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When particulate material in sea water is collected on a membrane filter, or in a settling cylinder, microscopic scrutiny always reveals the presence of many particles of irregular shapes and sizes. These particles, known collectively as detritus, consist of organic and inorganic material.

Organic particles (the concentrations of which are expressed amounts of particulate organic carbon-POC), are derived largely from metabolic waste the dissolution of pelagic organisms, and from dissolved organic matter drawn out of solution to form organic aggregates. Concerning the first and third sources, Sieburth under a section entitled ‘Fecal Fragments’, writes:

‘It is obvious that much of the amorphous and flake types of suspended debris occurring throughout the water column is the debris resulting from feeding. The increase in suspended organic debris following phytoplankton blooms led to the concept of organic aggregates in which large reserves of DOC formed flakes and amorphous particles through a foam tower or bubble-scavenging mechanism or through film collapse.

A revaluation of bubble-salvaging indicated that it could be a laboratory artifact. It has been shown that particle formation is mainly in the <4p.m range and is due to bacterial growth. This is in contrast to some 90% of the particles, which are in the 8-44(j.m size range.

‘Not all feces or their fragments are consumed by coprophagy. Just as there is a threshold level for DOC utilization there is also a minimum threshold level for particulates below which filter-feeding organisms will not feed. This ensures a more or less steady state input of debris into the sea floor. This material as well as larger debris undergoes biodeterioration on the sea floor to form a secondary production for the development of benthic ecosystems.’

The concentrations of both dissolved and particulate organic matter vary widely in the upper waters, but at depths below 200-500 metres there is relatively little change over most of the ocean.

Deep DOC ranges from 0.5 to 0.8 mgC/litre, while the corresponding figure for particulate organic carbon is 3-10 mgC/litre. Thus, DOC is hundreds of times the more plentiful, though nearly all of it seems to be unusable by living organisms. How nutritious, then, is particulate organic matter?

When, for instance, one looks in the house of an appendicularian, the gut of a copepod or the filtering-basket of a euphausiid, fragments of detritus are discernible. Deep-sea detritus, as histochemical tests reveal, contains proteins and carbohydrates, some of which should be digestible. But laboratory tests have at best provided conflicting evidence that zooplankters, such as copepods, thrive on a detrital diet.

This seems curious in view of Gordon’s finding that 20 to 25 per cent of deep organic detritus is hydrolysable by proteases. There is even more direct evidence from vertical profiles of the amounts of adenosine triphosphate (ATP) in the water column (Holm-Hansen and Booth, 1966; Holm-Hansen, 1970).

Since ATP, the high-energy basin of metabolism, is maintained in fairly uniform concentrations in all living cells, but is soon destroyed after death; measurement of total ATP concentrations should indicate amounts of living substance. Indeed, analyses in the laboratory show that the quantity of ATP should be multiplied by 250 to give the cellular content of organic carbon (Holm-Hansen, 1970).

Investigations in the eastern Pacific off California reveal that the ATP content, which is proportional to biomass, is very high in the euphotic zone but diminishes very rapidly between depths of 100 and 200 metres, below which there is a much more gradual decline to 3,000 metres.

Analyses of samples taken between 500 and 1,000 metres showed that 3 per cent or less of the particulate matter was alive but above 100 metres the live fraction ranged from 17 to 79 per cent. After concurring with other investigators that most of the dissolved and detrital carbon in deep water is unusable by micro-organisms, Holm-Hansen suggests that a small fraction of this organic carbon turns over rapidly in support of microbial populations that may well be important as the first stage in oceanic food chains.

What are the first animal stages of mesopelagic and bathypelagic food chains? At such levels the micro-zooplankton seems to be represented largely by foraminiferans and radiolarians; the remnants of which may be common in the guts of copepods, mysids, euphaussids and so forth.

Concerning the relatively large species of deep-sea zooplankton, the conclusions from Chindonova’s analyses of stomach contents is that most of the 64 species examined, which included 15 out of 18 species of copepods, were carnivorous. But her results show also, together with Harding’s more detailed studies, that some copepods feed at lower levels of the food chains. Harding examined deep-water copepods that were taken in closing nets fished in the Sargasso Sea and slope water off Nova Scotia.

The considerable range of food items that he found in about 80 species included mineral particles and detrital remains, phytoplankton protozoans, cysts and eggs, remains of cnidarians, copepods and euphausiids. He concludes that heterotrophic protists are not only the main food of filter-feeding species of deep-sea copepods, but also form much of the diet of omnivorous and carnivorous species.

In turn the heterotrophic forms’.. .must be supported by the dissolved and particulate organics, bacteria and possibly detrital remains of organisms present in deep waters’. Harding’s idealized chart, which outlines the food delations of filter-feeding, omnivorous and carnivorous species of deep-sea copepods and possible energy pathways.

Further studies of this kind coupled with investigations on the quantitative distribution and metabolic requirements of the main components in midwater food chains, should resolve some of the present uncertainties. Concerning the metabolic aspect, measurements of the oxygen consumption of crustaceans and fishes from different midwater levels showed, inter alia.

That species from about 1,000 metres respired at a tenth of the rate of shallow- water forms. Metabolic needs are evidently modest, but how are they met?