Primary production of freshwater microbial communities is affected by a cocktail of herbicides in an outdoor experiment

Primary production (PP) is a key variable to evaluate the quality of the ecological services provided by freshwater bodies because it gives information on the amount of oxygen and organic matter incorporated into the system. We analysed the impact of a mixture of commercial formulations of glyphosate- and 2,4-D-based herbicides (Roundup Max (R) and AsiMax 50 (R), respectively) on freshwater primary production. Primary production was studied through the oxygen exchange method. Four measurements were made during a 23-day experiment in outdoor mesocosms using the light and dark bottle method. High and low concentrations of the active ingredients were assayed to evaluate a concentration-dependent effect. Our results indicated that the mixture of Roundup Max (R) and AsiMax 50 (R) acted mostly additively on gross and net primary production. Moreover, we found a concentration-dependent effect of each herbicide on PP. Thus, AsiMax 50 (R) at low and Roundup Max (R) at high concentration induced a significant early decrease in respiration and gross primary production 4 h after application, attributable to physiological responses. Besides, significant increases in primary production were simultaneously recorded with increases in chlorophyll a concentration and micro + nano-phytoplankton abundance 7 days after the application of Roundup Max (R) at high concentration. This study contributes to the knowledge of the impact of widely used herbicides on freshwater ecosystems.