A 100-year perspective of trophic dynamics in a floodplain river

Floodplain rivers are among the most extensively modified and managed ecosystems in the world.  There has, however, been increasing interest over the past decade to restore some of their natural hydrogeomorphic features.  Unfortunately, there is a paucity of ecological data available for periods prior to modification from which to establish targets for rehabilitation.  We used fish, mussels, and snails from museum collections to obtain carbon and nitrogen stable isotope ratios to examine changes in trophic position of fish and changes in trophic structure of the community in several rivers over the past century.  Discussion will focus on the Upper Mississippi River.  Trophic position of most fish feeding guilds declined sharply after 1940, which followed completion of the lock and dam system, through 1965 before increasing again.  The exception to this was piscivores, which exhibited only limited fluctuation 1950 - 2000.  Measures of community trophic structure indicated that niche space increased over the 100+ yr period of the study while species packing decreased.  We propose that trophic omnivory allowed piscivores to maintain their trophic position.  Most changes within the food web occurred at lower levels and were likely caused by changes through introduction of new hydrogeomorphic patches within the riverine landscape.