Periphyton biomass and community composition in rivers of different nutrient status

Epilithic periphyton was investigated in riffle zones of 13 rivers in south ern Ontario and western Quebec to describe how algal biomass and community composition vary with nutrient concentration and water velocity during summ er. Algal biomass (milligrams chlorophyll a (Chl a) per square metre) was s trongly 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 u nrelated to water velocity over the range of 10-107 cm s(-1). Differences i n 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 we re not related to Rhodophyta and Cyanophyta biomass (p > 0.10). The relativ e proportions of taxonomic divisions varied with total standing stock. Perc ent Chlorophyta biomass increased with periphyton Chi a and was the largest fraction at moderately eutrophic sites. Rhodophyta contributed the most bi omass at sites with the lowest Chl a. Cladophora, Melosira, and Audouinella biomasses were positively correlated with total phosphorus concentration o ver the range of 6-82 mu g.L-1 (r(2) = 0.39-0.64, p < 0.005), and these gen era were dominant at sites with the highest nutrient concentrations.