Periphyton biomass and community composition in rivers of different nutrient status

Epilithic periphyton was investigated in riffle zones of 13 rivers in southern Ontario and western Quebec to describe how algal biomass and community composition vary with nutrient concentration and water velocity during summer. Algal biomass (milligrams chlorophyll a (Chl a) per square metre) was strongly correlated with total phosphorus concentration (r(2) = 0.56, p < 0.001) and conductivity (r(2) = 0.71, p < 0.001) of the overlying water but unrelated to water velocity over the range of 10-107 cm s(-1). Differences in periphyton Chl a were associated with changes in biomass of Chlorophyta (r(2) = 0.51, p = 0.001) and Bacillariophyta (r(2) = 0.64, p < 0.091) and were not related to Rhodophyta and Cyanophyta biomass (p > 0.10). The relative proportions of taxonomic divisions varied with total standing stock. Percent Chlorophyta biomass increased with periphyton Chi a and was the largest fraction at moderately eutrophic sites. Rhodophyta contributed the most biomass at sites with the lowest Chl a. Cladophora, Melosira, and Audouinella biomasses were positively correlated with total phosphorus concentration over the range of 6-82 mu g.L-1 (r(2) = 0.39-0.64, p < 0.005), and these genera were dominant at sites with the highest nutrient concentrations.