Reducing Ph Improves Copper Toxicity for Harmful Algal Bloom and Off-Flavor Management

Abstract

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 (CuSO4·5H2O) 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. This project evaluated the synergetic effects of low-dose CuSO4·5H2O treatments following artificial pH reduction with carbon dioxide (CO2) to improve water quality. 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, large ~1600 L mesocosms were placed in a hypereutrophic pond and treated with one of two doses, including a 20 µg/L Cu dose (0.08 mg/L CuSO4·5H2O) or an 80 µg/L Cu dose (0.33 mg/L CuSO4·5H2O), which are 5% and 20% of the alkalinity-based dosing recommendations, respectively. These Cu treatments were evaluated alongside untreated controls. Half of the enclosures received a CO2 addition that dropped the pH in the enclosures from 9.15 to 7.58 for the first 3 days of the experiment. The addition of CO2 for the 20 µg/L Cu dose resulted in 14% greater removal of cyanobacteria in the first 24 h and 20% more removal of off-flavors in the first 3 d, which was significantly more effective than treatments without CO2. After one week, treatments with CuSO4·5H2O and a CO2 addition continued to see a significant reduction in harmful cyanobacteria abundance (>94%) and off-flavors (>95%); furthermore, beneficial chlorophytes were significantly promoted (+512%) in these treatments, which was not observed in treatments without CO2 or the controls. This approach mitigated cyanobacteria and off-flavors, all while using just 5% of the copper dose that is typically recommended. These findings offer crucial insights for water resource managers regarding ways to best optimize CuSO4·5H2O use for effective and sustainable HAB mitigation.

Keywords: cyanobacteria, copper sulfate, sustainability, 2-methylisoborneol, geosmin, CO2