Diuron has no effect on the toxigenic cyanobacterium, Microcystis, relative to two other common algaecides: A field mesocosm experiment

Cyanobacteria threaten freshwater quality due to the production of nuisance secondary metabolites, such as the hepatotoxin microcystin. Chemical methods are commonly used to control harmful algal blooms (HABs) because they can reduce cyanobacterial abundance quickly. Nonetheless, a limited number of algaecides have been approved by the U.S. Environmental Protection Agency (EPA) to control HABs in food-fish aquaculture in five southern U.S. states. Although the EPA, via state registrations, has allowed fish farmers to use diuron (3-[3,4-dichlorophenyl]-1,1-dimethylurea) to manage cyanobacteria-induced off-flavor issues, few field studies have been performed to assess its efficacy to control cyanobacteria. Herein, we conducted a 21-day mesocosm experiment to evaluate the effectiveness of Karmex®DF (diuron, 11.49 µg L −1 ), along with two other commonly used algaecides, including Captain® (chelated copper, 0.4 mg L −1 as copper) and PAK®27 (sodium carbonate peroxyhydrate, 10 mg L −1 as hydrogen peroxide) in controlling a dense bloom of Microcystis . Captain® and PAK®27 significantly and rapidly decreased chlorophyll- a (CHL; measurement of all phytoplankton), phycocyanin (PC; measurement of cyanobacteria), and intracellular microcystin (measurement of toxic cyanobacteria). While PAK®27 and Captain® mesocosms had similar PC concentrations, the CHL concentration for PAK®27 was higher than that for Captain®, suggesting that Captain® suppresses phytoplankton more generally and effectively than PAK®27. Unexpectedly, phycocyanin concentrations significantly increased in the Karmex®DF mesocosms on day 3 but later declined to levels similar to the control. Overall, our study highlighted that although diuron is a widely used herbicide to control off-flavor in catfish aquaculture at a recommended concentration of 0.5 oz acre ft −1 , it was not effective in removing Microcystis in a hypereutrophic system (∼CHL 49.43–82.72 µg L −1 ). Therefore, further studies are needed to investigate the potential factors that can influence the effectiveness of diuron in managing Microcystis blooms, as well as other toxigenic and off-flavor producing cyanobacterial taxa. • A standard dose of Karmex®DF (11.5 µg L −1 as diuron) has no effect on cyanobacteria. • PAK®27 (10 mg L −1 as H 2 O 2 ) suppressed cyanobacteria more selectively than Captain® (0.4 mg L −1 as copper). • Regrowth of phytoplankton occurred after algaecide treatments. • Cyanobacteria began to return two weeks following PAK®27 and Captain® treatments.