Impact of interaction between Limnoperna fortunei and Roundup Max® on freshwater phytoplankton: An in situ approach in Salto Grande reservoir (Argentina)

Abstract The joint impact of the glyphosate-based commercial formulation Roundup Max ® and the invasive mussel Limnoperna fortunei on phytoplankton and water quality was assessed in Salto Grande reservoir, a scenario were both stressors coexist. We performed an in situ mesocosm approach, through a 7-day experiment using 400-L enclosures. The following treatments were applied by triplicate: addition of 250 mussels (M); addition of 5 mg L −1 of active ingredient (a.i.) in Roundup Max ® (R); addition of 250 mussels and 5 mg L −1 of a.i. in Roundup Max ® (MR), and controls, without any addition (C). R showed higher total phosphorus (TP) and ammonium nitrogen (N NH 4 + ) concentrations due to the herbicide input, and a significant increase in algal abundance, biovolume and chlorophyll a levels (Chl- a ). In M mussels grazed on phytoplankton, which resulted in subsequent phosphates (SRP) release. A decrease in species diversity was observed in R and M with respect to C. In MR, there were higher TP and N NH 4 + concentrations, a decrease in biovolume, an antagonistic effect on Chl- a and a synergistic effect on phytoplankton abundance. Species diversity and evenness showed a significant decrease due to the explosive growth of a small and opportunistic Chlorophyta, Spermatozopsis exsultans . The dominance of this species may be due to negative selectivity for S. exsultans and/or release of potential competitors by L. fortunei , and to the input of nutrients by Roundup Max ® and/or removal of competitors by its toxicity.