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Changes in the trophic status of fish feeding guilds in response to flow modification

Rivers are among the most extensively modified ecosystems globally. Identifying linkages between critical drivers of change and ecological response is challenging because of the myriad of ways rivers are modified. This study examines longer‐term relationships (>70 years) between the trophic status of fish and historical flow changes in rivers of the Mississippi Basin. The flow regime of each river is regulated but differs in terms of character of hydrological modification. Tissue samples from specimens obtained from museum collections were used for determination of carbon and nitrogen stable isotope ratios. Questions asked were as follows: (1) Are there differences in stable isotope ratios of fish feeding guilds prior to and following hydrological modification? (2) What hydrological attributes are associated with isotopic changes? And (3) are corresponding changes in stable isotope ratios and hydrology identifiable across different spatial scales? Significant and sustained changes in stable isotope ratios of fish feeding guilds occurred immediately following flow regulation in all rivers. These changes were not associated with human‐induced biogeochemical alterations. The subsequent response was complex because (1) different guilds exhibited pronounced shifts in isotopic ratios, (2) hydrological modification differed between rivers, and (3) differences in stable isotopic ratios varied between spatial scales.