Sensitivity of cyanobacteria to herbicides commonly used in aquatic systems: Implications for controlling harmful...

Water quality and eutrophication are major drivers of harmful cyanobacterial blooms. Issues arising from increased nutrient loads, poor water circulation, and temperature influence blooms of toxic cyanobacteria, such as Dolichospermum and Microcystis, which can be detrimental to aquatic organisms in natural and aquaculture systems. Copper sulfate, a U. S. Environmental Protection Agency-approved algicide, has been commonly used to control cyanobacterial blooms, especially in aquaculture systems. While effective, repeated annual use of copper can lead to accumulation in aquatic systems over time. Organic herbicides, which can degrade after application, used in aquaculture systems for weed control may also limit cyanobacterial blooms; however, their efficacy toward different cyanobacterial species is not well defined. Dolichospermum spp. and Microcystis aeruginosa were exposed to 15 herbicides (2,4-dichlorophenoxyacetic acid, bispyribac sodium, copper sulfate, diquat dibromide, diuron, endothall, hydrogen peroxide, imazapyr, imazamox, fluridon, flumioxazin, glyphosate, penoxsulam, topramezone, and triclopyr) to characterize their effects on cyanobacteria. Dose-response testing was conducted for each herbicide to measure growth rates, chlorophyll, and phycocyanin concentrations to derive inhibitory concentrations (IC50s). Results demonstrated that M. aeruginosa growth was inhibited by copper, diuron, diquat, hydrogen peroxide, and penoxsulam, with diuron having the lowest IC50, indicating it was the most effective chemical for controlling M. aeruginosa. Dolichospermum spp. growth was inhibited by bispyribac sodium, copper, diuron, diquat dibromide, H2O2, and penoxsulam, with both diuron and penoxsulam having lower IC50 values than copper. The research provides a comprehensive overview of how commonly used herbicides affect Dolichospermum spp. and M. aeruginosa, offering insight into alternative management strategies; however, additional research is needed to characterize the long-term efficacy of organic herbicides.