Trophic niche dimensions of fish communities as a function of historical hydrological conditions in a Plains river
Delong, Michael D.; Thorp, James H.; Thoms, Martin C.; McIntosh, Leah M.
The interplay of hydrology, geomorphology, and ecology within an aquatic-terrestrial landscape makes floodplain rivers among the world's most complex ecosystems. Their spatial and temporal variability create diverse communities with complex trophic interactions. Floodplain-rivers are, unfortunately, also among those environments most heavily impacted by human activities. Analysis of the extent of ecological change in these rivers as a direct result of altered hydrological and geomorphological conditions is typically limited to short-term, post-impact studies or comparisons between "similar" disturbed and undisturbed rivers. We used museum specimens of fish to develop a +70-yr timeline of carbon and nitrogen stable isotope ratios to quantify the response of the trophic organization of fish communities to changes in hydrological character below the last of the dams impounding the Missouri River, USA. Isotopic ratios of fish representative of four feeding guilds were used to calculate community measures of trophic structure. A series of hydrological indices, calculated from historical discharge data, were used to determine if alteration of hydrological conditions have influenced the trophic structure within the fish community. Data were examined using 5-yr intervals over the period for which museum specimens were available (1930-1989), with dam operation beginning 1950-1954. Hydrological indices for seasonal amplitude and seasonal periodicity exhibited the greatest change from the pre-dam to post-dam periods. This resulted from reduced peak flows and higher seasonal low flows, coupled with reversal of the annual high- and low-flow periods. Total community niche space and distance between niches occupied by individual species within the community niche space declined following commencement of dam operations, indicating a decrease in the range of basal resources driving the food web and shortening of the food chain. Species packing, however, did not change, suggesting that competition for resources did not increase. We propose that changes in the hydrological pattern resulting from dam operation reduced connectivity between hydrogeomorphic patches within the riverine landscape, thereby limiting access to potentially higher quality resource patches. Additionally, the shift in time of year of annual peak flow potentially altered availability and quality of autochthonous and allochthonous resources during a time when biological productivity should be greatest.