Ref ID : 1651
Steven A. Schoenberg; Short-term productivity responses of algae and bacteria to zooplankton grazing in two freshwater lakes. Freshwater Biology 23:395-410, 1990
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1. The specific productivities of algae and bacteria were measured in short-term (4 day) experiments consisting of enclosures with natural or reduced zooplankton biomass. Experiments were repeated five times over a season in each of two lakes that differed in the background concentration of dissolved organic carbon (DOC). 2. Algal biomass as estimated by chlorophyll-a was suppressed in enclosures with ambient grazer levels in six of ten experiments and enhanced in one experiment. Distribution of chlorophyll among net and nanoplankton was not significantly affected by grazing. 3. Relative to enclosures with reduced zooplankton, normal grazer biomass (97-466 µg l-1 dry weight) enhanced specific algal productivity in only one of five experiments in the low DOC lake and had no effect in all five experiments in the high DOC lake. The main effects of grazers on algae was through removal of biomass rather than through indirect changes in turnover rate. 4. Between experiments, bacterial density was either unaffected, or mildly enhanced (4-87%) in enclosures with ambient macrozooplankton compared to those with reduced levels. Bacterial productivity and turnover estimated by incorporation of [3H]thymidine into DNA showed different responses across experiments; increasing, declining or remaining the same with grazer manipulation. This variability was not related to differences in dissolved primary production or to background DOC between lakes or experiments. Comparison of bacterial productivities based on thymidine incorporation rates with changes in cell densities indicated that control of bacterial loss processes by macrozooplankton is more important than control of growth rates.