Life in the fast lane: fish and foodweb structure in the main channel of large rivers

We studied the main channel of the lower Illinois River and of the Mississi ppi River just upstream and downstream of its confluence with the Illinois River to describe the abundance, composition, and/or seasonal appearance of components of the main-channel community. Abundance of fishes in the main channel was high, especially adults. Most adult fishes were present in the main channel for either 3 or 4 seasons/y, indicating that fishes regularly reside in the main channel. We documented abundant zooplankton and benthic invertebrates in the main channel, and the presence of these food types in the diets of channel catfish and freshwater drum. All trophic levels were w ell represented in the main channel, indicating that the main channel suppo rts a unique food web. The main channel also serves as an important energet ic link with other riverine habitats (e.g., floodplains, secondary channels , backwater lakes) because of the mobility of resident fishes and because o f the varied energy sources supplying this food web. It may be more realist ic to view energy flow in large-river systems as a combination of 3 existin g concepts, the river continuum concept (downstream transport), the flood p ulse concept (lateral transport to the floodplain), and the riverine produc tivity model (autochthonous production). We urge additional research to qua ntify the links between the main channel and other habitat types in large r ivers because of the apparent importance of main-channel processes in the o verall structure and function of large-river ecosystems.