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

—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. Floodplain rivers are dynamic ecosystems characterized by high biological diversity, productivity, spatial complexity, and flow variability (Sparks 1995). Ecological processes in these systems are dominated by hydrology (Poff and Allan 1995), and conceptual models emphasize flood dynamics as a primary factor influencing fish productivity and recruitment (Junk et al. 1989; Humphries et al. 1999). Periodic inundation provides opportunities for aquatic organisms to access productive off-channel floodplain habitats, such as oxbow lakes that function as spawning and nursery areas for many fish species, provide aquatic organisms with refuge from disturbances in the river channel, and contribute to the biodiversity of the river–floodplain system as a whole (Halyk and Balon 1983; Sabo and Kelso 1991; Sabo et al. 1991; Hoover and Killgore 1998; Swales et al. * Corresponding author: [email protected] 1 Present address: Harmony Science Academy, 5435 South Braeswood Road, Houston, Texas 77096, USA. Received August 23, 2004; accepted May 10, 2005 Published online September 30, 2005 1999; Ward et al. 1999). Periodic connection to the river channel facilitates faunal exchange and tends to homogenize fish assemblages spatially (Rodriguez and Lewis 1997). As habitats become isolated, biotic interactions and physicochemical characteristics appear increasingly important in structuring fish assemblages (Tejerina-Garro et al. 1998; Winemiller et al. 2000). Modified river–floodplain systems have received comparatively little attention despite changes in fish distribution, abundance, and assemblage structure associated with the alteration of natural hydrologic regimes (Moyle and Light 1996; Tockner and Stanford 2002; Feyrer and Healey 2003). Proposed restoration strategies include increasing the connection between rivers and their floodplains and reestablishing a relatively natural hydrologic regime (Bayley 1991; Trexler 1995; Amoros and Bornette 2002). A major challenge for restoration efforts is to determine the ecological function of these systems under natural flow regimes (Bayley 1995; Richter et al. 1997; Michener and Haeuber 1998). Floodplain rivers exhibit a wide range of flow dynamics, and variable responses of aquatic fauna are expected