ENVIRONMENTAL FACTORS INFLUENCING THE DISTRIBUTION OF BENTHIC-ALGAL SPECIES IN FOUR LARGE RIVER BASINS OF THE UNITED STATES.

H.V. Leland. Water Resources Div., U.S. Geological Survey, 3215 Marine St., Boulder, Colorado 80303

Benthic-algal assemblages of streams and rivers in two large river basins of the temperate mid-western United States (White River, Indiana, and upper Illinois River) and two large river basins in semi-arid areas of the western United States (San Joaquin River, California, and Yakima River, Washington) were examined in relation to flow regime, land use, water chemistry and stream habitat in order to identify the environmental factors that influence growth and assembly of species in this biological community. The principal statistical methods used to examine sources of variation were canonical correspondence analysis, weighted-averaging regression and calibration, and two-way indicator-species analysis. There was substantial interaction between salinity and inorganic nitrogen as constraints on the distribution of benthic-algal assemblages in the San Joaquin and Yakima River Basins, whereas distribution of species in the upper Illinois River Basin was constrained primarily by available phosphorus. In the White River Basin, reactive phosphorus and inorganic nitrogen varied seasonally as constraints on the distribution of species. Flow regime strongly influenced benthic-algal assemblages in all of these basins, but resource supply (nutrient availability, substratum type, light) was of equal or greater importance. Salinity optima were comparable for most species that were abundant (>0.5% of algal biomass) in both the San Joaquin and Yakima River Basins. The ordering of nutrient optima for abundant species was generally predictable, but among-basin differences in nutrient optima were often greater (within tolerances determined) than differences in salinity optima.