Nutrient controls on phytoplankton composition and ecological function among hydrologically distinct habitats in the Upper Mississippi River

The Upper Mississippi River (UMR) is a productive floodplain river with a complex lateral habitat array. Three habitats (main channel, flow-through backwater (moderately connected), and single-connection backwater (least connected)) in a typical UMR reach (Pool 8 near La Crosse, WI) representing a connectivity gradient (distance to main channel) were studied. Differential responses by species of phytoplankton to diverse habitat conditions promoted compositional differences among contrasting habitats. Increases in cyanobacterial blooms have raised awareness that nutrient factors may be important to UMR phytoplankton dynamics. Four main goals of my research were to identify: (1) present (1999-2004) spatiotemporal patterns in phytoplankton species composition and related nutrient regimes in UMR Pool 8, (2) which nutrients control algal production across habitat and phytoplankton assemblage type, (3) the effect of nutrient enrichment on phytoplankton composition, and (4) how nutrient enrichment affects the ecological function (N2-fixation) of bloom-forming cyanobacteria. Nutrient addition bioassays were conducted in situ to test effects of nutrient enrichment on phytoplankton. Multivariate analyses aided in the identification of multi-species differences over space and time. Results indicated that seasonal change and to a lesser extent habitat differences affected phytoplankton species composition and that phosphorus (P) was the nutrient that best explained compositional patterns. Bioassay experiments indicated that net phytoplankton production was often N-limited or co-limited by N and P but that P alone was important in the single-connection backwater. The flow-through backwater was limited most frequently and had the greatest degree of limitation overall. In general, the strongest responses to nutrient enrichment by UMR phytoplankton occurred in response to NP-enrichment and resulted in greater species diversity. Often the addition of N alone or in combination with P resulted in lower cell densities and/or relative abundance of N2-fixing cyanobacteria, while non-N2-fixing cyanobacteria and chlorophytes responded positively to N and NP enrichment. Iron was found to be a factor determining cyanobacterial species composition. N2-fixation was found to supplement cyanobacterial N-requirements in single-connection and main channel habitats and was controlled by P supply. Management programs are going to need to account for the complex spatiotemporal dynamics between phytoplankton and nutrient supply identified in this study