Optimizing copper applications for the control of harmful algal blooms

Abstract

Harmful algal blooms (HABs) degrade water quality, disrupt aquatic ecosystems, and introduce off-flavors to aquaculture products and drinking water. Copper (Cu) sulfate pentahydrate is commonly used to control HABs due to its high efficacy and widespread federal approval. However, environmental concerns and the risk of phytoplankton developing copper tolerance with repeated use highlight the need for optimized treatment strategies. This thesis used large-scale mesocosm experiments to investigate Cu tolerance in phytoplankton communities and assess CO2-enhanced, low-dose Cu treatments as alternative approaches for HAB control. We compared phytoplankton responses to standard (350 µg/L Cu) and low (170 µg/L Cu) doses in a mesocosm experiment. Results showed that both standard and low doses achieved over 99% reduction in cyanobacteria within the first week. After three days, tolerance in phytoplankton to Cu increased significantly (up to 12.4x) in treated communities, especially among Cu-tolerant chlorophytes, suggesting a potential shift toward Cu-resilient communities. An additional mesocosm experiment was conducted using a CO2 addition to reduce pH. This addition was hypothesized to increase Cu toxicity making treatment effective at even lower Cu doses of 80 µg/L and 20 µg/L (20% and 5% of standard recommendations). The CO2-enhanced treatments further improved copper efficacy, achieving 94% cyanobacteria removal and 95% off-flavor reduction while promoting a 512% increase in beneficial chlorophytes, an effect not observed without CO2. These findings suggest that a CO2 addition can enhance copper treatment efficiency, reduce required dosages, and support beneficial phytoplankton. This combined approach provides valuable insights for water managers aiming to balance effective HAB control with long-term sustainability in aquatic ecosystems.