Biological Factor

The division of environmental factors into chemical, physical, and biological is, of course, arbitrary and only done for convenience. There is bound to be considerable overlap. In so far as the food of marine plants is mainly inorganic salts and carbon dioxide, these have already been dealt with under the section on chemical factors. The possible use of organic compounds by some plants was referred to in the same chapter. We are, therefore, mainly concerned with the food of animals in the present section. There is still doubt about the extent of utilization of solutions of organic compounds by nannoplankton. Although this is of importance in the basic understanding of the food cycle, it is, in a way, less important in understanding the needs of such forms. The existing doubts are largely concerned with the question of whether the cultures used were bacteria-free. Under natural conditions there are always bacteria present, and we know that nannoplankton grow particularly rapidly when organic matter is present, regardless of whether they assimilate it directly or through the intermediate step of bacteria. The same is true of many non-planktonic protozoa and probably of some higher forms too. A large group of marine animals, ranging from the simplest to the highest forms, consists of filter feeders. These remove particulate matter, detritus, or small animals and plants from the water by means of cilia, mucous nets, setae, etc. Our most extensive knowledge of the food requirements of these comes from the copepods.

Although there is evidence of a certain amount of selection of particular food items, in general they filter the water at a uniform rate regardless of its food content. The rate is of course dependent on such factors as temperature. Since the food content of the water does not affect the rate at which the food is passed through the filtering mechanism, when the phytoplankton is very abundant, it will be ingested at a greater rate than it can be digested, and partially digested food will be passed out in the feces. There is thus a limit to the rate at which the food supply can be utilized, but not to the rate at which it can be destroyed. Of course, such destruction is not permanent, unless the feces sink to the bottom and become permanently trapped in the bottom sediments, but it may involve the sinking of the contained nutrients below the photosynthetic zone, so that they will not again be available until such time as water movements bring them to the surface waters. It should be noted that various forms, even those as delicate as planktonic larvae, have been found on occasions to pass alive through the guts of other animals. It is probably true, though, that all but a small fraction of organisms eaten are killed. It is not known how widespread this principle of steady feeding rate is, but it may well prove to be of rather general application. This is in striking contrast to the carnivorous forms which are adapted to ingest a large quantity of food when they can find it and then pass long periods without any. A good example of this is found in the deep sea fish which inhabit very sparsely populated waters, but which, when they obtain prey, can swallow forms larger than themselves.

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