Abstract

Microcystin (MC) and chlorophyll a (chl a) concentrations were examined at three nearshore stations during two consecutive ice-free seasons in relation to nitrate-N (NN), orthophosphate-P (OP) and other parameters in the south basin of Lake Winnipeg, Canada. While average nutrient levels were high in both years, chl a levels were significantly higher in 2011 than 2010, while the reverse was true for MC. However within each year, chl a was correlated with MC. Chl a was highly correlated with OP and NN in 2010, but less strongly with OP and not with NN in 2011. The higher chl a levels of 2011 were more strongly related to preceding than to concurrent OP and NN. MC was also strongly correlated with OP and (less) with NN in 2010, but only with chl a in 2011. The NN/OP ratio (NPR) was unrelated to chl a in either year, but was inversely correlated with MC in 2010. Cross-correlation analysis for 2010 showed that while MC lagged behind changes in chl a, OP, NN, and inversely behind NPR, correlations for these parameters (except NPR) were stronger for concurrent rather than delayed MC values. More vigorous blooms, as reflected by the phaeophytin/chl a ratio, were more toxic. While in a given season shifts in nutrients and NPR may signal impending changes in bloom density and toxicity, in some years additional factors besides nutrients appear to modify phytoplankton nutrient response. Nutrients remain a primary target for lake refurbishment strategies.

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