Nektonic invertebrate dynamics and prolonged summer flooding on the lower Illinois River

Prolonged extreme flooding during mid-summer 1993 provided an opportunity t o investigate nektonic invertebrate dynamics in lower Illinois River floodp lains and backwater lakes. We used plankton nets to sample flooded grass sh orelines, flooded forests, and open water habitats during rising and fallin g stages of the flood. Transects oriented perpendicular to shore were sampl ed to investigate community composition along the floodplain gradient exten ding riverward. Invertebrate densities differed between samples collected o n the rising stage of the flood (mean = 11,584 individuals m(-3)) and on th e falling stage of the flood (mean = 78 individuals m(-3)). Density estimat es from samples collected at the shoreline of the rising flood waters excee ded estimates from open water and the falling flood shoreline by two orders of magnitude. Corixids were the most abundant taxa found (78%) at flooded shorelines. Densities were highest in inundated grass habitats at the risin g edge of the flood. Flooded trees had the next highest densities, followed by floating macrophytes and open water. Our findings exemplify the flood p ulse hypothesis in that productivity, as measured by invertebrate density, increased dramatically on the rising flood but then fell just as dramatical ly on the falling flood.