Repeated low-dose copper algaecide treatments promote beneficial phytoplankton and zooplankton communities

Abstract

Effective control of harmful algal blooms (HABs) is critical for improving and maintaining water quality. Common treatment strategies, such as copper sulfate pentahydrate with alkalinity-based dosing, often prescribe higher doses (>250 μg/L Cu) than may be necessary to achieve HAB control in many systems. The present study evaluated the performance of a previously developed multiple linear regression (MLR) based copper dosing method when compared to alkalinity-based doses after three repeated copper treatments spaced every two weeks. A 42-day field based mesocosm experiment was conducted in a hypereutrophic aquaculture pond where the alkalinity-based dose (350 μg/L Cu), full MLR based dose (80 μg/L Cu), and half of the MLR dose (40 μg/L Cu) were applied every 14 days (3x across the experiment) to mimic common application practices in aquaculture, recreational pond, and drinking water management. Initially, all copper treatments saw an identical ∼95% reduction in cyanobacteria, indicating that smaller doses are as effective as the larger alkalinity-based doses for HAB control. Moreover, the comparatively smaller, MLR-based doses caused less harm to beneficial green algae (chlorophytes) and zooplankton (important phytoplankton grazers) over the course of the experiment. Notably, it was observed that the alkalinity-based dose showed decreasing efficacy with each subsequent treatment, indicating that lower doses offer better control of harmful phytoplankton genera over time. Overall, the results of the present study demonstrate that repeated low MLR-based Cu dosing strategy offers superior long-term control of HABs with less ecological impact than higher, alkalinity-based Cu dosing.

Keywords: Algal bloom; Copper sulfate; Cyanobacteria; HABs; Multiple linear regression; Zooplankton.