The role of environmental parameters in the structure of phytoplankton assemblages and cyanobacteria toxins in two hypereutrophic lakes

We evaluated the variability of cyanotoxins, water chemistry, and cyanobacteria communities in two hypereutrophic drowned river mouth lakes (Spring Lake and Mona Lake; summer 2006) in west Michigan, USA. Even with considerable geographical and watershed similarity, local variations in nutrient concentrations and environmental factors were found to influence the differences observed in cyanobacteria assemblages and cyanotoxins levels between the two lakes. Limnothrix sp. dominated the phytoplankton community in Spring Lake (82% of biovolume) and was negatively correlated with total phosphorus (TP) concentrations. Although Spring Lake was treated with alum during the previous year, Limnothrix sp. was able to bloom in the lower P environment. In contrast, the N2-fixing cyanobacterium, Anabaena flos-aquae, dominated the phytoplankton in Mona Lake (64% of biovolume). N2-fixing cyanobacteria dominance in Mona Lake was correlated with higher TP lower dissolved nitrogen levels. Cylindrospermopsis raciborskii was found in both systems; however, the toxin-producing polyketide synthetase gene was not present in either population. The higher TP in Mona Lake appeared to account for the 3-fold increase in cyanobacteria biovolume. Restoration plans for both lakes should include assessments of internal loading and continued phytoplankton monitoring to track the temporal distribution of cyanobacteria species and cyanotoxin concentrations.