Biomass distribution in marine planktonic communities

Patterns in primary production and carbon export from the cuphotic zone suggest that the relative contribution of planktonic heterotrophs to community biomass should decline along gradients of phytoplankton biomass and primary production. Here, we use an extensive literature data survey to test the hypothesis that the ratio of total heterotrophic (bacteria + protozoa + mesozooplankton) biomass to total autotrophic biomass (H : A ratio) is not constant in marine plankton communities but rather tends to decline with increasing phytoplankton biomass and primary production. Our results show that the plankton of unproductive regions are characterized by very high relative heterotrophic biomasses resulting in inverted biomass pyramids, whereas the plankton of productive areas are characterized by a smaller contribution of heterotrophs to community biomass and a normal biomass pyramid with a broad autotrophic base. Moreover, open-ocean communities support significantly more heterotrophic biomass in the upper layers than do coastal communities for a given autotrophic biomass. These differences in the biomass structure of the community could be explained by the changes in the biomass-specific rates of phytoplankton production that seem to occur from ultraoligotrophic to eutrophic marine regions, but other factors could also generate them. The patterns described suggest a rather systematic shift from consumer control of primary production and phytoplankton biomass in open ocean to resource control in upwelling and coastal areas. One of the earliest tenets in community ecology is the pyramidal distribution of biomass in increasing trophic levels, with a broad autotrophic base supporting increasingly smaller strata of heterotrophs (Elton 1927). Planktonic communities in the ocean, however, are the textbook example of “inverted pyramids” (Odum 1971; Jumars 1993). There is evidence that the biomass of zooplankton (Eppley et al. 1977; Holligan et al. 1984; Alcaraz et al. 1985) and bacteria (Fuhrman et al. 1989; Cho and Azam 1990; Simon et al. 1992) can exceed or at least be equivalent (Li et al. 1992; Buck et al. 1996) to that of phytoplankton in the ocean. The high biomass of bacteria and zooplankton, together with the substantial biomass of heterotrophic protists (e.g. Sorokin 1977), form an inverted pyramid in which the compounded biomass of heterotrophs exceeds that of autotrophs. The question that is yet unresolved is the generality of the phenomenon of inverted biomass pyramids in marine plankton and whether it can be extended to all marine pelagic communities. The existence of communities where heterotrophic biomass exceeds autotrophic biomass has been explained by the high turnover rate of the autotrophic pool (Odum 197 1;