Confluences and land cover as agents of change: habitat change modifies the movement and assemblage stability of headwater fishes

Interspersed inputs of wood and sediment bring about morphological change at confluences and the extent to which these processes are modified by anthropogenic disturbance has ramifications for stream fish assemblages. In this study, we used a 2X2 design and mark-recapture methods to assess the influences of confluence size (> 0.6, < 0.6) and upstream land cover (dominantly forested or urban) on reach-scale habitat change, the movement rate of three functional groups of stream fishes, and fish assemblage change at four headwater confluences in a Gulf Coastal Plain drainage in the southeastern United States. We found little evidence to suggest that confluence size contributed to differences in upstream mainstream and downstream mainstem habitat characteristics. Land cover did cause reach-scale, dissimilarities in channel morphology, streamflow, and physical complexity. Therefore, we suggest that physical complexity affected fluvial geomorphic processes and regulated base flow, thus influencing the magnitude of reach-scale habitat change. Accordingly, greater movement by water-column specialists in our urban reaches suggest that land cover indirectly modified this functional group’s ecological response to reach-scale habitat change. Furthermore, our results indicated that the degree of intra-reach, assemblage change was likely a corollary of channel morphology stability. Our understanding of the extent to which land cover alters the geomorphic and ecological gradients associated with headwater confluences will be critical to ensure the conservation of sensitive species whose fitness is dependent on the integrity of these habitats.