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Abstract
Microcystin is a cyanobacterial hepatotoxin of global concern. Understanding the environmental
factors that cause high concentrations of microcystin is crucial to the development
of lake management strategies that minimize harmful exposures. While the literature
is replete with studies linking cyanobacterial production of microcystin to changes
in various nutrients, abiotic stressors, grazers, and competitors, no single biotic
or abiotic factor has been shown to be reliably predictive of microcystin concentrations
in complex ecosystems. We performed random forest regression analyses with 16S and
18S rRNA gene sequencing data and environmental data to determine which putative ecological
drivers best explained spatiotemporal variation in total microcystin and several individual
congeners in a eutrophic freshwater reservoir. Model performance was best for predicting
concentrations of the congener MC-LR, with ca. 88% of spatiotemporal variance explained.
Most of the variance was associated with changes in the relative abundance of the
cyanobacterial genus Microcystis . Follow-up RF regression analyses revealed that
factors that were the most important in predicting MC-LR were also the most important
in predicting Microcystis population dynamics. We discuss how these results relate
to prevailing ecological hypotheses regarding the function of microcystin.