Effects of the herbicide metazachlor on locomotor activity and biochemical responses of Ephemera danica larvae

Abstract Background The widespread use and frequent detection of the herbicide metazachlor (MTZ) in aquatic environments have raised concerns about its impacts on non-target organisms. Despite its regular presence in freshwater ecosystems, the ecotoxicological effects of MTZ on benthic macroinvertebrates are mostly unknown. We investigated the sublethal impacts of two environmentally relevant concentrations of MTZ (5 µg/L and 100 µg/L) on sediment-dwelling larvae of mayfly Ephemera danica , collected from natural habitats over two consecutive years during their critical physiological stage of emergence, while also considering their natural parasite infections. Results We observed a significant increase in the mortality of individuals exposed to MTZ, following two separate ten-day exposure experiments conducted in 2022 and 2023 under controlled conditions. Furthermore, in both trials, MTZ-induced stress responses at the pre-emergence developmental stage resulted in significant decreases in larval glycogen content, while glutathione S-transferase activity increased. Infections of E. danica with microsporidian parasites had no significant impact on the sublethal endpoints; however, the number of infected individuals was low in the different treatment groups (low statistical power). Moreover, microsporidian infection rates in E. danica differed significantly between the two years. Conclusions MTZ affects the survival and biochemical responses of E. danica larvae at field-realistic concentrations, while the varying prevalence of microsporidian infection highlights the potential effects of parasitism on study outcomes. Thereby, we advocate the use of ecologically relevant testing approaches for a better understanding of the ecological implications of herbicides in aquatic ecosystems. Graphical abstract