Microcystin-LR Degradation Following Copper-Based Algaecide Exposures

When copper-based algaecides are used in aquatic systems to decrease cyanobacteria densities, endotoxin fate is a concern, due to the potential for human health and ecological risks. Pulse exposures of algaecides can result in episodic low dissolved oxygen (DO) concentrations (< 2 mg L⁻¹), due to oxygen consumed via microbial oxidation of algal detritus. Research objectives of this study were to determine the influence of declining DO levels on microcystin-LR (MC-LR) degradation and changes in resident bacterial assemblages. It was hypothesized that cyanobacteria cell densities would be positively correlated with rates and extents of DO decline based on the oxygen required for bacteria to degrade cyanobacteria detritus following exposure to copper-based algaecides. In addition, it was hypothesized that total MC-LR concentrations would increase proportionally with increasing cyanobacteria cell densities. Mesocosm experiments were conducted in a pond in Anderson, SC, that frequently experiences cyanobacteria blooms. Three densities of a cyanobacteria assemblage were exposed to a copper ethanolamine algaecide. DO and total MC-LR concentrations were measured with time following algaecide exposures to determine rates and extents of declines. As anticipated, DO concentrations had the highest rate of decline in the highest cell density treatment, followed by medium and low cell densities. MC-LR degradation occurred at similar rates (half-lives 1 to 1.9 days) among cell densities. Acinetobacter and Aeromonas were dominant in treatments following copper exposures. The relationship between cyanobacteria densities and MC-LR half-lives demonstrates the benefits of managing cyanobacteria in early growth stages to minimize MC concentrations.