Environmental factors influencing microcystin distribution and concentration in the Midwestern United States

During May–September 2000–2001, physicochemical data were collected from 241 lakes in Missouri, Iowa, northeastern Kansas, and southern Minnesota U.S.A., to determine the environmental variables associated with high concentrations of the cyanobacterial hepatotoxin microcystin (MC). The study region represents a south–north latitudinal gradient in increasing trophic status, with total phosphorus (TP) and total nitrogen (TN) values ranging between 2–995 and 90–15870 μg/L, respectively. Particulate MC values, measured by ELISA, ranged from undetectable to 4500 ng/L and increased with increasing latitude. Despite latitudinal trends, environmental variables explained <50% of the variation in MC values. Inspection of MC–TN and MC–Secchi bivariate plots revealed distinctly nonlinear trends, suggesting optima for maximum MC values. Nonlinear interval maxima regression indicated that MC–TN maxima were characterized by a unimodal curve, with maximal (>2000 ng/L) MC values occurring between 1500 and 4000 μg/L TN. Above 8000 μg/L TN all MC values were <150 ng/L. MC–Secchi maxima were characterized by exponential decline, with maximal MC values occurring at Secchi depths <2.5 m. The development of empirical relationships between environmental variables and MC values is critical to effective lake management and minimization of human health risks associated with the toxin. This study indicates MC values are linked to the physicochemical environment; however, the relationships are not traditional linear models.