THE INFLUENCE OF ENVIRONMENTAL FACTORS ON SPATIAL AND TEMPORAL VARIATION OF FISH ASSEMBLAGES IN THE LOWER BRAZOS RIVER, TEXAS

Large floodplain rivers are spatially heterogeneous and temporally dynamic ecosystems. However, few studies have quantified the variation or species-environment relationships of fish assemblages in the main-channel of large rivers. Fishes were collected along a 10-km reach of the lower Brazos River, a large floodplain river in Texas. Collections targeted the 15th, 30th, and 50th percentile discharge rates of summer and winter seasons. My objectives were: (1) to compare fish assemblage structure in shallow river-margins versus deepwater habitats, (2) to evaluate the spatial and temporal variability of fish assemblages in these two habitats, (3) to identify species-environment relationships that likely structure these assemblages, and (4) to quantify the relative variation in assemblage structure as related to environmental versus seasonal sources. A total of 41 species and 28,469 individual fishes were collected. Assemblages were less variable than levels typically reported for streams and had weak species-environment relationships. Temporal variability of the shallow river-margin fish assemblage was primarily the result of juvenile recruitment, displacement of individuals following spates, or seasonal immigration by Mugil cephalus. Among the deepwater assemblages, increased movement associated with reproductive activities increased temporal variation. Spatial variation was detected only among deepwater assemblages and was related to velocity. Eighteen commonly collected species were evaluated for relationships with environmental variables and season. Shallow river-margin assemblages were dominated by habitat-generalists and were most strongly differentiated by season, discharge and conductivity. Deepwater samples were dominated by Lepisosteus osseus and L. oculatus and were most strongly differentiated by velocity. For shallow river-margin and deepwater assemblages, environmental variables uniquely explained more of the total variation than season. Results of this study point to biotic factors as probably explaining a large proportion of the unexplained variation.