Introduction

Heterotrophic microorganisms, here defined as bacteria and protozoa, account for a major portion of secondary production and nutrient remineralization in aquatic ecosystems (Stockner & Porter, 1988; Sherr & Sherr, 1991). These organisms are potentially regulated by both their predators and by the supply of organic carbon and inorganic nutrients. In general, prior studies of the trophic cascade have not considered heterotrophic microbial processes (e.g. Carpenter et al., 1987; McQueen et al., 1989; Benndorf, 1990; but see Riemann & Søndergaard, 1986). In previous chapters of this volume, the significance of cascading trophic interactions in determining the structure of zooplankton communities and in explaining shifts in phytoplankton biomass and primary production has been demonstrated. In this chapter, I consider the question, are heterotrophic microbes and the biogeochemical processes mediated by them strongly influenced by these same top-down controls?

One difficulty in answering this question arises from uncertainty about the trophic interactions and recycling processes connecting heterotrophic microbes with phytoplankton, zooplankton and fish. It is an oversimplification to consider the heterotrophic microbial food web as a separate ‘loop’ wherein bacteria utilize dissolved organic matter and are consumed by protozoa, with energy and nutrients there by returned to the phytoplankton–zooplankton–fish food chain (Azam et al., 1983). Rather, a more current view of microbial food webs is one of a complex interacting community including phytoplankton, bacteria and protozoans that collectively accounts for primary carbon fixation, nutrient regeneration and production to support metazoans (Sherr & Sherr, 1991).