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 cyanobac terial toxin microcystin-LR (MC-LR) in phytoplankton is regulated by enviro nmental factors that affect both the biomass of the main producer of the to xin, Microcystis aeruginosa, and the concentration of the toxin in the cell s. Of all environmental factors examined, total phosphorus was the stronges t correlate of both M. aeruginosa biomass and cellular MC-LR (expressed as micrograms per gram of M. aeruginosa). Microcystis aeruginosa biomass was a lso strongly negatively related to the total nitrogen to total phosphorus r atio (TN:TP) and inorganic nitrogen (NO2- + NO3-, NH4+). A univariate regre ssion model of TN:TP explained the most variation in MC-LR concentration (e xpressed as nanograms of cellular toxin per litre) in mixed phytoplankton c ommunities. This study indicated that MC-LR dynamics in phytoplankton of la kes was related to changes in the concentration and ratio of phosphorus and nitrogen.