2,4-D and Glyphosate affect aquatic biofilm accrual, gross primary production, and community respiration

2,4-Dichlorophenoxyacetic acid (2,4-D) and glyphosate are widely used agricultural herbicides commonly found in surface waters near cultivated land. Field experiments were conducted to determine the effects of 2,4-D and glyphosate on biofilms in a pond next to agricultural land in Athabasca, Alberta. Contaminant-exposure substrates (CES) consisted of GF/C glass fiber or a cellulose filter paper substrates placed on specimen jars filled with agar that contained low levels of nitrogen and phosphorus, and different concentrations (15, 9.0, 1.5 mM) of either 2,4-D or glyphosate. Nutrients and herbicide diffused freely through the agar to the substrate surface. CES arrays were deployed 15 cm below the water surface for 22 days, after which biofilms were collected and biomass (chlorophyll a), autotroph gross primary production (GPP), and heterotroph community respiration (CR) were measured. 2,4-D (15 mM) caused significant decreases in rates of biomass accrual (−22%), GPP (−34%), and CR(−63%). Glyphosate (15 mM) also caused significant decreases in rates of biomass accrual (−50%), GPP (−67%), and CR (−47%). For the contaminant concentrations used, mean flux rates are estimated to be between 50–700 ng cm⁻² min⁻¹.