Monitoring organic pollution and eutrophication in the Grand River, Ontario, by means of diatoms

Diatom analysis discriminates between the impacts on river water quality of treated urban wastewaters and diffuse nutrient sources from farmland. 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 central reaches that are influenced by both urban discharges and agricultural runoff have the lowest water quality. The ranking of sites according to both diatom indices is consistently independent of substratum and seasonality. Ordination of samples by multivariate analysis results in a primary separation of the spring and summer communities and a secondary separation of upper, central, and lower reaches. Substratum differences are only apparent at the tertiary level, and mainly for midsummer 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 temperature 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 conducted in the 1960s, the Grand River shows clear signs of increased eutrophication.