Ecosystem response to mercury mitigation and forbidden fishing: A 20-year chronosequence of fish contamination in Baihua Reservoir

Environmental remediation including mercury (Hg) source closure, cage-farming prohibition, and sediment capping has been implemented at Baihua Reservoir (BHR) since 2007, which strongly influences Hg bioaccumulation in fish. In this study, total Hg (THg) and methylmercury (MeHg) concentrations were determined in abiotic matrices and fish in 2015–2016 and 2018–2019 and compared with data extracted from previous studies referring to 2002–2006 and 2008–2009. Following these remediation measures, the water quality improved, eutrophication declined, and environmental Hg levels dropped dramatically. By 2018–2019, the aqueous THg and MeHg decreased by 82 % and 89 %, and sediment THg and MeHg decreased by 99 % and 86 %, respectively, compared to 2002–2006 levels. However, prohibition of cage fish farming in BHR shifted fish diets toward plankton, slowing their growth rate and lengthening the food chain by one level. Paradoxically, average fish THg during 2008–2019 increased significantly by 73–168 %, whereas the increase in MeHg (33–137 %) was not statistically significant. The relative proportion of MeHg to THg (%MeHg) in fish transiently peaked at 56 ± 23 % in 2015–2016. The bioaccumulation factor for fish Hg was high, while the trophic magnification rates of Hg were lower and remained low compared to global averages, reflecting efficient initial uptake into the food web but constrained transfer efficiency. This unexpected rise in fish Hg is likely due to the dietary shift to Hg-rich plankton, reduced somatic growth dilution in wild fish, and enhanced Hg trophic transfer. Therefore, ongoing monitoring and updated fish consumption advisories are recommended to mitigate potential health risks. • Mercury mitigation significantly reduced Hg in water and sediment in BHR(82 %-99 %). • Fish THg & MeHg increased up to 1.68x & 1.37 x despite environmental Hg drop. • Cage fish farming ban shifted fish diets to plankton, slowing growth and lengthening food chain. • Hg bioaccumulation and biomagnification in fish intensified post-remediation. • Fish Hg remained below China's safety standards, posing no current risk.