Nektonic Invertebrate Dynamics and Prolonged Summer Flooding on the Lower Illinois River

Abstract

ABSTRACT Prolonged extreme flooding during mid-summer 1993 provided an opportunity to investigate nektonic invertebrate dynamics in lower Illinois River floodplains and backwater lakes. We used plankton nets to sample flooded grass shorelines, flooded forests, and open water habitats during rising and falling stages of the flood. Transects oriented perpendicular to shore were sampled to investigate community composition along the floodplain gradient extending riverward. Invertebrate densities differed between samples collected on the rising stage of the flood (mean = 11,584 individuals m−3) and on the falling stage of the flood (mean = 78 individuals m−3). Density estimates from samples collected at the shoreline of the rising flood waters exceeded 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 rising edge of the flood. Flooded trees had the next highest densities, followed by floating macrophytes and open water. Our findings exemplify the flood pulse hypothesis in that productivity, as measured by invertebrate density, increased dramatically on the rising flood but then fell just as dramatically on the falling flood.