterotrophy will first be reviewed, and then the problem of the general balance between production and consumption will be reconsidered. (a) Heterotrophy. Until recently most estimates of the numbers of bacteria in deep ocean waters were based on plate counts, which greatly underestimate the total population. Sorokin (1964) examined some stations in the central Pacific by direct counting methods, and a t a depth of 2 000 m his numbers ranged from 0.8 t o 1.03 x l o 6 bacteria per litre. Other investigators have obtained more or less similar results. Water samples from the deep ocean commonly contain a few algae. Most oceanographers have supposed that they represent merely a remnant that has sunk into deep water and is doomed to quick death. However, Wood (1956) demonstrated viability of diatoms taken from ocean depths. More recently Fournier (1966) and Hamilton et al. (1968) have found that large concentrations of small cells (2-4 p ) of somewhat uncertain systematic position are regular inhabitants of ocean deeps. Bernard (1967) reported large numbers of coccolithophores in Mediterranean waters, but Fournier ( 1 968) has cast serious doubt on the validity of these observations. An earlier section mentioned a paper by Holm-Hansen and Booth (1966) who developed a method for assessing relative quantities of living and non-living organic matter by means of ATP analyses. They listed data for fractional partition of this kind a t a station located approximately a t 33"N, 119"W. Cellular organic carbon at depths of 10 and 30 m was respectively 33.6 and 95 pgllitre, constituting 42 and 79%, respectively, of total particulate carbon. At 103 m corresponding figures were 4.6 pg C/litre and 14%. With minor exceptions deep water samples fell within fairly narrow limits and averaged 1.7 pg C/litre and 3.7% of total carbon. Hamilton and Holm-Hansen (1967) extended the examination of the ATP content of marine bacteria, and Hamilton et al. (1968) published additional data from several stations in tropical and subtropical waters of the Pacific Ocean. There were clear indications of a middepth maximum in ATP and a decrease in deep water. The amount of carbon in living organisms, as indicated by the usual ATP conversion factor, was of the order of 1-2% of total particulate carbon in waters below 1000 m. These authors apparently believed that the small coloured cells were quantitatively more important than bacteria. They also mentioned the fact that small green cells from deep water collections had been established in autotrophic culture, but they stated that " the nature of the coloured spheres seen on the membrane filters

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remains an enigma and more work is needed to establish their identity and assess their importance, if any, in the bathypelagic food web ”. Holm-Hansen ( I 968) found a distinct decrease in ATP with depth in the deep ocean, suggesting a corresponding decrease in the biomass of heterotrophs. Possibly it is too early to generalize on these results, but there are indications that although total organic carbon does not decrease systematically with depth, its capabilities for the support of living organisms may decline as it sinks toward the bottom. Nevertheless, one must conclude from this brief summary of recent literature that the quantity of heterotrophs in deep water is much larger than has been supposed in the past. Most of these organisms have not been identified, and there is still little information available on the kinds of substrates that they depend upon. Some information is, however, now being obtained about possible rates of uptake of organic substances and minimum concentrations required to support growth. Parsons and Strickland (1962b) undertook an investigation of heterotrophic processes by measuring the uptake of C14 labelled glucose and acetate, Sophisticated biochemical and microbiological concepts were involved, which will not be elaborated here. It suffices to say that they established beyond reasonable doubt that natural substrates of 10-7 moles or less of glucose or acetate were adequate for heterotrophic growth. Their experiments were performed with natural sea water taken off the coast of British Columbia (49”12’N, 123”58’W). Most of the samples were taken from depths of 5-35 m, and the rate of uptake varied from 0.004-0-3 pg C/litre in an hour, averaging 0.015. Two samples from a depth of 175 m had values of 0.003 and 0.007. I n discussing their results the authors concluded: “ Values for a Michaelis-Menton type constant of less than 10- M have been reported but are uncommon. Yet it should be reasoned that marine microorganisms must have evolved so as to utilize substrates a t the concentrations occurring in nature, with possibly the mediation of an adsorptive concentration onto particulate matter. More work with a wider variety of samples and substrates is required to decide . . . but the method looks promising.” Further work along these lines was reported by Vaccaro and Jannasch (1966). They used both pure cultures and natural populations in studies of glucose assimilation and obtained results similar to the earlier ones in that natural substrate levels appeared t o be of the order of 10-7 M or even as little as l o - * . They were also able t o estimate the in situ rate of substrate removal. At five stations in the tropical North Atlantic, samples collected in the upper 50 m gave values

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of 0.01-0.18 pg C/litre in an hour, and four samples from 100 m ranged from 0.001-0.062 pg, with three of the four toward the lower end of the range. These results were on the whole somewhat higher than the ones obtained by Parsons and Strickland, and clearly more data are needed in order to give us a clear idea of natural ranges of variations and their reasons. Also these tracer methods, though elegant and provocative, do not yet provide a generalized picture of heterotrophic absorption of the complex array of carbon sources existing in sea water. Parsons and Strickland found that the rates of uptake of glucose and acetate were quite different, and a few experimental results cannot be extrapolated to generalities. However, if as much as l-lOyo of the total dissolved organic matter is biologically usable and is as readily available as glucose, this would provide a substrate concentration of 10-8-10- M. Thus it is not entirely unreasonable to suppose that the dissolved fraction might supply most or all of the necessary substrates for heterotrophs, and also, again quoting Parsons and Strickland, " the mediation of an adsorptive concentration onto particulate matter '' is a distinct possibility, particularly since that is where most of the bacteria are living. Hamilton et al. (1968) dealt with some of these problems and quoted unpublished references t o experiments on heterotrophic uptake by deep water organisms. One of these experiments was credited to J. D. H. Strickland (ref. Antia et al., 1962) who showed that the rate of uptake from 500 pgllitre of radiocarbon-labelled glucose, acetate, succinate and citrate a t i n situ temperatures was less than 0.001 pg C/litre/hr a t depths greater than 1 0 0 0 m. Similar results were also obtained by Hamilton, using 20 pg/litre of radiocarbon-labelled glucose, proline and glycine. These experiments were mentioned in passing without further details and presumably will be described more extensively in a later publication. Hamilton et al. had reported that the biomass of deep water heterotrophs in terms of organic carbon was of the order of 0.3-0.5 pgllitre. Thus if the uptake is less than 0.001 pg C/hr, the daily consumption will be less than 10% of the weight of the organisms. This is a remarkably low rate of turnover for small organisms ; however, in an environment that is obviously marginal for the support of heterotrophs one would expect a low metabolic rate, and these results may be realistic. Some algae are much less eficient in heterotrophic uptake than bacteria. For example Wright and Hobbie (1965) found that some freshwater algae they were studying required substrate concentrations several orders of magnitude larger than those required by bacteria.

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They concluded that the algae were lacking in active transport systems and were dependent upon simple diffusion for heterotrophic growth. However, Hellebust (1968) reported that some species are capable of heterotrophic uptake a t substrate concentrations comparable to the minimum levels required by bacteria, and there now seems to be little doubt that some algae at least have effective transport systems. Relative biomasses of the different kinds of cells, as reported by Hamilton et al. (1968), might even suggest that deep sea algae are more effective than bacteria in their competition for available substrates, although admittedly there is still considerable question as to what proportion of the small coloured cells reported by these authors are actually living algae. (b) Variations in production and consumption with depth. Returning t o the problem of allocating the available production in the marine environment to the various kinds of consumers, the situation in the Sargasso Sea will be examined once more, for this area provides more complete data than any other place in the open ocean. This is largely a matter of reworking and amplifying the analyses described in items (1) to (4)of the introductory section. Unfortunately, however, the problem seems more difficult now than it did when Riley et al. ( I 965) published the original a,nalyses, and the results will be less clear cut. Item (1)was an estimate of net phytoplankton production obtained by Menzel and Ryther (1960). Their average was 200 mg Cam-2.day-1 on a mean annual basis. I n recent years the C14 method has been criticized because of the difficulty of proper correction for self absorption on filters, and the International C14 Laboratory a t Charlottenlund Slot has recommended that all of the older values should be increased 45% t o bring them more nearly in line with modern information, Whether this correction is really adequate remains to be seen. Goldman (1968) has recommended burning the material and determining absolute activity by gas counting methods. He compared the two methods, using cultures of various algae. Results varied considerably, but in general the values obtained by absolute counts were about three times as high as those analyzed by the traditional method. The fact that there were systematic differences among different species of algae raised some doubts as to whether the results obtained with natural populations might also vary in a rather unpredictable way depending upon the species composition of the population. This is where the situation stands at the moment. It remains to be seen whether the accumulated C14 data can be salvaged by application of an arbitrary correction factor or will have to be discarded. I n

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the present case the correction factor recommended by the C14 Laboratory is applied provisionally t o the Sargasso Sea data, bringing the mean value to 290 mg C.m-2.day-1. I n the light of Goldman’s recent results, the correct figure might be as high as 600 mg C, but the writer is inclined to doubt this. Even the old light and dark bottle oxygen techniques, which have been criticized for giving results that are too high, indicated that the general range of net production in oceanic waters is of the order of 400-1 000 mg C.m-2.day-l (Riley, 1944), and the Sargasso Sea should be near the minimum limit of the range. The total estimate for usable organic production by phytoplankton also should include extracellular metabolites, which would not be included in measurements of productivity by routine C14 tests. Hellebust (1965) reported experimental results on the production of external metabolites by a large number of algal species. The list included Coccolithus huxleyi, an important species in the Sargasso Sea. At nominal light intensities of 3-10 x lo3 lux, 1.3-4*4y0 of the photoassimilated carbon was released, and the value increased to 1 7 % in full sunlight. Dinoflagellates ranged from 4.4-1070. Values of up to 25% were obtained for some of the diatoms, but these algae are not important constituents in the Sargasso Sea except in winter and spring. From these data one might suppose that a reasonable estimate for extracellular release might be of the order of 10% of photoassimilated carbon. However, (2.1. Choi of this laboratory (personal communication) has obtained considerably higher values in experiments with natural populations. These were in situ C14 experiments performed a t a series of depths a t stations between the Newfoundland-Nova Scotia shelf and the Gulf Stream, followed by measurements of organic C14 in the particulate matter and in the filtrate. At the three southernmost stations filterable organic C14 averaged 25% of the C14 incorporated in particulate matter. Even higher values were common a t the more northerly stations. I n short, extracellular release in the natural environment can be 1 0 - 3 0 ~ 0 or more, and the total organic productivity a t the Sargasso Sea station under present consideration is estimated to be 320-380 mg C.m-2.day-1, with some possibility that the true value might be more. The next step is t o attempt to equate organic production in the surface layer with consumption in the water column as a whole. The two physical oceanographic computations in items ( 2 ) and (3) gave closely concordant values of 48-53 mg C.m-2.day-1 for consumption below 300 m, and the others varied around these values. As a guide to the remainder of the analysis, it will be assumed that these

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numbers are approximately correct, and for convenience a value of 50 mg will be chosen arbitrarily, although the later discussion will indicate that this may be an underestimate. An additional simplification arises from the fact that the analyses were not quite comparable as to depth ranges that were selected. Zooplankton data by Menzel and Ryther (1962) were based on tows made from 500 m t o the surface, so that their calculation of zooplankton consumption was lumped into a single estimate for this depth range. I n item (4),deep water estimates of zooplankton Consumption were calculated from 300 m to the bottom, of which about 20 mg can be assigned t o the depth range of 300-500 m. To avoid counting part of the zooplankton twice, this number will be subtracted from the estimate based on the data of Menzel and Ryther. The estimates for consumption by zooplankton will then be 110 mg C in the upper 300 m, and the remainder of vertical column will remain the same. Data for estimating heterotrophic consumption are scanty, as was evident in the last section. Observations by Vaccaro and Jannasch (1966) yield the following averages: 1-10 m, 0.045 pg C.litre-l.hr-l; 35-50 m, 0.061 ; 100 m, 0.004 (excepting one anomalously high value of 0.062). Further, assuming that consumption from 100-300 m is about 0.001, their lowest value for 100 m, the daily carbon requirement for the upper 300 m would be 120 mg C/m2. A similar treatment of the observations of Parsons and Strickland (1962) gives a value of 45 mg C/day. The agreement is not particularly close, and of course neither estimate is directly applicable to the area in question. There is the further question of whether experimental results of this type are indicative of the natural rate of absorption of the more complicated array of substances available in nature. Nevertheless, for what it may be worth, the combined estimate for consumption by zooplankton and heterotrophs in the upper 300 m plus the assumed deep water value of 50 mg comes to 205-280 mg C, which is about two-thirds to threequarters of the postulated surface production. Direct measurements of consumption by heterotrophs would be desirable, but there are grave dangers of introducing experimental artifacts into such determinations. Ordinary dark bottle incubation is useless because of the well known problem of extensive bacterial growth in bottles of stored sea water. Pomeroy and Johannes (1968) have attempted to get around this problem by developing a method for gentle filtration of large volumes of sea water, permitting short term measurements of oxygen consumption in the concentrate. Their paper contains an interesting descriptive account of the collections, which has been mentioned earlier, in addition t o the experimental

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measurements. The latter were obtained from collections off Peru and in subtropical latitudes of the western North Atlantic, an area well to the west of the Bermuda station that is under present consideration and which probably is somewhat richer biologically. The results that they obtained were considerably higher than the present writer’s estimates of in s i t u oxygen consumption in the central Atlantic basin as determined by physical oceanographic calculations, and the question is not fully resolved as to whether this represents real regional differences or is due t o errors in one method or the other. Pomeroy and Johannes determined changes in rate of oxygen consumption with varying concentrations of their material and found that the curves extrapolated to the origin. Thus there were no obvious indications of density dependence in the results. Even their minimum values are equivalent to a carbon consumption of about 300 mg.m-2.day-l in the upper 300 m and 500-600 mg in the whole of the surface and mid-depth column. These figures do not fit comfortably into the carbon balance sheet for the Sargasso Sea, but as indicated earlier, they were dealing with more productive regions. There is another way to look at the situation which might be more informative. Their measurements in the Peru offing were accompanied by experiments on C14 uptake and they obtained a mean P / R ratio of 1 - 5 : 1 , where P is uncorrected C14 uptake, and R is total carbon consumption in the upper 500 m as determined by measurements of oxygen consumption. Their experiments in the Atlantic did not include C14 measurements, but they reported that there was about a 1 : l ratio between earlier C14 estimates in the area and their determinations of respiratory requirements. If the same sort of ratio is applicable to the Bermuda region, the carbon consumption by ultraplankton in the upper 500 m would be about 133-200 mg.m-2.day-1. Total utilization in the water column would then be 290-360 mg, a range that is essentially in agreement with production figures, although the indirect methods used t o derive it leave much to be desired. Below 300 m there are three major oceanographic regimes, biologically speaking. There is a mid-depth zone which includes the oxygen minimum layer and a mid-depth assemblage of animals which comes up close to the surface a t night, and migrates downward some hundreds of metres to a daytime position that more or less corresponds with the depth of the oxygen minimum layer. Below the main thermocline the species composition changes and the total quantity of animals is much reduced. The deep water animal population will be referred t o here simply as bathypelagic animals

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although this is an over-simplification, for a detailed study of populations requires subdivision into several depth zones. Finally there is an assemblage of benthic and epibenthic animals and an associated bacterial flora. The benthic population of deep ocean waters is not large, but it is probably about equal to the total quantity in the sparsely inhabited bathypelagic zone. The mid-depth zone properly includes the whole thermocline region. Drawing an arbitrary dividing line at 300 m as was done above is an artificiality imposed by the nature of the analyses. Physical oceanographic analyses were predicated on assumptions of a steady state and had to be limited to waters in which there were no seasonal changes in properties, and analyses of the upper 300 m cannot be partitioned with enough precision to fill in the gaps in a very satisfactory way. There is of course the further difficulty that the top of the thermocline varies seasonally from 50 m or less to more than 150. However, with some slight over-simplification a range of estimates of biological consumption can be obtained for the depth range of 100-900 m, which constitutes the major part of the mid-depth zone in this area. This depth range will be examined briefly, and a more detailed balance sheet will be drawn up later for the whole of the vertical column below 300 m. A generalized estimate for oxygen consumption within this layer can be derived from physical oceanographic analyses. The net decrease in oxygen from the compensation depth to 300 m (item 2 above) plus the decrease from 300-900 m (the same data used in compiling item 3) are equivalent t o a total carbon consumption of 119 mg.m-2.day-1. For reasons given below this figure may be too small. A further estimate based upon respiratory rates requires an assumption about partitioning of zooplankton respiration within the upper 300 m. The total utilization of 110 mg C/day as described above is assumed to be uniform with respect t o depth on the grounds that the population is subject t o diurnal migration, so that 73 mg can be allotted to the 100-300 m depth range. To this is added 28 mg for animalrespiration bet,ween 300 and 900 m and an allotment for heterotrophs and associated ultraplankton which could be as much as 70-100 mg if the vertical variations in this segment of the population are comparable .to those described by Pomeroy and Johannes (1968). Vertical flux into this layer (Table X, p. 85) was estimated to be 109 mg C.m- 2.day- l, and the flux into deep water below was 9.5 mg ; thus the loss within the layer was estimated to be about 100 mg. This again is a crude and possibly minimal estimate. From the earlier discussion of this problem it will be apparent that as in the case of phytoplankton, the larger particles may be lost from the surface layer a t a

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rate several times that of their apparent sinking rate, although there is some question as to what proportion of total carbon will be so affected. So again there is a considerable range in possible estimates, from about 109 to 200 mg C.m-2.day-1 in the 300-900 m depth range. The most likely value is toward the upper end of this range, and a major proportion of the consumption is believed t o occur between 100 and 300 m. With regard to deep water below the transition zone, physical oceanographic computation of oxygen consumption (Riley, 195 1 ) indicated that carbon utilization in the bathypelagic zone and on the bottom totalled 9.4 mg C/m2. A somewhat higher value of 14-16 mg is indicated for metabolic consumption by bacteria and animals in the bathypelagic zone and on the bottom. Again there is some variation in results, but this amount of agreement is reassuring, because the earlier estimate based on oxygen consumption was drastically over-simplified. The problem is simply that oxygen consumption in deep water is so slight that extensive averaging is necessary in order to get a body of data that will yield consistent results. I n this case data were combined into a single north-south profile of the Atlantic Ocean. Within this profile the deep water showed an apparent oxygen utilization, as defined by Redfield (1942), which averaged about 2 ml/litre. If this were due entirely to biological utilization, the rate of consumption could be calculated in terms of the age of the sea water. The age probably falls within the general range of 250-750 years. Calculated carbon utilization in the whole deepwater column would be about 12-33 mg C.m-2.day- l . These are maximum values because part of the loss in oxygen is due to transfer upward toward the oxygen minimum. The analysis utilized estimates of vertical and horizontal eddy coefficients derived from the salinity distribution to correct for physical dissipation of oxygen, and the residual value was the estimate given above. It will be apparent from the method of calculation that this is a generalized statistic, and its application to any particular area is questionable. However, it is the only information of its kind available. This analysis possibly should be reworked, incorporating new data and using a finer network of stations. The broad grid that was used failed to delineate narrow, deep ocean currents, which indeed were not known to exist a t that time, so that total advective circulation was underestimated. Sverdrup et al. (1942) and Riley (1951) computed the average age of the deep water as about 1000 years, which now appears to be an over-estimate, and in both cases this was due to the assumption that the advective pattern was broader and more diffuse.

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Correction of the figures for advection would require adjustments in coefficients of eddy diffusivity in order t o maintain a conservative balance of salt, and this would lead in turn to alterations in estimates of biological rates of change. Possibly the alterations would be quite local, in association with major deep water currents, or they might be more general. Intuitive methods are not equal to the problem, but conceivably all of the mid-depth and deep water biological rates of change that have been listed here should be doubled, more or less. I n deep water the values are so small that no conclusions would be seriously altered. I n the mid-depth region from 100 to 900 m it makes some difference as to whether consumption is one-third or two-thirds of the total surface production. This discussion makes it quite apparent that the writer does not subscribe to the viewpoint expressed by Menzel and Ryther (1968a), who regarded the oxygen distribution in lower mid-depths and deep water as being essentially conservative. Their hypothesis seems to be that oxygen deficits develop in the upper reaches of the thermocline and are translated horizontally and to greater depths by isentropic mixing, and apparently they regarded the whole of the oxygen minimum layer of the Atlantic Ocean as having been derived from Equatorial upwelling areas off Africa. Reasons why this conclusion is unacceptable have been presented earlier. Their conclusion that oxygen consumption in deeper water is conservative requires further discussion. They analyzed the distribution of oxygen along the axis of the Antarctic Intermediate water in the South Atlantic, a water mass lying just above the upper boundary of the deep water that is under present consideration. In their north-south profile, which extended from approximately 10’ to 36’5, there was a decrease in oxygen of about 2.65 ml/litre from south to north. They used the so-called core ” method to analyze oxygen at intermediate points. This method assumes that mixing takes place only along the axis, so that the salinity at any intermediate point can be determined as percentage mixtures of water at the two ends. These percentages are then applied to the two end values for oxygen to determine intermediate concentrations. Menzel and Ryther found that these more or less agreed with observed values and concluded that oxygen is a conservative property. Earlier Riley (1951) had obtained a n estimate for annual oxygen consumption of about 0.01 ml/litre in this water mass, a modest amount although considerably larger than the deep water average quoted above. However, even this small rate of change could lead t o 66

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a considerable oxygen deficit during the slow northward movement of the water. The difficulty with the core method is that it is incapable of detecting this biological deficit except under certain special circumstances. The observed oxygen concentrations a t the two ends of the profile are arbitrarily chosen to compute intermediate values. These concentrations show an apparent oxygen utilization, but whether it has taken place locally or a t the point of origin or how rapidly it has occurred are unknown. Menzel and Ryther begin with values that do not deny their hypothesis but do not support it either. And if local utilization has occurred, the use of arbitrary reference values will hide any utilization that has taken place a t intermediate points, provided this utilization proceeds a t a reasonably constant rate with respect to time and mixing rates along the path of flow. Only pronounced anomalies and non-linearities can be detected. There is, unfortunately, no solution to this problem except the treatment developed by Sverdrup et al. (1942), requiring evaluation of advection and diffusivity in terms of conservative properties and application t o non-conservative distributions to derive a residual rate of biological change. Ideally this is done as a three-dimensional problem, although a two-dimensional profile can serve the purpose. This type of analysis, developed by Riley (1952), is not a very dependable solution, because there are so many complexities and uncertainties in the application. However, vertical and horizontal diffusion are both important enough so that one-dimensional analysis of deep water is seldom effective, even aside from pitfalls of the kind described. Returning to a consideration of biological problems in waters below 300 m, total consumption was postulated t o be of the order of 50 mg C.m-2.day-1, and earlier estimates indicated that 38 mg are consumed by pelagic animals and 4 mg on the bottom. However, no real confidence can be placed in these figures. They are largely derived by extrapolation of experiments on surface water forms. Little is known about effects of pressure and nutritional state on the respiratory rates of these organisms. Some speculation is necessary in order to try t o fit the pieces of the puzzle together. One can suppose that animal respiration might consume as much as 38 mg C/day, or perhaps it might be only half that much. This would leave 8-27 mg/day for heterotrophs. Converting to more familiar terms, this is 1-4 x 10-4pgC.litre-l.hr-l as an average for the deep water mass as a whole, a range that is one order of magnitude less than observed uptake a t depths of 100-175 m, two orders less than in near-surface waters, and essentially similar to the

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value of <0.001 quoted by Hamilton et al. (1968) for deep water. The daily uptake would be less than 10% of the carbon content of the organisms (judging the latter by ATP analyses). This consumption is small compared with metabolic rates that are commonly reported for bacteria, but it is in accord with the low levels of bacterial activity associated with long term laboratory experiments, as well as with the experiments on heterotrophic utilization by natural populations. Presumably both particulate and dissolved organic matter provide carbon sources for these heterotrophs, and the latter is the larger potential supply, although not all of it is likely to be assimilable. Nevertheless, these rates have some bearing on the question of the residence time of dissolved organic matter in the deep sea. Analyses of dissolved organic matter by wet oxidation methods have indicated a carbon content of 0.5 mg/litre more or less. Its distribution in the North Atlantic (Duursma, 1960) shows some slight indications of a relation with oxygen distribution ; for example, there is a reduction in the region of the oxygen minimum layer. I n view of the general similarity but less extreme variation, Riley et al. (1965) suggested that the mean residence time is fairly long but not as long as that of the water itself. I n retrospect this conclusion seems too simple, and it should be modified. Heterotrophic carbon utilization is directly related with oxygen consumption, but this is not true of animal metabolism, which is more likely t o increase the dissolved organic content than to decrease it. Some investigators have felt that mid-depth concentrations of animals are very largely responsible for the development of the oxygen minimum layer, and it is perhaps significant in this respect that the decrease in dissolved organic matter is slight or lacking. For example, Menzel and Ryther (1968a) found no mid-depth reduction in dissolved organic carbon in the tropical South Atlantic where animal populations are known t o be unusually large (Jespersen, 1935). As to the average age of the dissolved organic matter, the length pg C.litreof time required to use 0.5 mg C/litre a t a rate of 1-4 x is 140-570 years. The larger value falls within the somewhat elastic range of estimates that has been proposed as the age of the deep water of the Atlantic Ocean. As a generalized statistic this range seems reasonable ; however, the true situation is likely to be a rapid rate of turnover of a small fraction of readily assimilable substances and a much longer residence time for more refractory compounds. Webb and Johannes (1967) and Johannes and Webb (1968) have investigated the kinds and quantities of dissolved organic compounds that are released by copepods and other invertebrates. The product

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includes free amino acids and other substances which should be readily utilizable by heterotrophs. Thus the general stock of dissolved organic matter in the deep sea includes materials that have been recently produced in situ by the local assemblage of bathypelagic animals. Theoretically, such things as amino acids could either be utilized directly by heterotrophs or could be adsorbed on particulate matter, which would make them available either to bacteria or to filter feeders. The general concept that dissolved organic matter originates a t the surface and has a long residence time in deep water is true as a generalization, but these recent observations suggest that internal recycling of “ young ” materials could be a significant aspect of the biological economy of the deep sea. However, measurements of excretory rates thus far have been limited to zooplankton collections taken in the surface layer, and the results cannot be extrapolated to a quantitative estimate of excretion under the semi-starvation conditions of the bathypelagic zone. As indicated earlier, most of the nitrogenous material in deep water is in the form of amino N, yet much of the older material must be combined in forms that make it resistant t o biological attack. The low concentration occurring in nature cannot be the only reason for its inability to support active heterotrophic growth. Barber (1968) increased the concentration by pressure dialysis but found no significant utilization in experiments of 1-2 months’ duration. Of course the method is not precise enough t o detect small changes with certainty. Rakestraw (1947) reported small but significant reductions in oxygen in long term B.O.D. experiments. The carbon equivalent of this consumption was of the order of 0.1 mg/litre in experiments lasting considerably longer than Barber’s. While this amount is small compared with the B.O.D. of surface waters, it is in fact much more rapid than estimated in situ rates of consumption. Presumably the bacteria were growing more effectively in the bottles than they do in the sea, as is usual in this kind of experiment. Degens (1968) has noted that ratios of C13 t o C12 are essentially similar in living plankton, in non-living particulate organic matter and in recent marine sediments, and there is a reduction in the ratio in dissolved organic matter and in ancient marine sediments. Biological utilization of the organic matter presumably has resulted in isotope fractionation. For example, Degens quoted evidence that the CO, released during decarboxylation of amino acids can be significantly enriched in C13 in relation to the remainder of the molecule, and the remaining organic matter of course will be correspondingly depleted. Degens concluded that the sediments were derived primarily from

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particulate matter in the water column rather than from the dissolved fraction. There is a further implication in his results that the dissolved matter is older and has gone through more metabolic transformations than the particulate fraction. This sort of conclusion could be valid as a generalization without denying the concept developed above that a small fraction of the dissolved material is new and metabolically usable. The problem of estimating zooplankton food requirements and feeding rates is not an easy one. Experimental estimates vary widely, and probably there are similar variations in nature. The problem is particularly difficult in the present case because most of the experimental information has been obtained from surface forms. Our opinion about what bathypelagic organisms eat is largely a matter of inference, based upon the character of their mouth parts. The question is largely unresolved as to whether there are local concentrations of food that ameliorate the average picture of extreme scarcity. There is as yet no clear answer to the question of whether food requirements and metabolic rates in the deep sea are essentially similar to those of nearsurface waters. Attempts to investigate pressure effects have led to somewhat equivocal results. Napora (1964) reported that pressure slightly increased the respiratory rate of Xystellaspis debilis, a prawn which is a member of the mid-depth population that migrates to the surface a t night. He was working with animals that live in the Sargasso Sea, where the thermocline is not particularly steep, and the effect of decreasing temperature as the animals moved downward was approximately balanced by the pressure effect, so that the animals maintained an essentially constant respiratory rate a t all depths. Teal and Carey (1967) found a significant pressure effect with Thysanopoda monocantha, a euphausiid which normally lives a t mid-depths and apparently does not come t o the surface a t night. However, they tested several migratory euphausiids and found that pressure effects were operative only a t temperatures that were too high to be of any significance a t the daytime level. Pearcy and Small (1968) examined two euphausiids and a prawn, all migratory forms, and found no significant pressure effects. I n short, no consistent pattern emerges among the mid-depth forms, and the writer has failed to find any information on truly bathypelagic animals. Provisionally the problem must be considered in terms of what is known about surface forms although the deep sea environment may have a significant effect on physiological rates. Present estimates of zooplankton respiratory requirements have been based upon measurements of oxygen consumption of surface

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forms a t low temperature as indicated in item (4) of the introductory section, together with data from Leavitt (1938) on the quantity of zooplankton in deep water. The latter collections were made with a twometre stramin net. Recent work with finer mesh sizes indicates that there is a significant fraction of small zooplankton that would not be included in the present estimats. The true value for deep water consumption by animals probably is somewhat larger than the one given here, and as indicated above, physical oceanographic computations may also underrate total consumption. However, present information provides a basis for a general discussion of feeding problems in deep water. One of the early attempts to express grazing rates of herbivorous plankton in a simple form suitable for quantitative application was that of Riley et al. (1949). The formula, restated in a slightly different notation, was G = 235 R where G is a grazing coefficient indicating ml/day of water swept clear of particulate food by a quantity of zooplankton containing 1 mg C, and R is the respiratory consumption in mg C/mg of zooplankton carbon. This formula as expressed conformed with experimental observations that grazing rates increased with temperature in much the same way that respiratory rates do, and the constant in the equation was more or less in the middle of the observed range of grazing coefficients. The equation was used with a fair degree of success in theoretical computations of regional plankton abundance in various areas of the western North Atlantic, including the Sargasso Sea. Recent work has added a great deal of knowledge about grazing and assimilation rates and respiratory requirements under various conditions, but these investigations have not yet produced a definitive statement of quantitative relationships, so that the formula given above still serves as a rule of thumb in gauging the effectiveness of a given food supply in supporting a crop of herbivorous zooplankton. With regard t o the surface layer of the Sargasso Sea, Menzel and Ryther (1961) found that the respiratory requirement of the zooplankton in terms of carbon was about 12% of zooplankton dry weight so that R = ca 0.06 mg C/day, and G = 1.4 litres. The general range of particulate organic carbon as determined by cruise averages for various seasons (Riley et al., 1965) was 46-168 pg C/litre. A reasonable annual mean would be about 85 pg, leading to an estimated daily consumption of 120 pg C/mg of zooplankton carbon. This is probably an adequate food supply, on the average, for a t least half of the particulate carbon in the surface layer is likely to be assimilable. During the poorer part of the year the supply would seem to be somewhat inadequate, but as

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indicated earlier, there are likely t o be micro-concentrations of food in convective downdrafts that are not clearly demonstrated by routine sampling procedures. Most of the mid-depth organisms in this area perform long diurnal migrations of some hundreds of metres. Varying food habits are represented. The herbivorous fraction presumably gets most of its nourishment in the surface layer a t night. At the daytime level there is probably little more particulate food available than in the deep sea, although there may be layering in small discontinuities in the main thermocline as indicated earlier. Food relations of bathypelagic animals have been discussed by many authors, but this remains as one of the most poorly understood problems in biological oceanography. Early investigators wrote that deep sea animals are fed by a " rain of detritus from the surface layer )', a phrase that has been quoted too often, for it is a superficiality that tends to sweep the problem under the rug. It is presumably a simplistic expression of the valid concept that all deep sea life is supported directly or indirectly by surface production, but the statement was made with essentially no knowledge of the quantity or food value of detritus sinking from the surface, or of the further transformations that must occur in order t o develop the observed assemblage of deep sea animals. A considerable proportion of this assemblage is of course carnivorous or necrophagous. Wheeler (1967) has found that dead zooplankton constitutes a significant fraction of total net catches in deep water. This is not generally the case in near-surface catches, where presumably the population is sufficiently concentrated so that any animals dying a natural death will be consumed very quickly. The implication in Wheeler's finding is that bathypelagic organisms which die a natural death may not be found and eaten immediately and in the meantime will sink into deeper water. Even in the case of capture of living prey, vertical movement of animals and random capture a t different depths can lead t o a sort of " biological diffusion " of food materials from middepths down into the sparser deep waters, a concept mentioned by Riley (1951) and developed more fully by Vinogradov (1962). Some bathypelagic copepods have foliaceous appendages which appear t o be suitable for a filter feeding habit. Animals with mouthparts of this type probably are more common than has been suspected, because most of the collections of deep water zooplankton have been made with coarse nets that do not catch small copepods effectively. However, it remains an open question as t o whether indiscriminate filter feeding in the bathypelagic zone will supply an adequate amount

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of food. Work by Conover (1964) on Calanus hyperboreus is useful in evaluating this problem. This copepod is a surface form in high latitudes and is a minor constituent of the bathypelagic fauna of lower latitudes. It is well adapted to the rigors of its normal habitat a t the surface, where food is abundant in summer but virtually absent during much of the year. Conover has demonstrated that there is a high rate of accumulation of stored fat when it is fed a t an optimal level, and it can survive starvation for some months with a reduced rate of metabolism that conserves its stored food. Conover found that the normal daily food requirement a t low temperature is about I yo of the weight of the animal ; under starvation conditions carbon consumption is reduced to as little as 0.4%. According to average grazing estimates presented earlier a 1yo respiratory requirement would be accompanied by a filtering rate of about 235 ml of water per milligram of zooplankton carbon in a day. This filtration rate would require a concentration of 17 pgllitre of assimiIable carbon to supply a maintenance ration even a t a reduced level approaching starvation metabolism. Most of the values that have been reported for deep water particulate carbon fall within the general range of 1050pgC/litre, which appears t o be inadequate unless it is more completely assimilable than is indicated by experiments with enzymes (Gordon, 1970b). As far as present information is concerned, there is little likelihood that bathypelagic organisms can support themselves solely by indiscriminate filter feeding. According to Conover ( I 964) C. hyperboreus is capable of either selective capture or filter feeding, and the present discussion points toward a tentative conclusion that this kind of feeding habit is essential for bathypelagic copepods with foliaceous appendages. Possible food materials would include smaller zooplankton, living and dead, large aggregates, and faecal pellets. Reports by divers of " marine snow " indicate that there are organic aggregates of visible size in the deep ocean. These presumably have a fairly rapid sinking rate, and the number delivered into deep water may be much greater than might be inferred from the relative scarcity of large aggregates in water samples. Faecal pellets of visible size can sink at rates of 50-375 m/day (Smayda, personal communication). They, too, may be a source of particulate food that is much more important than would be suspected on the basis of their relative abundance. These various sources can only be mentioned in passing. Little can be said about their nutritive value or rate of supply. Certainly the sparse nature of the bathypelagic population is sufficient evidence that the food supply is minimal.

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Coming back to the earlier comments about a “ r a i n of detritus from the surface ”, faecal pellets derived from the surface can sink quite rapidly and conceivably could arrive in the bathypelagic zone before their organic content, such as it is, has been seriously depleted by bacterial activity. Most of the other suspended matter would have a residence time ranging from one year t o many years, and there is little likelihood that any biologically usable fraction of it could escape consumption by one means or another during that period. The continued existence of particles containing a significant fraction of usable substances (as indicated for example by hydrolysis experiments quoted above) must be due to in situ processes. There are indications that particles can form de novo. There are also indications of interchanges of simple molecules such as amino acids between the dissolved organic pool and various kinds of particulate matter, living and dead. The pathways are complex and probably have not all been properly identified. Our knowledge of probable rates of exchange is very limited. The rates are low, due to the dilute nature of the medium, and it will be a very difficult job indeed to obtain realistic information on natural rates of exchange. Possibly carnivorous activity is quantitatively more important than heterotrophic processes in energy transfer and vertical flux of materials toward the bottom. This remains t o be seen. Present information is crude and equivocal. However, heterotrophy is more important than was realized unti