Physical and biological controls on abundance of Microcystis, a harmful algal bloom (HAB) species

Background/Question/Methods: Microcystis and other buoyant, toxin producing, algae are sensitive to changes in lake turbulence and light availability, thus climate change may effect the occurrence of such harmful algal blooms. Simultaneous changes in lake nutrient, zebra mussel (ZM), and transparency status may also influence algal competition. Our long-term goal is to develop a predictive model of Microcystis bloom occurrence for small to mid-sized lakes based on lake morphometry, chemistry, ZM occurrence, and mixing calculated from weather forecasts. To meet this goal we have conducted a survey and time series study of chemistry, physics, and algal occurrence in Michigan lakes of diverse size, nutrient level, and ZM density.

Results/Conclusions: A survey of 28 Michigan lakes, ranging in size from 1.8-830 ha, and containing approximately paired gradients of nutrient levels with and without zebra mussels, showed that Microcystis bloomed in low nutrient lakes (TP < 20 μg/L) only in the presence of zebra mussels. Results of this survey, combined with weekly monitoring results from Gull Lake, MI (surface area 830 ha, TP 5 μg/L), were compared to a previously published model of HAB formation based on turbulence and light competition. This model was an insufficient predictor of Microcystis abundance in low nutrient lakes, predicting blooms where none were observed. Moderately strong Microcystis blooms were observed, with particle attached microcystin toxin concentrations reaching 6 μg/L in open surface waters and >10x higher concentrations near shore in protected bays. Empirical results to data indicate that turbulence, light, nutrients, and zebra mussels must all be included in a Microcystis model for lakes in the Grate Lakes region.