Evidence supporting the importance of terrestrial carbon in a large-river food web.

Algal carbon has been increasingly recognized as the primary carbon source supporting large-river food webs; however, many of the studies that support this contention have focused on lotic main channels during low-flow periods. The flow variability and habitat-heterogeneity characteristic of these systems has the potential to significantly influence food web structure and must be integrated into models of large-river webs. We used stable-isotope analysis and IsoSource software to model terrestrial and algal sources of organic carbon supporting consumer taxa in the main channel and oxbow lakes of the Brazos River, Texas, USA, during a period of frequent hydrologic connectivity between these habitat types. Standardized sampling was conducted monthly to collect production sources and consumer species used in isotopic analysis. Predictability of hydrologic connections between habitat types was based on the previous 30 years of flow data. IsoSource mixing models identified terrestrial C3 macrophytes (riparian origin) as the primary carbon source supporting virtually all consumers in the main channel and most consumers in oxbow lakes. Small-bodied consumers (<100 mm) in oxbow lakes assimilated large fractions of algal carbon whereas this pattern was not apparent in the main channel. Estimates of detritivore trophic positions based on delta15N values indicated that terrestrial material was likely assimilated via invertebrates rather than directly from detritus. High flows in the river channel influenced algal standing stock, and differences in the importance of terrestrial and algal production sources among consumers in channel vs. oxbow habitats were associated with patterns of flooding. The importance of terrestrial material contradicts the findings of recent studies of large-river food webs that have emphasized the importance of algal carbon and indicates that there can be significant spatial, temporal, and taxonomic variation in carbon sources supporting consumers in large rivers.