Food web dynamics of fishes in relation to hydrological connectivity gradients in the Upper Mississippi River

Structurally complex rivers consist of the main channel and various slackwater habitats which differ in hydrological connectivity within the riverscape. Differences in current velocity and other factors alter abiotic and biotic conditions along lateral gradients from main channel to backwater habitats. Fish communities using habitat patches along lateral gradients also change in response to behavioral and life history adaptations and dynamic habitat conditions. My study focused on the importance of lateral connectivity gradients in structuring aquatic food webs in the Upper Mississippi River during spring and summer sampling periods. I used stable isotope and fish gut content analyses to determine patterns of trophic position in four feeding guilds of fish. Over 1000 tissue samples were taken in four lateral sites (main channel, secondary channels, and tertiary channels, and backwaters) in each of two reaches of two river "reaches" during June and August of 2004. Trophic position differed significantly among feeding guilds and across lateral gradients of connectivity. Food chain length was similar for all feeding guilds across the lateral connectivity gradient during the higher flow spring sampling period. However, during the lower flow summer period variability in food chain length for piscivores, invertivores, and omnivore/detritivores increased in similar habitat patches within the connectivity gradient, particularly in backwater habitats. Backwater habitats also exhibited significantly lower food chain length compared to tertiary and main channel habitats during this season. Temporal changes in current velocity occurring from the flow pulse seem important in structuring food webs along lateral connectivity gradients. While variation in trophic position was not different between seasons, variability in trophic position among fishes in feeding guilds was greatest in habitats with intermediate connectivity. Because of increased habitat heterogeneity, patches with intermediate connectivity may allow a greater diversity of competitors to feed on a variety of trophic levels.