Item

Abstract

The microbial loop, now widely accepted as an important component of aquatic food webs, includes bacteria, cyanobacteria, and a variety of amoeboid, flagellated and ciliated protists. Most metazoan zooplankton are relatively inefficient at capturing bacteria-size particles, but the sizes of the planktonic protozoa cover a range such that nearly any metazoan zooplankter is able to feed on some members of this diverse group. Thus protozoa serve as a necessary trophic link in the transfer of bacterial biomass to some larger organisms. We review data indicating that many marine and freshwater zooplankton species, either throughout their lifetime or during early life history stages, may include protozoa as an important part of their diets. Numerous metazoans grow and/or reproduce when fed protozoa. Not all protozoa are nutritionally adequate food sources, however, and the "food quality" of a protozoan species may vary for different consumers. Nevertheless, protozoa do contribute to the nutrition of zooplankton, and metazoan predation can alter the abundance and species composition of protozoa. Conversely, some protozoa are able to prey on metazoans, although it is probably qualitatively insignificant to metazoan populations in most situations.