Fish assemblage structure in relation to environmental variation among Brazos River oxbow lakes

Fish assemblages and habitat conditions of oxbow lakes and the main channel of the middle-lower Brazos River, a meandering lowland river in east centr al Texas, were investigated during summer 1994. All oxbows were eutrophic, with chlorophyll-a levels of up to 640 mu g/L. Assemblage structure showed large between-lake variation that was explained by both physical and biotic variables, with combinations of water depth, dissolved oxygen, dissolved n utrients, turbidity, and plankton densities accounting for 45-59% of the va riation in abundance of the dominant species. Water depth and dissolved nut rient concentrations were the best predictors of species diversity and fish abundance. Periodic desiccation of shallow, vegetated oxbows created harsh conditions that favored small fishes that are efficient colonizers. The tw o youngest oxbows were relatively deep and contained a high diversity and b iomass of fishes. Of the 42 fish species collected, several were largely re stricted to oxbow lakes, and others were either entirely restricted to or c ommon only in the river channel. The hood dynamics of Brazos River floodpla in habitats are less predictable (both intra- and interannually) than are t hose of large temperate rivers that receive runoff from snowmelt or predict able spring rainfall. As a result, Brazos River oxbow lakes remain separate d from the river channel for many months or years, such that faunal exchang e between oxbows to the channel should be pulselike and irregular.