Relation between biomass and body weight of plankton in a steady state oceanic ecosystem l

In the size range from lO-4 to 10’ pg (carbon) body weight, the biomass of plankton in the euphotic layer of the North Pacific Central Gyre decreases as an allometric function of body weight. Even in a steady state ecosystem such as that analyzed here, there is variability in space and time; this suggests that one must be careful in extrapolating the relation to less predictable marine areas. In obtaining dynamic information from biomass spectra, one must distinguish changes due to the flow of energy within the spectrum (growth, predation, reproduction) from changes due to emigration from or immigration into the spectrum of the particular area sampled, such as those due to the diel vertical migration of macrozooplankton in the largest size classes. As a basis for a “particle-size” approach to the description of pelagic ecosystems, it is important to establish whether there is a simple relation between the biomass (material per unit environmental volume or area) of organisms in any size category and the individual body size of those organisms. Although this approach has been applied mainly in aquatic ecology (in part because of the tendency for pelagic predators to swallow their prey whole but to be otherwise rather nonselective), Platt (1985) traced its origin to Elton (1927) and analyzed the reasons for the long hiatus until the 197Os, when this dormant approach began to develop with Odum’s (197 1, fig. 3.6) comparison between aquatic and terrestrial size distributions of biomass. The accumulated empirical evidence obtained from marine and freshwater ecosystems has differed greatly in both amount and quality. Sheldon et al. (1972) carried out an extensive oceanic survey which, when combined with data from the literature, revealed an apparently general feature of the pelagic ecosystem, namely, “The tendency for roughly similar amounts of particulate material to occur in logarithmically equal 1 Research was supported by CAICYT Project No. 1185181 and by a Fulbright grant to J.R., Department of Energy grant DE-AT03-82-ER60031 and various NSF and ERDA grants under which samples were collected. 2 Present address: Department0 de Ecologia, Facultad de Ciencias, Universidad de Malaga, Spain. size ranges . . .” (Sheldon et al. 1972, p. 336). The data presented by them (fig. 12) suggest a slight decrease in biomass with increasing individual size, but the basic data almost certainly included detrital as well as living particles in the l-100~pm range, and they hypothesized that the spectrum for living organisms was flat (0 slope). The underlying idea was extensively used as the basis for three types of study: the theoretical analysis of ecosystem structure (Kerr 1974); the estimation of standing stocks of fishes in the Gulf of Maine, the North Sea, and the Peruvian upwelling (Sheldon et al. 1977); and as the marine point of reference for much more solid empirical analyses of freshwater ecosystems (Sprules et al. 1983). However, theoretical modeling of particle-size spectra has reached a high level of sophistication in biological oceanography. In contrast to Kerr (1974), whose analysis considered the energy flows between discrete trophic levels, Platt and Denman (1977, 1978) based their modeling process on a continuous flow completely independent of the trophic level concept; at the same time, they discarded the mode of data presentation of Sheldon et al. (1972). Platt and Denman approached the problem by analyzing the flow of energy or material through the pelagic ecosystem from the smallest particles to those of larger sizes; the turnover of material within each size class was assumed to be controlled by the reproductive and respiratory rates of organisms with the nominal weight which typified that size class.