VARIABILITY OF THE HEPATOTOXIN MICROCYSTIN-LR IN HYPEREUTROPHIC DRINKING-WATER LAKES

The patterns of occurrence of the peptide hepatotoxin microcystin-LR ( MC-LR) was studied in three hypereutrophic hardwater lakes (Coal, Drie dmeat, and Little Beaver) in central Alberta, Canada, over three open- water seasons. MC-LR concentration was based on high-performance liqui d chromatography detection and expressed as mu g . g(-1) of total plan kton biomass, ng . L(-1) of lake water, and mu g . g(-1) of Microcysti s aeruginosa Kuetz. emend. Elenkin. MC-LR was highly variable temporal ly (differences up to 3 orders of magnitude) within each lake over an individual year, between years in an individual lake, and between lake s in any year. Seasonal (within-year) changes in MC-LR concentration ( expressed in the preceding units) were positively correlated to the ab undance and biomass of the cyanobacterium M. aeruginosa (r = 0.60-0.77 ), total and total dissolved phosphorus concentration (r = 0.46-0.59), pH (r = 0.38-0.58), and chlorophyll a (r = 0.25-0.59). Surprisingly, there was no relationship between MC-LR concentration and water temper ature (range: 7 degrees-24 degrees C, r = -0.13 to 0.02) and a negativ e correlation with nitrate concentration (r = -0.27 to -0.34). In two synoptic surveys examining spatial variability, MC-LR concentrations w ere quite variable (CV of 185 and 36% between sampling sites for Coal and Little Beaver lakes, respectively). Spatial distribution of MC-LR on any one day was correlated with the abundance and biomass of M. aer uginosa. Over a 24-h period, MC-LR concentration in M. aeruginosa decr eased more than G-fold at night relative to daytime concentrations. In general, analytical and within-site variation of MC-LR was relatively small (CV < 4 and 9%, respectively) but greatest both within and betw een years in a lake followed by diel and spatial variation.