The Ocean's Food Web, A Changing Paradigm

Few of us may ever live on the sea or under it, but all of us are making increasing use of it either as a source of food and other materials, or as a dump. As our demands upon the ocean in- crease, so does our need to understand the ocean as an ecosystem. Basic to the understanding of any ecosystem is knowledge of its food web, through which energy and materials flow. Flux of both energy and essential elements shapes or limits ecosystems, but only energy and organic compounds are con- sidered here. The related problems of limiting supplies of essential elements (N, P, Si, Fe) will be considered else- where (Pomeroy'). Although the ocean's food web has been studied for more than a century, several recent discoveries lead us to believe that the classical textbook de- scription of a chain from diatoms through copepods and krill to fishes and whales may in fact be only a small part of the flow of energy. Recent studies of microorganisms, dissolved organic mat- ter, and nonliving organic particles in the sea suggest the presence of other pathways through which a major part of the available energy may be flowing. Marine scientists have been approaching this view of the food web cautiously for decades, and caution is to be expected whenever an established paradigm is questioned (Kuhn 1962). Now there are many lines of evidence which suggest that a new paradigm of the ocean's food web is indeed emerging. THE ROLES OF MICROORGANISMS ocean, with 90% of the total area and the lowest mean rate of photosynthesis, accounts for 81.5% of primary produc- tion. Coastal waters over the continental shelves, with 9.9% of the total area and twice the rate of photosynthesis of the open ocean, account for 18% of primary production. The major upwellings, with 0.1% of the total area and a rate of photosynthesis nearly 10 times that of the open ocean, account for 0.5% of primary production. The real accuracy of these estimates is not as good as the three digit numbers suggest. The usual methods of estimating the photo- synthetic rate of phytoplankton do not measure all organic matter produced, and there is rarely enough replication to give us confidence limits for the values. The most obvious plants in the sea are the seaweeds, but they probably are not the most significant primary pro- ducers. Ryther (1963) estimates that seaweeds account for 10% of the pri- mary production of the ocean. Recent data on photosynthetic rates of kelp and other large seaweeds (Mann 1973) make that estimate seem high, although it is still difficult to make a good estimate on a planetary scale. Certainly, seaweeds and sea grasses are of major importance in the coastal zone. On a planetary scale phytoplankton are the major producers. Phytoplankton have been divided into two size groups, net plankton and nannoplankton. The separation is arbitrary, based on the aperture of what was once the finest bolting cloth for making plankton nets. Net plankton (>60 pm) have received considerable attention and form the basis of the established paradigm of the food web, but nannoplankton (<60 pm) have proven more difficult to study and until recently they have been neglected. Like net plankton, they are so sparsely distributed in the water that it is neces- sary to concentrate them in some fashion to see them at all. Because many of them cannot be preserved well, they must be studied at sea while alive. Only recently has high magnification, oil- immersion microscopy at sea permitted the study of living nannoplankton aboard ships. Efforts to understand the relative importance of net and nannoplankton have been made by several investigators, using the 14C technique (Steeman- Nielsen 1952), by counting separately the radioactivity retained on discs of bolting cloth and on fine membrane filters. This was a crude separation. Some organisms larger than the aperture of the bolting cloth would be forced through it in fragments, and if a suf- ficiently thick layer of plankton ac- cumulated on the bolting cloth, nanno- plankton would be retained by it. In spite of these shortcomings, at least a dozen studies produced consistent re- sults showing that the large diatoms and other net plankton, although highly visible and beautiful, account for a small fraction of total primary production. In a majority of cases nannoplankton ac- count for more than 90% of total photosynthesis (Table 1). This is true not only in the central gyres of the ocean, but in upwellings, coastal waters, and estuaries. No one seems to have made the separation in polar regions. However, Digby (1953) found that in Scoresby Sound, Greenland, nanno- plankton chlorophyll was always equal