Modeling the freshwater ecological response to changes in flow and thermal regimes influenced by reservoir dynamics

Capturing the ecological effects of climate-induced shifts in hydrologic and thermal regimes in regulated river systems remains a challenge in regional-scale studies. In this study, we used a well-established species distribution model to analyze the results of a process-based hydrologic modeling approach that accounts explicitly for regulation impacts on river flow and temperatures. We also accounted for the stream network fragmentation resulting from dam blockage. This combined model framework can be used to predict historical environmental suitability of river reaches for specific fish species and estimate changes in suitability in response to changes in climate and reservoir operations. As a case study in the highly regulated and fragmented Tennessee River system, we examined the environmental suitability for nonnative rainbow trout (Oncorhynchus mykiss), a species of high recreational value, and native blackfin darter (Etheostoma nigripinne), an endemic species of unique ecological importance. To quantify the impacts of reservoir regulation, we examined the historical environmental suitability, climate change signals, and the environmental drivers of change between the regulated and unregulated model setups. By the end of the 21st century (2080s), historically suitable streams for rainbow trout will disappear due to higher river temperatures. Only 8% of historically suitable streams for blackfin darter will remain but over half of them will be unreachable by current populations due to dam blockage. For river reaches influenced by reservoir regulations, the regulated model projects decreasing environmental suitability for blackfin darter while the unregulated model projects the opposite because of higher river temperatures. The contradictory climate signals between regulated and unregulated models highlight the necessity of considering reservoir regulation.