Quantifying the influence of initial nutrient conditions on phytoplankton growth in response to low-level glyphosate exposure

Abstract

Glyphosate, the active ingredient in RoundUp®, is the most common herbicide in the United States. Glyphosate is efficient at eliminating nuisance terrestrial plants. However, the nitrogen and phosphorus that make up glyphosate are made bioavailable upon decomposition into aminomethylphosphonic acid (AMPA) and phosphate. In a natural environment, glyphosate is prone to rapid degradation. The impact of glyphosate on phytoplankton growth has been both positive and negative, with few definite results. This project investigated the influence of various parameters on the ability of glyphosate to impact phytoplankton, and therefore, what systems are most at risk of eutrophication due to glyphosate addition. Multiple laboratory bioassays were performed as well as a three-pond mesocosm field experiment conducted in Auburn, AL. Lab experiments, tested on a laboratory-cultured Microcystis aeruginosa strain, found systems with low pH (<7) and dissolved organic matter (DOM) had the most growth following glyphosate addition; these environments promoted up to a 30% increase in chlorophyll-a compared to the control. Glyphosate, in the form of RoundUp®, was added to field mesocosms in levels of 100 µg/L and 700 µg/L. The field experiment found the mesotrophic and eutrophic ponds had 50% more chlorophyll-a in the 700 µg/L treatment than the control. These results show that low levels of RoundUp® (<1 mg/L) can increase nutrient values significantly and be effective at causing excess phytoplankton growth and in some cases eutrophication.