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

Abstract.—Fish assemblages and habitat conditions of oxbow lakes and the main channel of the middle-lower Brazos River, a meandering lowland river in east central Texas, were investigated during summer 1994. All oxbows were eutrophic, with chlorophyll-a levels of up to 640 mg/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 nutrients, turbidity, and plankton densities accounting for 45–59 % of the variation in abundance of the dominant species. Water depth and dissolved nutrient 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 two youngest oxbows were relatively deep and contained a high diversity and biomass of fishes. Of the 42 fish species collected, several were largely restricted to oxbow lakes, and others were either entirely restricted to or common only in the river channel. The flood dynamics of Brazos River floodplain habitats are less predictable (both intra- and interannually) than are those of large temperate rivers that receive runoff from snowmelt or predictable spring rainfall. As a result, Brazos River oxbow lakes remain separated from the river channel for many months or years, such that faunal exchang