Distribution of benthic algae in the upper Illinois River basin in relation to geology and land use

1. Benthic-algal distributions in the upper Illinois River basin, IL, U.S.A ., were examined in relation to geology, land use, water chemistry and stre am habitat using (detrended) (canonical) correspondence analysis, autecolog ical metrics and indicator-species analysis in order to identify the major environmental gradients influencing community variation. 2. Ionic composition and major nutrient [i.e. nitrogen (N) and phosphorus ( P)] concentration of surface waters, salinity (Na-Cl type), substratum type and physiognomic form of dominant species were primary factors contributin g to variation in benthic-algal assemblages of the basin. Basin geology was a significant contributing factor, but the explained variance associated w ith this factor was less than that related to land use. 3. Proportions of algal biomass consisting of cyanophytes, filamentous chlo rophytes, halophilic diatoms and diatoms which utilize nitrogen heterotroph ically were greater in eutrophic river segments than in less nutrient-enric hed segments. Composition of the benthic flora indicated meso-eutrophic or eutrophic conditions throughout the basin; there were few diatoms indicativ e of hypertrophic waters. Shifts in diatom-assemblage structure in response to nutrient loading provided an incomplete representation of the community -response curve. 4. A weighted-averages regression model based on total P and benthic-algal abundances tall divisions included) yielded a highly significant correlatio n (r(2) = 0.83) between species-inferred [WA((tol))] and observed total P, with systematic bias (increased deviation of residuals) occurring only at c oncentrations greater than similar to 1.0 mg L-1 total P. This result indic ates that total P regression and calibration models can be predictable for a river basin receiving excessive loadings of phosphorus.