Experimental Evaluation of Microbial Biomass and Production in Limnetic Food Webs Dominated by Planktivorous Fish

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This research will determine how microbial biomass and production (bacteria, algae less than 5 um in diameter, and protozoa) respond to changes in the remainder of the limnetic food web brought about by planktivorous fish. Experiments will be conducted in 18 large outdoor tanks to compare the effects of three planktivorous fish (Dorosoma cepedianum, Tilapia aurea and Menidia beryllina) on microbial and other components of the limnetic food web. Because these fish have different selectivities on zooplankton and phytoplankton, we expect significant indirect responses of microbial biomass and production, and in the transfer of microbial production to other food web components. Experiments will be conducted in different seasons to compare fish effects on plankton communities representative of different weather regimes. During each tank experiment, grazing rates of protozoa on bacteria and algae, and grazing rates of metazoan zooplankton on bacteria, algae, and protozoawill be determined by measuring ingestion of radioisotope-labelled bacteria and algae by protozoa and metazoan plankton. The grazing impact of planktivorous fish on protozoa will be measured in short-term feeding trials associated with each tank experiment. These experiments provide an integrated experimental approach to the determination of the role of the microbial loop in planktonic food webs.