Release and Degradation of Microcystin-LR Following Exposures of Microcystis to Copper-Based Algaecides

Microcystins (MCs) are hepatotoxic endotoxins produced by cyanobacteria such as Microcystis that can cause both ecological and human health risks, as well as economic losses in freshwater resources. The use of copper-based algaecides to manage MC-producing cyanobacteria is sometimes limited due to perceived risks of MC release from cyanobacterial cells and persistence of toxins in water resources. The aim of this research was to investigate the influence of copper concentration and form on MC release and persistence, as well as potential consequences of “no-action” management decisions (i.e. no treatment). To achieve these objectives, a tiered approach was employed utilizing both laboratory and field experiments and the commonly problematic microcystin-LR (MC-LR) producing cyanobacterium, Microcystis. In experiments that investigated MC release, a positive relationship was measured between copper concentrations and aqueous MC in both laboratory and field experiments. The positive relationship between copper concentration and aqueous MC-LR was observed at effective algaecide concentrations (i.e. concentrations that decreased cell density and chlorophyll a), demonstrating that lower copper concentrations can be as effective as relatively higher concentrations in controlling Microcystis while minimizing the proportion of MC-LR released from the cellular to aqueous phase. In experiments that investigated MC degradation, copper exposures influenced rates and extents of MC-degradation. However, these effects were concentration dependent and only occurred at copper exposures greater than concentrations registered for algaecidal use (i.e. 2-5 times greater than registered label concentrations [~1.0 mg Cu/L]). Influences of copper form on MC release and degradation were tested in laboratory experiments, but did not significantly influence either MC release or degradation rates. Total MC concentrations were also measured throughout each experiment in untreated controls and compared against total MC concentrations in copper treatments. Results indicated that total MC concentrations either increased or remained stable in untreated controls throughout all experiments, compared to copper treatments where MC concentrations typically decreased to background (depending on exposure concentration); thus demonstrating the potential consequence of “no-action” decisions. Results of these studies provide a more thorough understanding of the influences of copper algaecide concentration and formulation on MC-LR release, MC-LR degradation, and potential consequences of “no-action” management decisions. These studies can be used to inform more accurate risk evaluations and use of copper-based algaecides for management of MC-LR producing Microcystis.