ENVIRONMENTAL FACTORS INFLUENCING MICROCYSTIN DISTRIBUTION AND CONCENTRATION IN MIDWESTERN LAKES

Jennifer L. Graham (grahamjl@missouri.edu)

Microcystin (MC), a potent cyanobacterial hepatotoxin, is a potential health risk in Midwestern drinking water supplies and recreational resources. The development of empirical relationships between environmental variables and MC concentration is critical to effective lake management and minimization of human health risks associated with the toxin. To this end, a study of 252 lakes in Missouri, Iowa, northeastern Kansas, and southern Minnesota was conducted during summers 1999-2001 and Mozingo Lake, a Missouri reservoir, was studied during summer 2001 to determine the environmental factors associated with MC distribution and concentration.

MC was detected in 77% of the Midwestern lakes sampled. The study region represents a south-north gradient in increasing trophic status, with TP and TN values ranging between 2-1994 and 90-35340 micro-g/L, respectively. MC values ranged from undetectable to 4500 nano-g/L and increased with latitude. Despite latitudinal trends, environmental variables explained < 50% of the variation in MC values; however, bivariate plots showed distinct nonlinear trends, suggesting environmental optima for maximum MC concentrations. Nonlinear interval maxima regression indicated that MC-TN maxima were characterized by a unimodal curve, with maximal (>2000 nano-g/L) MC values occurring between 1500-4000 micro-g/L TN. Above 8000 micro-g/L TN all MC values were < 150 nano-g/L. MC-Secchi maxima were characterized by exponential decline, with maximal MC values occurring at Secchi depths < 2.5 m. This study indicates MC values are linked to the physicochemical environment; however, the pattern is not linear.

Environmental factors associated with the spatiotemporal variation of MC values in Mozingo Lake were studied during May-September, 2001. MC values increased during summer, ranging from 20-1220 nano-g/L. Seasonal trends were similar to net chlorophyll and cyanobacterial biovolume trends, and were associated with increased dominance by Microcystis and Anabaena. MC values showed strong negative correlations with dissolved nitrogen (r=-0.95) and cations (r=-0.98), and strong positive correlations with net chlorophyll (Chl > 11 micro-m r=0.92; Chl > 35 micro-m, r=0.91). There was a high degree of autocorrelation among variables, due to similar seasonal patterns. Once the lake stratified, MC and chlorophyll values remained uniform throughout the photic zone, and decreased significantly (p<0.05) in the aphotic zone. Field experiments indicated MC values did not change in response to decreased light or increased nutrients. Mozingo Lake MC values were tightly coupled with seasonal lake processes and cyanobacterial community composition, abundance, and distribution in the water column; thus, biological, physical, and chemical variables all played a role in determining MC values.