Role of Chemical and Physical Variables in Regulating Microcystin-Lr Concentration in Phytoplankton of Eutrophic Lakes

A survey of eutrophic to hypereutrophic hardwater lakes in central Alberta was conducted to test the hypotheses that the concentration of the cyanobacterial toxin microcystin-LR (MC-LR) in phytoplankton is regulated by environmental factors that affect both the biomass of the main producer of the toxin, Microcystis aeruginosa, and the concentration of the toxin in the cells. Of all environmental factors examined, total phosphorus was the strongest correlate of both M. aeruginosa biomass and cellular MC-LR (expressed as micrograms per gram of M. aeruginosa). Microcystis aeruginosa biomass was also strongly negatively related to the total nitrogen to total phosphorus ratio (TN:TP) and inorganic nitrogen (NO2- + NO3-, NH4+). A univariate regression model of TN:TP explained the most variation in MC-LR concentration (expressed as nanograms of cellular toxin per litre) in mixed phytoplankton communities. This study indicated that MC-LR dynamics in phytoplankton of lakes was related to changes in the concentration and ratio of phosphorus and nitrogen.