Overall impacts of bisphenol S (BPS) on the three-spined stickleback (Gasterosteus aculeatus) in a mesocosm experiment: Relationships between biomarkers, life-history traits, and population-level effects

Effects of Bisphenol S (BPS), a widespread substitute for Bisphenol A, were investigated on the three-spined stickleback (Gasterosteus aculeatus) across multiple biological levels and timescales in mesocosms (semi-natural rivers). By comparing short- and long-term exposures, the aim is to identify early predictive biomarkers and establish mechanistic links between sub-organismal and population-level effects of the BPS. This study thereby contributes to the integration of these results into Adverse Outcome Pathway (AOP) and supports Environmental Risk Assessment (ERA). Two complementary approaches were conducted: a 215-day mesocosm experiment exposing sticklebacks to BPS (1, 10 and 100 µg/L) throughout full stickleback life cycle, and a 21-day caging assay designed to evaluate short-term biomarker responses. During the mesocosm experiment, measured BPS concentrations in the 100 µg/L scenario were lower than expected, with only minor changes in water pH and no detectable effects on macrophytes, macro-invertebrates, or zooplankton. This long-term exposure resulted in higher female mesocosm-born body mean length at low concentrations, and a dose-dependent decrease in Fulton's condition index in juveniles. The caging experiment revealed early warning signals of effects of exposure to BPS, i.e. higher gonadosomatic index (GSI) and hepatosomatic index (HSI) in females, higher HSI and lower nephrosomatic index (NSI) in males and activated oxidative stress biomarkers in both sexes. Together, these findings demonstrate that combining long-term mesocosm studies with short-term caging assays provides complementary insights into effects of BPS, by highlighting mechanistic links between biomarkers, life-history traits, and population dynamics, and offering an approach to integrate early biological signals into AOPs for improved ERA.