Cooperative Fish and Wildlife Research Units Program: South Carolina
Education, Research and Technical Assistance for Managing Our Natural Resources

Hydrology, lateral connectivity and stable isotope ratios of fishes and mussels in the Guadalupe River, Texas, were analyzed for evidence of spatial food web subsidies between the active channel and oxbow lakes in the floodplain. During and following lateral connections, aquatic organisms residing in the channel may assimilate material from sources imported from oxbows, and oxbow residents may consume and assimilate material imported from the channel. During surveys conducted between March 2016 and April 2017, fish, mussel, algae, and riparian plants samples were collected in and around two oxbows and adjacent channel sites for analysis of stable isotope ratios. Biplots of d¹³C and d¹⁵N were graphed for basal sources and specimens of Longnose Gar Lepisosteus osseus, Gizzard Shad Dorosoma cepedianium, Threadfin Shad Dorosoma petenense, Bullhead Minnow Pimephales vigilax, Smallmouth Buffalo Ictiobus bubalus, White Crappie Pomoxis annularis, sunfishes (Lepomis species combined), and two mussel species (combined) captured from oxbows and the channel. Within each graph, a source polygon was drawn to indicate the space occupied by animals that could have assimilated feasible combinations of source materials from the oxbows or river channel. Based on positions of animals within source polygons, riparian plants were the most important source of organic matter supporting biomass of fishes and mussels both within the channel and oxbows during every survey period, with algae contributing minor percentages. Most organisms had isotopic signatures consistent with assimilation of in situ sources, or else were inconclusive with regard to cross-habitat exchanges. Between-habitat food web subsidies were indicated for 7.6% of 314 organisms from oxbows and 11.2% of 231 from the channel, and these cases followed high flow pulses that connected oxbows for extended periods. Several issues that compromise inferences from stable isotope analysis were identified, and attempts to estimate spatial food web subsidies in fluvial systems could be enhanced by analyzing additional biomarkers, such as isotopic ratios of other elements and compound-specific stable isotopes, as well as additional sources, time-specific biotracers, and experimental approaches that directly track movement of sources and organisms in spatially structured food webs.