Biochemical and populational responses of an aquatic bioindicator species, Daphnia longispina, to a commercial formulation of a herbicide (Primextra (R) Gold TZ) and its active ingredient (S-metolachlor)

tThe growing demand of human populations for food supplies has led to an increase in the use of syntheticproducts, mainly pesticides, which induce adverse effects not only to target organisms, but also to non-target biota of agroecosystems. Aquatic ecosystems in the proximity of agricultural areas are particularlyvulnerable to pesticides, which cause underperformance or extinction of non-target sensitive species.Once in the aquatic system, these chemicals can affect biological processes at multiple levels (molecular,individual, populational), causing ecosystem imbalance across multiple scales. In this study, the effectof a commercial formulation of a herbicide (Primextra® Gold TZ) and its main active ingredient (a.i.,S-metolachlor) was studied on a freshwater cladoceran species (Daphnia longispina), at different levels ofbiological organization and temporal scales. S-metolachlor is used in many herbicide formulations appliedin corn/maize cultures, which is a relevant culture worldwide. As a first step, the acute and chronic effectsof both commercial formulation and a.i. were quantified, and both formulations negatively affected thecladoceran’s survival and reproductive parameters (age at first reproduction, number of offspring andnumber of broods), as well as the population’s rate of increase. Whilst acute effects were comparable,the commercial formulation was slightly more toxic (EC50was two-times lower) than the a.i. in chronicexposures, being prejudicial to D. longispina populations above 4.0 mg/L of S-metolachlor. In a secondexperimental step, we focused on the potential multi-generational impacts of the exposure to the a.i.alone on biochemical (lipid biomarkers, namely fatty acids) and populational responses, because of therelevance of S-metolachlor as a biosynthesis inhibitor in many herbicidal formulations. The herbicidecaused a significant decrease in Daphnia fecundity (in the size of the 1st clutch), but no concomitantalterations were found in fatty acid profiles of mothers or offspring. More important, this experimentshowed that S-metolachlor did not cause effects in the subsequent generation, thus suggesting thatbiotic communities may recover after exposure to the xenobiotic