Cascading Trophic Interactions in Lake Ecosystems: Effects on Bacteria and Their Consumers

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Heterotrophic microorganisms process a substantial portion of the primary production of aquatic ecosystems. This research will examine how food web interactions control microbial productivity. Whole lake experiments are being conducted at the University of Notre Dame Environmental Research Center that manipulate fish populations that in turn modify the planktonic food web. This project will study the response of the heterotrophic microbial community. The effect of planned manipulations will be to decrease zooplankton biomass and increase phytoplankton biomass and productivity. It is hypothesized that bacteria, protozoa, bacterial production, and grazing on bacteria will all increase in response to these manipulations. The enhanced microbial activity in the manipulated lakes will also lead to higher rates of total water column respiration. Measurements of the biomass, productivity, grazing by heterotrophic microbes as well as community respiration will be made to test these hypotheses at the ecosystem scale. The direct and indirect effects of the food web on heterotrophic microbes will be further examined in enclosure experiments and modeling studies. Results from this study will likely be of major significance in terms of the basic understanding of the biotic dynamics of lakes and may be applicable to new strategies for lake management.