Evaluating the tolerance of harmful algal bloom communities to copper

Harmful algal blooms (HABs) cause severe economic and environmental impacts, including hypoxic events and the production of toxic and off-flavor compounds. Chemical treatments, such as copper sulfate pentahydrate (CuSO₄·5H₂O), are often used to mitigate the damaging effects of algal blooms. However, treatment effects are usually short-lived leading to waterbodies requiring repeated CuSO₄·5H₂O applications to control persistent algal blooms, particularly in highly eutrophic systems, such as aquaculture ponds or small agricultural impoundments. We hypothesized phytoplankton communities routinely treated with Cu develop community tolerance to treatment making algal blooms more difficult to manage over time. Pollution-induced community tolerance (PICT) is a method for measuring how a community can withstand selective pressures to a toxicant. To test whether phytoplankton develop community tolerance to algaecidal treatment, the toxic effects of Cu were evaluated at a standard dose (1.37 mg/L CuSO₄·5H₂O or 0.35 mg/L total Cu) and a low dose (0.69 mg/L CuSO₄·5H₂O or 0.17 mg/L total Cu) relative to untreated controls. Treatments were applied once to 1300 L mesocosm enclosures installed in a productive aquaculture pond and monitored for 28 days. Acute toxicity bioassays measured photosynthetic efficiency across a wide range of Cu concentrations (0.05-300 mg/L). The PICT bioassay results were used to generate dose-response curves for median effective concentrations (EC50s) to assess phytoplankton community tolerance to Cu toxicity. The results of this study showed that both doses of Cu led to over 99% removal of cyanobacteria in the first seven days and maintained a reduction in cyanobacterial abundance by at least 70% throughout the experiment. After three days of exposure, the phytoplankton communities in the standard and low-dose treatments exhibited a 12.4x and 5.2x increase in Cu community tolerance, respectively, compared to controls. This increase in community tolerance was driven by Cu-tolerant chlorophyte species. These findings suggest that, while community tolerance to Cu may alter the perceived effectiveness of treatment over time, it can promote a beneficial shift from cyanobacteria to chlorophyte species, ultimately contributing to a more sustainable system.