Seasonal changes in composition of the cyanobacterial community and the occurrence of hepatotoxic blooms in the eastern townships, Quebec, Canada

Four eutrophic lakes in the eastern townships (Québec,Canada) were sampled on a biweekly basis between May and October2001 to examine seasonal changes, and to study the role of taxonomicand environmental factors in cyanobacterial toxin production.Microcystin-LR (MC-LR) equivalent content was determined usinga protein phosphatase inhibition assay on extracts of lyophilizedplankton. Three of the lakes showed a similar pattern of maximumwater column toxicity in late summer, while in the fourth, toxicitywas highest in spring and then declined over the year. Variationsin water toxicity level could be attributed to the abundanceof two potentially toxigenic genera, Microcystis and Anabaena.A multiple regression model explained 75% of the variation inmicrocystin (MC) concentration, based on water column totalnitrogen concentration (TN) and the biomass of these two genera.Microcystis and Anabaena genera appeared to be similarly toxicin all lakes. Increased water column stability, higher lightextinction coefficient and a lack of dissolved nutrients wereall associated with increased total biomass of toxigenic cyanobacterialgenera.