Lowering pH enhances copper toxicity: A novel strategy for controlling harmful algal blooms and off-flavors

Harmful algal blooms (HABs) severely disrupt aquatic ecosystems by degrading water quality and producing unpalatable off-flavors in aquaculture products and drinking water. Copper sulfate pentahydrate (CuSO₄·5 H₂O) has been used as an affordable and FDA-approved algaecide to manage HABs for decades. However, growing concerns about long-term negative environmental impacts and the diminishing efficacy of treatments over time have prompted the exploration of alternative treatment strategies. Copper toxicity is hypothesized to increase with lower pH as the cupric ion, which is most often associated with toxicity, becomes the most dominant form of copper. To rigorously evaluate this effect, ∼1600 L mesocosms were placed in a hypereutrophic pond and treated with a 20 µg/L Cu dose (0.08 mg/L CuSO₄·5 H₂O) or an 80 µg/L Cu dose (0.33 mg/L CuSO₄·5 H₂O). Half of the enclosures received a CO₂ addition that dropped the pH in the enclosures from 9.15 to 7.58 prior to Cu treatment. The addition of CO₂ for the 20 µg/L Cu dose resulted in 14 % greater removal of cyanobacteria in the first 24 h and a 20 % reduction of off-flavors in the first 3 d, which was significantly more effective than treatments without CO₂. After one week, treatments with CuSO₄·5 H₂O and a CO₂ addition continued to see a significant reduction in cyanobacteria (>94 %) and off-flavors (>95 %); furthermore, beneficial chlorophytes were significantly promoted (+512 %), which was not observed in treatments without CO₂ or the controls. This approach mitigated cyanobacteria and off-flavors and promoted beneficial phytoplankton, all while using just 5 % of the copper dose that is typically recommended.