MONITORING ORGANIC POLLUTION AND EUTROPHICATION IN THE GRAND RIVER, ONTARIO, BY MEANS OF DIATOMS

Diatom analysis discriminates between the impacts on river water quali ty of treated urban wastewaters and diffuse nutrient sources from farm land. Evaluation of organic pollution and trophic levels in the Grand River, Ontario, using two diatom indices shows a clear differentiation among 10 sampling sites over a distance of 214 km. Sites in the centr al reaches that are influenced by both urban discharges and agricultur al runoff have the lowest water quality. The ranking of sites accordin g to both diatom indices is consistently independent of substratum and seasonality. Ordination of samples by multivariate analysis results i n a primary separation of the spring and summer communities and a seco ndary separation of upper, central, and lower reaches. Substratum diff erences are only apparent at the tertiary level, and mainly for midsum mer samples from the upper reaches. Canonical correspondence analysis shows that the largest portion of the observed variability in species composition can be explained by a seasonal gradient related to tempera ture and by longitudinal gradients of nitrate-nitrogen, conductivity, and chloride. Differences in species composition between nutrient-rich sites and polluted sites are identified. Compared with a study conduc ted in the 1960s, the Grand River shows clear signs of increased eutro phication.