Seasonal patterns of bacterivory by flagellates, ciliates, rotifers, and cladocerans in a freshwater planktonic community

Bacterivory in eutrophic Lake Oglethorpe, Georgia, was determined by direct observation of tracer particle uptake by all members of the planktonic community. Heterotrophic flagellates dominated grazing at all times, accounting for 49-8 1% of grazing on an areal basis and up to 98% of grazing at some depths. Pigmented (mixotrophic) flagellates were major grazers during winter and spring blooms, when they contributed up to 45% of community grazing on an areal basis and 79% at depths of maximum abundance. In late spring to early summer, rotifers and ciliates were responsible for as much as 25 and 30% of bacterivory at some depths, but averaged 3 and 11% over the year, respectively. Grazing impact of cladoceran crustaceans was generally < 1% of the total. Bacterivory by copepods was not detected. Total bacterial mortality due to grazing ranged from 11 to 162% of bacterial cell production estimated from the incorporation of [3H]thymidine. Use of 0.57-pm microspheres as tracers gave similar estimates ofingestion to fluorescently labeled bacteria in this system. The use of 4% ice-cold glutaraldehyde mixed 1 : 1 with the water sample was found to be equally effective to an acrolein-tannic acid mixture for reducing egestion of particles by protists. We therefore consider our findings to be representative of bacterivory by the planktonic community in this eutrophic system. The microbial loop is now recognized as a dynamic component of pelagic marine food webs (Pomeroy 1974; Azam et al. 1983; E. Sherr et al. 1986). In it, heterotrophic and autotrophic picoplankton and their protistan grazers recycle particulate and soluble nutrients released by the classical pelagic food chain of algae, crustacean zooplankton, and fish. In lakes, 50-100% of phytoplankton carbon production may pass through bacterioplankton (Simon 1987) and bacteria may constitute > 50% of the combined picaand nanoplankton biomass (CaI Present address: Philadelphia Academy of Natural Sciences, Division of Environmental Research, Philadelphia, Pennsylvania 19 103. 2 Present address: Department of Biology, University of North Carolina, Chapel Hill 275 14. 3 Present address: Savannah River Ecology Laboratory, Drawer E, Aiken, South Carolina 29802. Acknowledgments This study was supported by NSF grant BSR 84-