Going With the Flow: How Direct and Indirect Alterations to Streamflow Influence Headwater Fish Communities in the Southeast

Streamflow has been deemed the “master variable” regulating riverine communities as it can influence the chemical, physical, and biological components of river systems. Anthropogenic changes have and will continue to alter streamflow through direct (e.g. road culverts) and indirect (e.g. climate change) processes. Understanding how these alterations impact riverine communities is essential for maintaining and restoring ecosystem properties like biodiversity. Our objective is to integrate ecological theory and conservation approaches to understand how direct and indirect alterations to streamflow influence freshwater fish communities within the biodiverse headwater streams of the southeastern U.S. In a test of a standardized barrier protocol we found that the protocol was not an effective estimate of fragmentation effects measured by mark-recapture at road culverts. However, we did show that altering the calculation of the protocol to reflect local conditions may improve the scores efficacy. Standardized protocols are a promising method of identifying structures which alter streamflow, but more research is needed if the management goal is to accurately quantify locally specific movement. We also investigated the influence of direct alterations to streamflow on communities across multiple scales. Communities were more influenced by environmental filtering rather than fragmentation across both scales, supporting the network position hypothesis and emphasizing that restoration should be prioritized lower in watersheds. Lastly, we investigated the influence of indirect alterations to streamflow by testing hypotheses informed by life history theory. We did not find evidence that individual traits were selected for by streamflow conditions, but we did find that one of the three life history strategies (equilibrium) within the tripartite life history model was correlated with lower streamflow variability. This suggests that selection may act differently on life history strategies and individual traits. Overall, we learned that integrating theory into management can be challenging as ecological theory is often developed under strong environmental gradients which are not always present. Despite this, it was important to include as it provided context for our results and informed management decisions. Each of these studies outline novel approaches to detect ecological patterns related to streamflow and serve as a foundation for future studies to build upon.