Interactions Between Planktonic Community Structure and Nutrient Cycling Processes in Freshwater Ecosystems

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This project will continue an investigation on the major interactions between planktonic community structure and nutrient cycling in aquatic ecosystems. Because community composition is tightly coupled with nutrient dynamics in a number of complex cause-effect relationships, the relative importance of different interactions in any particular habitat can be difficult to assess. For example, phosphorus fertilization, water column N:P and planktivorous fish abundance may each independently influence (directly or indirectly) phytoplankton and zooplankton abundance and species composition. The latter variables may, in turn, influence nutrient cycling rates and routes (either directly through water-column effects, or indirectly through sedimentation and benthic regeneration). Three major variables can be experimentally manipulated to identify circumstances under which particular processes are important: the abundance of zooplanktivorous fish, the ratio of nitrogen to phosphorus available for uptake by phytoplankton, and the absolute amount of phosphorus added to the environment. Using a set of 16 large experimental ponds, researchers will manipulate two variables at a time with 4 replicate ponds per treatment. At present, N:P (high vs low) and fish (presence vs. absence) are being manipulated. This increment will be a further 1-year experiment to manipulate the third variable, P- loading (high vs. low) again + fish. Research will contribute to basic ecological concepts and will provide data useful in lake management.