Harmful algal blooms (HABs) are a significant and growing problem worldwide in both freshwater and marine ecosystems. In many freshwater systems, cyanbacteria blooms can cause significant risks to public health due to the production of potent toxins. An important objective in managing harmful phytoplankton blooms is determining what factors influence the toxicity of bloom-forming species. In this dissertation, I combined lab and field experiments with field measurements in Lake Erie to determine whether the production of microcystin (a potent liver toxin) by the cyanobacterium Microcystis is strongly affected by nutrient limitation. Taken together, the field and lab experiments demonstrated that nitrogen-limited conditions led to significantly reduced microcystin quota relative to phosphorus-limited or nitrogen and phosphorus saturated conditions. Results from an extensive seasonal survey in the western basin of Lake Erie revealed a similar influence of nitrogen availability on microcystin quota, indicating that nitrogen limitation appears to have a robust influence on the toxin quota of Microcystis. These results are consistent with stoichiometric considerations in that production of nitrogen-rich secondary metabolites, such as microcystins, should be reduced disproportionately under nitrogen limitation. The results of this work may be used to inform public decision making about managing nitrogen and phosphorus inputs into freshwater lakes.
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