Monday, June 4, 2007 - 3:45 PM
51

Spatial and Temporal Patterns in the Isotopic Composition of Phytoplankton in a Large Floodplain River

Michael D. Delong, Large River Studies Center, Winona State University, Biology Department, Winona, MO 55987 and James H. Thorp, PhD, Ecology and Evolutionary Biology, University of Kansas, 2101 Constant Avenue, Lawrence, KS 66047-3759.

Food web studies have generated increasing evidence that phytoplankton and benthic microalgae are principle drivers of energy flow in large rivers. Questions still remain, however, concerning the temporal variability of autotrophs in complex systems and the possible implications to food web dynamics on an annual basis. This study examined changes in phytoplankton in different habitats of the Upper Mississippi River through analysis of carbon and nitrogen stable isotope ratios from April – September 2004. Rather than follow expected climatic-based shifts, temporal changes in both the carbon and nitrogen isotope ratios responded to more strongly to hydrological conditions in main channel, secondary channel, and slackwater habitats. Nitrogen isotope ratios of phytoplankton from backwaters did exhibit a temporal shift, but changes more closely resembled a response to seasonal conditions.  Carbon isotopic signatures remained unchanged over the course of the study period.  The results suggest that hydrological processes, specifically flow pulses, influence the forms of inorganic carbon and nitrogen delivered to highly connected riverine habitats, whereas, nutrient depletion may be important in habitats, such as backwaters, that are only susceptible to large-scale connectivity with riverine habitats during flood pulses.  Preliminary results from an ongoing study support this hypothesis.

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