Cyanobacteria and microcystin-LR in a complex lake system representing a range in trophic status: Lake of the Woods, Ontario, Canada

Euphotic water samples were collected monthly from 16 sites in Lake of the Woods, Ontario, Canada from July to September, 2006 and 2007. Sites represented a range of inorganic nitrogen and phosphorus concentrations and mixing regimes, which was reflected in the spatial variability in phytoplankton biomass and community composition, and cyanobacterial toxin [microcystin-LR (MC-LR)] concentrations. Nutrient concentrations decreased along a south to north transect, concomitant with distance from the main river inlet at the south end of the lake. Filamentous, nitrogen-fixing Aphanizomenon spp. and Anabaena spp. were dominant at most sites: Aphanizomenon was more abundant at southern sites and Anabaena was more abundant at northern sites. MC-LR was detected at all sites at least once (maximum concentration 0.69 μg L−1). However, there was no correlation between MC-LR concentration and the biomass of Anabaena, a documented MC-LR producer. For the pooled data set, total phosphorus and ammonium concentrations were the two parameters most strongly related with MC-LR concentration. Principal components analysis was used to analyze the data set; it indicated that two factors accounted for 81% of the variability among the 12 environmental parameters tested. Factor 1 represented physicochemical characteristics and phytoplankton biomass and Factor 2 represented Anabaena and Woronichinia biomass.