Manipulating pH improves copper toxicity for harmful algal bloom and off-flavor control

Authors: HENNESSEY, A - Auburn University; MCDONALD, M - Auburn University; JOHNSON, P - Auburn University; GLADFELTER, M - Auburn University; MERRILL, K - Auburn University; TENISON, S - Auburn University; GANEGODA, J - Auburn University; HOANG, T - Auburn University; Torbert Iii, Henry; Beck, Benjamin; WILSON, A - Auburn University

Submitted to: World Aquaculture Society Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 10/5/2024
Publication Date: 3/6/2025
Citation: Hennessey, A.V., Mcdonald, M.B., Johnson, P.P., Gladfelter, M.F., Merrill, K.L., Tenison, S.E., Ganegoda, J.S., Hoang, T.C., Torbert III, H.A., Beck, B.H., Wilson, A.E. 2025. Manipulating pH improves copper toxicity for harmful algal bloom and off-flavor control [abstract]. World Aquaculture Society 2025, March 6-10, 2025, New Orleans, LA.

Interpretive Summary:

Technical Abstract: Harmful algal blooms cause severe economic and ecological problems, including fish mortality and the production of toxins and off-flavor compounds. Chemical treatments, such as copper sulfate pentahydrate (CuSO4·5H2O), are often used to mitigate the damaging effects of algal blooms. Given that waterbodies often require repeated CuSO4·5H2O applications to control algal blooms, we hypothesized that phytoplankton communities treated with CuSO4·5H2O develop tolerance to treatment, making agals blooms more difficult to manage over time. To test whether phytoplankton develop tolerance to algicidal treatment, the toxic effects of CuSO4·5H2O were evaluated at a standard dose (1.37 mg/L CuSO4·5H2O or 0.348 mg/L Cu) and a low dose (0.69 mg/L CuSO4·5H2O or 0.174 mg/L Cu) relative to untreated controls. Treatments were applied once to 1,300 L mesocosm enclosures installed in a productive aquaculture pond and monitored for 28 days using the pollution-induced community tolerance (PICT) methodology. Acute short-term bioassays were conducted measuring photosynthetic efficiency across a broad range of copper concentrations (0.05 to 300 mg/L). The results of the PICT bioassays were used to create half-maximal effective concentration (EC50) dose-response curves that were utilized to determine phytoplankton community tolerance to copper toxicity. Results from this experiment indicated that both doses of CuSO4·5H2O resulted in >99% removal of cyanobacteria in the first seven days and reduced cyanobacterial abundance by at least 70% throughout the experiment. After three days, communities in the standard dose and low dose treatments were 12.4x and 5.2x more copper tolerant, respectively. Tolerance peaked 14 days after the treatments were applied and remained higher than the control throughout the experiment. The data demonstrates that a lower dose of CuSO4·5H2O is equally effective at selectively removing harmful cyanobacteria, but treatment leads to higher copper tolerance in the system, making algal blooms more difficult to manage with a chemical algaecide.