Integrating the Microbial Loop and the Classic Food Chain Into a Realistic Planktonic Food Web

The objectives of this paper are (1) to summarize recent findings about feeding relationships among the microbial components of the freshwater planktonic food web; (2) to make the microbial components easily understood through simplified, yet valid, trophic groupings and the application of standard food web terminology; and (3) to show that the microbial components are not part of a distinct microbial loop but are inextricably integrated, by omnivory and mixotrophy, with the classical algal-based food chain. New concepts presented are that (1) bacteria and their consumer can exert both direct and indirect effects as supplemental food resources, competitors, and predators, at all levels of the classic planktonic food chain; therefore (2) consumers of bacterial production can exert top-down and lateral as well as bottom-up control; and (3) seasonal shifts in the dominance of autotrophic and heterotrophic production determine the relative strength of the microbial dominance of the planktonic food web. Microbial components of the planktonic food web will be enhanced in (1) later stages of seasonal succession following intense algal blooms that go ungrazed; (2) in pulsed systems with frequent algal blooms, and (3) in systems where input of allochthonous organic matter from the watershed, littoral zone, and benthos enhances heterotrophy.