Salinity Stresses Along a Complex River Continuum: Effects on Mayfly (Ephemeroptera) Distributions

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Abstract
The saline—stressed Washita River in western Oklahoma was evaluated to test the River Continuum Concept (RCC). Physical, chemical, and biological, variables were monitored at six sites in the river over 14 sampling periods during 1980 and 1981. Physical parameters represented stream discharge, sediment particle sizes, organic content of the sediment, and related factors. Chemical variables were conductance, pH, and concentrations of major salts (calcium, sulfate, sodium, and chloride). Biological variables were densities of mayfly genera from dredge samples. Two—dimensional ordinations were produced for the physical, chemical, and biological groups of variables using the ALSCAL model of three—way non metric multidimensional scaling. The ALSCAL model summarized variation over the 14 sampling periods. Rank correlations of the dimensions from all ordinations suggest that both river gradient and local effects influence the structure of the benthic community. River gradient dimensions for physical and chemical variables were highly correlated with one biological dimension. The remaining biological dimensions was correlated with local salinity effects. Benthic community structure is influenced by physical gradients. Saline effects, although governed by geomorphological processes, do not conform to the typical river gradient. This localized saline impact influences the benthic community structure. Therefore, the RCC must be modified to allow for multiple gradients if it is to be useful in the generation of ecological models in regions with high river salinity.