Reevaluating copper sulfate pentahydrate dosing for harmful algal bloom control

Abstract

Copper sulfate pentahydrate is the most widely used chemical means of controlling nuisance algal growth that often lead to harmful algal blooms (HABs). However, its efficacy and ultimate ecological impact are closely tied to key water quality parameters known as toxicity-modifying factors (TMFs). A substantial body of research has demonstrated limitations in current copper dosing methods, which rely predominantly on alkalinity and frequently result in the application of higher doses than necessary to manage nuisance algae. To establish a more precise and efficient copper algaecide dosing strategy, a multiple linear regression (MLR) model was developed to relate copper toxicity to phytoplankton with respect to key TMFs, including pH, dissolved organic carbon (DOC), alkalinity, and hardness. The quantified relationships led to a novel dosing equation that integrates site-specific pH and DOC measurements for targeted copper applications. In Chapter 1, I tested this MLR-based dosing approach against the standard alkalinity-based dosing method in a large-scale field mesocosm experiment. Results indicated that the MLR-based dose, which was 60% lower than the traditional alkalinity-based dose, selectively reduced harmful cyanobacterial populations while minimizing overall ecological impact. Chapter 2 extended this analysis over time by examining the efficacy of the MLR-based dosing approach through multiple applications in a 42-day mesocosm experiment. Findings from this study confirmed that the MLR-based doses continued to outperform alkalinity-based doses, with a half-MLR dose, almost nine times lower than the standard alkalinity dose, producing the best water quality (i.e., highest transparency) throughout the experiment. Importantly, non-target effects of copper on zooplankton and beneficial green algae were nearly eliminated. Together, these studies establish a foundation for a more sustainable, targeted, and cost-effective copper algaecide dosing strategy. The MLR-based dosing approach offers a promising advancement for HAB control that optimizes copper use while reducing environmental impact for long-term water quality management.