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peroxide (H₂0₂), produced by SOD activity, is in turn scavenged by CAT and APX (Gunes et al., 2007). Increased activity of these enzymes could help some phytoplankton species to cope with ROS generation and ROS-induced burst, although this was not sufficient to avoid lipid peroxidation.

De Lorenzo et al. (1999) mentioned that phytoplankton community structure and functional aspccts were altered when exposed to atrazine, deethylatrazinc, endosulfan and chlorpyrifos, these pesticides inducing compositional shifts in photoautotroph species. Our rcsults indicate that glyphosate-based herbicide, Factor540^JO, also did alter the physiological and structural aspects of the freshwater phytoplankton community, as expressed by the changes of the chlorophyll a and of chi ,es in the dominant species of the community. Overall, the Shannon diversity index decreased following the increase of glyphosate concentrations, which indicates a loss of species diversity of the phytoplankton community when exposed to the herbicide glyphosate (figurę 3.3). An exposure to Iow concentrations of glyphosate-based herbicide (5 and 10 pg I¹) reduced the diversity of the phytoplankton dominant species, while glyphosate concentrations of 50 pg I*¹ and higher favored the appcarance of new species becoming dominant.

3.8 Conclusion

Glyphosate negatively affects freshwater phytoplankton community from the studied agricultural strcam. A progressive decrease in biodiversity was clearly identified by the decrease of the Shannon’s index when the original phytoplankton community was exposed to incrcasing glyphosate concentrations. However, some phytoplankton species, such as Nitzschiapalea proved to be resistant at high glyphosate concentrations, becoming dominant under these conditions. These results indicates that glyphosate-based herbicides affects the structure of phytoplankton community at Iow concentrations that are representative of environmental concentratic as monitored in different studies (Giroux et Pelletier, 2012; Scribner et al, 2007; Struger et al, 2008; Dubois et al, 2010). The obscrved short-term effeets of Iow concentrations of glyphosate-based herbicides on freshwater phytoplankton communities, questions the effectivcness of the chronic toxicity standard for aquatic life (65 pg I'¹ of glyphosate; CCME, 2008) for risk management of glyphosate-based herbicides.