Relationship Between Base Flow Magnitude and Spring Fish Communities
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Base flow is the portion of stream flow attributed to groundwater, and few studies quantify the pure effects of base flow reductions on stream fish communities. Spring complexes within the karst terrains of the Edwards Plateau Region of central Texas offer a unique opportunity to test hypothesized relationships between base flow and stream fish communities. Spring complexes are numerous within the Edwards Plateau, providing multiple independent observations, stable hydrographs dominated by base flow conditions, similar groundwater sources, and support endemic fishes that are associated with the spring complexes (i.e., spring-associated fishes). Primary objectives of this study were to assess spring-associated fish richness, relative abundances, and densities across a gradient of base flow magnitudes with predictions that metrics of spring- associated fish communities would linearly decrease with reductions in base flow. To control potential confounding variables, additional objectives were to test for the presence and strength of parapatry that is hypothesized to exist between spring-associated fishes and riverine-associated fishes (i.e., fishes with distributions not typically associated with spring complexes). Patterns in richness, relative abundances, and densities indicated parapatric distribution between spring-associated and riverine-associated fishes. Strength of parapatry depended upon base flow magnitude. Correspondingly, differences in spring-associated fish richness, relative abundances, and densities along a base flow gradient were detected, but only densities were linearly related to base flow. Richness and relative abundances of spring-associated fishes were non-linearly related to base flow, suggesting that spring complexes have a level of buffering capacity against base flow reductions. The relationship between spring-associated fish communities and base flow gradient was used to support the reported parapatry between spring-associated fishes and riverine-associated fishes with in the area and to highlight the conservation value of spring complexes to regional fauna. Predictive models generated in this study can be used to evaluate spring-associated fish community integrity within the Edwards Plateau Region and to predict future changes in Edward Plateau spring complexes related to increases in groundwater extraction.