Spatial and temporal patterns in the food web structure of a large floodplain river assessed using stable isotopes

We assessed naturally occurring stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) for available food resources and consumers in the mainstream channel of the Mississippi River. Isotopic ratios were assessed for organic sources and organisms at two different sites during a fall, spring, and two summer seasons. Terrestrial C4 plants did not appear to be an important carbon source for consumers in the mainstream channel. A mixing model, IsoSource, indicated that terrestrial C3 vegetation, suspended algae, and epixylon were at times important food resources for large river consumers. Many consumer signatures fell outside the mixing polygon defined by these sources, indicating that there was a 13C-depleted food resource for which we did not account. We could not distinguish precisely whether downstream allochthonous and autochthonous carbon, or in situ production, was the dominant food resource supporting consumers in these systems. However, our data suggest that in situ organic matter sources can be important. Consumer δ13C and δ15N signatures intermediate between several sources indicated widespread omnivory in the river reaches that we studied. To fully understand food web structure and energy sources in complex large river ecosystems, an integrative approach that combines related empirical data sets is needed.