Deepsea food supply

Whereas in shallow water much of the food supply of the sea bottom comes directly from the surface layers, in very deep water it is unlikely that much surface plankton reaches the bottom intact because most of it is consumed by pelagic

Table 6.2 Percentages of Dry Weight Biomass Contributed by Different Animal Groups in Macrobenthos of Western English Channel.

Group Percentage

Lamellibranchs 35.46

Polychaetes and Nemertines 25.79

Echinoderms 17.8

Crustacea 10.51

Coelenterates 7.00

Polyzoa 2.46

Protochordates 0.88

Gastropods 0.02

organisms on the way down. Between the productive surface layers and the deeper parts of the ocean there is a food pyramid with many links in the food chain, and the quantity of food available to support the population at deep levels can be only a very small fraction of the surface production. Knowledge of the distribution of deep-sea benthos is scanty although increasing, but population studies in deep water generally indicate that the benthos is sparse and that the biomass decreases with depth. Broadly, the deeper the water and the further from land, the smaller the weight of animals on the sea bottom. There are certain exceptions to this, namely hydrothermal vent communities (see Section 6.4.4).

Occasionally, even at great depths, large pieces of meat, the remnants of bodies of whales or very large fish, must reach the sea bottom. This supports a population of relatively large scavengers (Issacs and Schwartzlose, 1975), including fish, amphipod and decapod crustacea and ophiuroids. Beneath areas of low production these scavengers are often more numerous than might be expected from the size of the surface population. The quantity of readily digestible food reaching the sea bottom must depend partly on the numbers of midwater organisms devouring it on the way down.

Apart from the bodies of large animals, most of the organic material of surface origin that eventually settles on the bottom in very deep water must consist of structures which cannot be digested by pelagic animals during descent; for example, cell walls, shells and skeletons. Pieces of wood and other materials from the land also find their way to the deep sea floor, sometimes carried off the continental shelf by turbidity currents (see Section 5.7). Further decomposition of these materials depends upon the activities of bacteria, which occur in the superficial layers of deposit even in the deepest water and constitute an essential link in the organic cycle. Their food is drawn from many sources. Most of them are heterotrophic, obtaining energy from oxidation of organic compounds, but they can act upon a far wider range of materials than animals are able to digest, including chitin, keratin, cellulose and lignin. Even at the deepest levels these substances are likely to reach the sea floor, and here they are digested and assimilated by bacteria. In this way, organic materials which animals cannot make use of directly are transformed into bacterial protoplasm, and in this form become assimilable by numerous animals which feed on bacteria.

A further source of food available to benthic animals is dissolved organic matter (DOM) (see Section 4.3.3). The ways in which DOM can be utilized by planktobacteria, and their role in the 'microbial loop' and the food web, are described in Section 5.1.2. However, it is believed that some benthic animals can directly utilize DOM. Some shallow-water invertebrates can utilize dissolved amino acids but it is in deep-sea benthic animals that uptake of DOM is best developed. Pogonophoran worms do not have an internal alimentary system and part of their energy requirements are thought to be met by uptake of DOM. They also utilize symbiotic chemosynthetic bacteria (see Section 6.4.5). There is experimental evidence that many other marine invertebrates, though possessing feeding mechanisms which ingest solid food, are also capable of taking in aminoacids, glucose and fatty acids from dilute solution by direct absorption through the epidermis (Sorokin and Wyshkwarzev, 1973; Southward and Southward, 1970 and 1972; Southward et al., 1979). Southward and Southward (1982) have estimated that DOC (dissolved organic carbon) provides 30 per cent of the energy required by the deep water seastar Plutonaster and the polychaete Tharyx. It is not known to what extent this facility provides a useful supply of food to these organisms in normal conditions, but it is at least reasonable to suppose that when particulate food is scarce the intake of dissolved organic compounds is of some value.

Colourless flagellates as well as certain groups of pigmented plants, chiefly coccoliths and blue-green algae, sometimes occur far below the illuminated layers - even to 4000 m - in such large numbers as to suggest that they are living and multiplying there, presumably feeding as saprophytes. Many bacteria also draw nutriment from the organic constituents of the water, and the bacterial population includes chemosynthetic autotrophs which do not require organic material but are able to obtain energy by oxidizing inorganic compounds such as ammonia, nitrites and sulphides. These forms can function as primary producers of organic matter below the photosynthetic zone. It is likely that bacterial and protistan populations contribute a significant proportion of the food for animals at great depths. There is also some addition to the biomass of deep water from larval and juvenile stages which develop nearer the surface, drawing on the more abundant food available there before descending to deeper levels.

These sources of food originating far below the surface may go some way to explaining the fact that although populations of deep sediments are small compared with those of most shallow areas, they appear none the less to have a somewhat greater biomass than would be expected from food chains originating only at the surface. An overall estimate of benthic biomass at depths below 3000 m is about 0.2 g dry wtm~2; but in certain areas dry weights of some 1-2 gm~2 of animal material occur on the bottom at depths of 5000-8000 m. These values are not so much less than mean values in shallow areas, and indicate that supplies of food must be available in deep water in addition to particulate matter sinking from above.

The food relationships of benthic communities are complex and not well known, but enough has been said to emphasize that bacteria and saprophytes are an extremely important source of food for the animal population at all depths. The microbenthos includes many flagellates, amoebae and ciliates which feed mainly on bacteria; some also ingest minute particles of organic debris, and the larger flagellates and amoebae can probably take whole diatoms. For the meiobenthos and macrobenthos, bacteria comprise an appreciable part of the food of many suspension and surface deposit feeders. Animals which obtain food from the sediment presumably derive a substantial amount of their nutriment from the bacteria and protozoa it contains. Some interconnections of the marine food web are illustrated in Figure 6.2.

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