Fluvial geomorphic evolution and stream fish community trajectories in the Bayou Pierre, Mississippi, U.S.A.

Abstract

Abstract Human activities have often altered sediment dynamics in rivers, leading to aquatic habitat degradation. The dominant paradigm in fish–sediment interactions focuses on excessive fine sediments as indicators of habitat and water quality degradation. In contrast, geomorphologists have provided frameworks linking altered sediment dynamics to much broader and more complex changes to channel morphology and stream habitats across spatial scales. In this paper we use an interdisciplinary approach with historical and contemporary channel morphology and fish community datasets to examine how altered sediment dynamics have affected stream fish community evolution in the Bayou Pierre, Mississippi, U.S.A. Erosional process regimes have advanced upstream consistent with models of knickpoint propagation and channel evolution, leaving most of the catchment in a state of increased sediment transport and local sediment deficits. Fish communities in modern samples have less α and β diversity and fewer habitat specialist taxa than in historical samples. Whole‐community analysis via non‐metric multidimensional scaling found a strong gradient of homogenisation towards a fauna comprised of small‐bodied taxa adapted to large rivers. Comparisons of community and geomorphic change on this gradient found that reaches that transitioned between process regimes had more community change, and that local habitat factors relating to advanced channel evolution were strongly positively related to total community change. These results demonstrate that a catchment‐scale geomorphic analysis provided a strong tool for understanding fish community change on a decadal scale. Our findings further demonstrate concordant ecological responses to a complex sequence of disturbances and suggest that a broader view of sediment dynamics offers a rich toolkit for ecological analyses.