We examined potential relationships between hydrology, lateral connectivity, and food-web dynamics in the lower Guadalupe River and oxbow lakes within its floodplain. Ratios of stable isotopes of carbon and nitrogen were analyzed to infer the potential for alternative basal production sources to support fish and mussel biomass in oxbow lakes and sites in the river channel under contrasting flow conditions. Of particular interest was evidence that fishes and mussels in the river channel had assimilated material from basal sources from oxbows, and that fishes and mussels in oxbows had assimilated material originating from the river channel. Fishes and mussels were collected from two oxbow lakes and adjacent sites in the river channel during six surveys conducted between March 2016 and April 2017. Samples of riparian vegetation, seston, and periphyton also were collected for stable isotope analysis. The discharge level associated with establishment of lateral connectivity was estimated for each oxbow.
Results/Conclusions
Turbidity was high throughout the study period, which was a relatively wet year, and aquatic primary production apparently was low. Stable isotope analysis indicated that riparian vegetation was the most important basal production source supporting biomass of most fishes and mussels in both oxbows and the river channel during every survey. Isospace polygons indicated that periphyton and seston could have contributed only minor proportions to consumer biomass at most sites during most periods. Isotopic evidence of cross-habitat exchange of basal sources or consumers was found for 7% of 316 specimens from oxbows and 11% of 231 specimens from the river channel. Most of the specimens that were analyzed had isotopic signatures that were either consistent with assimilation of material from the habitat from which they were captured, or inconclusive with regard to cross-habitat exchanges. Estimates of discharge that induce lateral connectivity at Oxbow 1 indicate that cross-habitat exchange of resources and aquatic organisms would occur for the following flow pulse categories within the state’s current environmental flow standards for the Guadalupe River at Cuero: 2-per-season pulse in spring, summer and fall, and 1-per-season pulse in all four seasons. Given the current standards for the Guadalupe River at Victoria, lateral connections at Oxbow 2 could be achieved by 2-per-season pulses only during spring and fall, and 1-per-season pulses in all four seasons.