Phytoplankton abundance and methane emissions are minimally impacted by environmentally-relevant glyphosate concentrations in small-scale outdoor mesocosms

Glyphosate is one of the most widely applied agrochemicals in North America and can be directly transported via runoff into non-target aquatic habitats. Yet, our understanding of how this herbicide affects aquatic ecosystems is limited; past studies have often focused on single species effects and/or used herbicide concentrations several orders of magnitude higher than what has been reported in contaminated aquatic systems. Further, glyphosate in aquatic systems has the potential to alter greenhouse gas emissions (methane) if it is broken down for phosphate utilization by bacteria under specific environmental conditions (i.e., oxygen, nutrient concentrations). In this study, we assessed the temporal changes in nutrients, phytoplankton, copy number of genes associated with breakdown of glyphosate or production of methane (phnJ, mcrA), and methane concentrations in 12-day mesocosms with amendments of glyphosate, nitrogen, and/or phosphorus. We found glyphosate at environmentally-relevant concentrations (∼4 ug/L) did not confer changes in overall ratios and total concentrations of nutrients, or abundance of any major phytoplankton group (cyanobacteria, diatoms, green algae), methane concentration or flux, or gene copy numbers of phnJ and mcrA. Our results suggest that the relatively low concentrations of glyphosate we used (relative to levels used in toxicological studies) did not cause major changes over short time periods in mesocosms, and that the potential for glyphosate to increase greenhouse gas emissions in aquatic systems requires specific conditions to occur and may not be universal in contaminated systems.