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Meta‑analysis and field trials show repeated copper dosing can trigger cyanobacteria rebound by killing zooplankton
Summary
A meta‑analysis and a 56‑day field trial presented at CC Hub found that while chemical algaecides reduce algal biomass short‑term, repeated or high copper dosages killed zooplankton, caused ammonia spikes, and led to later increases in cyanobacteria.
Alan Wilson, an aquatic ecologist at Auburn University, presented a meta‑analysis of treatment studies and a follow‑up field experiment that together question common assumptions about chemical treatments for HABs.
"I think pretty much everyone's using too much copper," Wilson said, describing patterns seen across published trials and his own experiment. "If you treat with chemicals, especially if you overtreat, you're gonna lead to more cyanobacteria."
Wilson's synthesis of 39 field studies found that treatments (biological, chemical, plant and physical) produced large reductions on the most effective day (median ≈70% lower chlorophyll on that peak day) but much smaller reductions by the end of experiments (≈29% lower overall). Chemical treatments comprised the bulk of available data, and when analysts split treatment types only chemical algaecides consistently showed statistical significance — likely because of the much larger sample size for chemical trials.
To test mechanisms, Wilson's team ran a 56‑day, 1,700‑liter dock experiment with four treatments (control, copper sulfate, Phoslock‑type lanthanum clay, and a half‑doses combination). They added a second copper dose partway through. Results showed that while copper caused rapid short‑term drops in cyanobacteria indicators, repeated or high copper dosing corresponded with zooplankton crashes, ammonia spikes and later rebounds in chlorophyll and phycocyanin. In several replicates a double copper dose led to higher cyanobacteria indicators weeks after treatment than control treatments.
Wilson emphasized the mechanistic link: copper can reduce grazer populations (large cladocerans like Daphnia), removing biological control on phytoplankton and enabling opportunistic cyanobacteria to proliferate when nutrients are available. His field data showed SRP (soluble reactive phosphorus) and ammonia tended to rise after large copper treatments, creating nutrient pulses that support rebound growth.
Wilson recommended managers reassess dosing strategies, explore lower or alternate non‑chemical approaches, and prioritize experiments that measure grazer responses and fish/community impacts rather than only short‑term chlorophyll declines.
The presentation concluded with an appeal for more coordinated meta‑analysis work and larger, standardized field experiments to evaluate tradeoffs and ecological impacts of common mitigation tactics.
