Despite their importance, little information is available on the ecology of protists and bacteria in freshwater wetland ecosystems. Microbial communities were compared monthly at eight sites in a lotic wetland within the coastal plain region of the southeastern United States, from upstream, through an alder swamp, reed marsh, and water lily pond, to a downstream site. Protist abundances and biomass, and bacterial abundances, biomass, and productivity were all generally greater in the wetland habitats compared to up stream and downstream sites, especially in summer. Among wetland habitats, planktonic microbial populations were most productive in the reed marsh and among submerged aquatic plants, and lowest in the alder swamp. The planktonic microbial communities were primarily heterotrophic. Dissolved organic carbon (DOC) derived from aquatic plant community production during summer in the wetland pond stimulated planktonic bacterial production, which served as a primary food source for protozoa. The wetland habitats affected the lotic system as a whole by substantially increasing the amount of microbial biomass transported downstream.

In the wetland pond, DOC concentrations, bacterial abundance and productivity, and protist abundance were more than an order of magnitude higher during warm months than cool months. Out-of-phase oscillations between microbial populations suggested protist grazing pressure strongly affected bacterial abundance during the warm months. Experiments using natural microbial communities showed similar uptake rates of macrophyte leachate by both grazed and ungrazed bacteria. However, in the plankton, grazing of bacteria by nanoflagellates resulted in greatly increased rates of carbon mineralization to carbon dioxide (CO2) rather than making this carbon available to other trophic levels.

OPHRYDIUM VERSATILE, a mixotrophic, colonial ciliate was studied to determine seasonal changes in its distribution, primary productivity, and rates of bacterivory. Summer rates of primary production and bacterial consumption were higher than any other season on the basis of colony surface area. However, high bacterial productivity and limited ciliate distribution diminished their importance to the pond ecosystem in summer. During the winter, these ciliates functioned primarily as bacterivores, and low planktonic bacterial productivity combined with a wide distribution of large ciliate colonies made O. VERSATILE capable of clearing up to one-third of the water column of bacterial production daily.