Figure 16.6 Seven ecosystem processes in the open ocean that depend on biodiversity rather than physical processes. Arrows indicate levels of interactions between processes. Nutrients are returned from the benthos to the epipelagic ecosystem by upwelling currents (i.e. physical processes)

of inorganic nutrients (Lenz 1992). The heterotrophic microbial loop in some ocean systems may account for over 70% of the total carbon and nitrogen in the euphotic zone (Fuhrman and Capone 1991). It may be appropriate to regard trophic relationships among macroscopic organisms (Lenz's "classical food web") as a phenomenon that flourishes only under exceptional circumstances, as dictated by local surfeits in food, or the concentrating effects of wind and current. The microbial loop is thought to be limited by grazing (Ducklow 1992). It is important to note that in pelagic systems, viral predation is an important component of grazing on the microbial biota. Host - parasite dynamics between bacteria and viruses is a new research field. Because of the large numbers of parasitic viruses found in open occans (Bergh et al. 1989; Proctor and Fuhrman 1990), diversity of bacterial lineages may be important in stabilizing the microbial loop.

Ducklow (1992) has argued that functional variation among ocean habitats may be related to both bottom-up (i.e. differences in availability of organic carbon, under the influence of a host of environmental factors) and top-down (predation and grazing pressure) effects. However variation at smaller scales, i.e. within habitats, will almost invariably be due to top-down effects.

Chemical pollution could have a serious effect on microbial diversity by differentially impacting specific species. Circumstantial evidence suggests that microbial processes such as degradation and the use of specific nutrients are most efficiently performed by specific microbes. Although the specific mechanics are not known, changes in the abundance of these organisms could alter the microbial loop.

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