Response of Brazos River Oxbow Fish Assemblages to Patterns of Hydrologic Connectivity and Environmental Variability

Abstract

Three oxbow lakes with different connection frequencies and an adjacent reach of the middle Brazos River, Texas, were surveyed quarterly from summer 1993 to summer 1996 to examine the effects of hydrology and physicochemical attributes on fish assemblage structure. During flood events, oxbows usually were colonized by about 20 fish species from the river channel, and several fluvial specialists were rarely or never sampled from oxbows. Multivariate analyses of seine samples revealed divergent patterns of assemblage structure during periods of isolation that also were associated with a gradient of maximum water depth, temperature, and conductivity. In contrast, analysis of gill-net samples that targeted large fishes revealed low between-site and temporal variation in assemblage structure among oxbow lakes but significant differences in the structure of oxbow versus channel assemblages. The shallowest oxbow (which dried out with greatest frequency) had the most variable fish assemblage across seasons and was dominated by small colonizing species, whereas deeper oxbows and the river channel tended to have higher species richness and more stable assemblages. Despite large differences in connection frequency and distance from the river channel, the deepest oxbows had the most similar fish assemblage structure across all seasons. Multiple regression analysis suggested that the timing and frequency of flood events (yielding colonization) in the most frequently connected oxbow interact with predator abundance (yielding mortality) to influence the abundance of common lotic-adapted minnows (red shiner Cyprinella lutrensis and bullhead minnow Pimephales vigilax) that periodically dominated the species assemblage of that oxbow after floods. Our results suggest that current hydrologic and geomorphologic dynamics in the middle Brazos River produce oxbow lakes with a range of physical characteristics yielding different disturbance and colonization regimes that strongly influence fish species assemblages.