Effects of a Declining Hydrograph on Instream Habitats and Fish Communities in a Semi-Arid Karstic Stream

Curtis, Stephen G.
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Low to subsistence flows are natural components of riverine hydrographs, but their effects on fish communities are poorly understood. Instream habitats and fish community responses to a declining hydrograph were assessed along upper, middle, and lower reaches of the Llano River, Texas, and among tributaries. Using general stream theory, I predicted that greatest hydrological variability among reaches during a declining hydrograph would be associated with increased habitat variability, decreased species richness and diversity, and shifts in fish-habitat associations. The control was the upper reaches of the Llano River with hydrologically stable flows attributed to large outflows of a karst terrain aquifer. Hydrological variability differed (P < 0.01) among mainstream sites and tributaries with the greatest hydrological variability occurring in the lower reach. Biotic indices (catch per unit effort, diversity, and evenness) were inversely related, whereas taxa richness and community stability were directly related to hydrological variability. Available habitats and fish-habitat associations were independent of hydrological variability, although riffle specialists were restricted to current velocities> 0.6 mis and run specialists to current velocities> 0.4 mis less than expected approaching subsistence flows. Collectively, patterns in hydrological variability, instream habitats, and fish community responses suggest that karst spring outflows alter expected patterns in hydrological variability along a river network, but hydrological variability was supported as the primary driver of instream habitats and fish communities. In addition, fish communities and habitat associations were similar as flows approached subsistence levels. Following subsistence flows, flows within the lower reach ceased and the reach rapidly dewatered. Once flows returned, the lower reach was initially recolonized by large-bodied fishes from downstream and hydrologically stable sites of the Colorado River and secondarily by small-bodied fishes from the hydrologically stable upper Llano River. Consequently, this study demonstrated that persistent flows of the upper reach contribute to the downstream community and therefore illustrates the influence of karst aquifers on communities near and distant from the spring source.
Hydrology, Streamflow, Fishes
Curtis, S. G. (2012). <i>Effects of a declining hydrograph on instream habitats and fish communities in a semi-arid karstic stream</i> (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.