Changes in contaminant bioaccumulation and biochemical responses of Carcinus maenas in response to ecosystem restoration measures

An environmental restoration programme based on Zostera noltii transplantation aimed to rehabilitate a historically multi-metal contaminated estuarine system. This study evaluates early responses of the crab Carcinus maenas during the first two years following seagrass transplantation (2021-2022), by combining tissue mercury burdens and biochemical biomarkers. Although several potentially toxic metals co-occur in the system, sediment quality guideline screening (Probable Effect Levels) identified mercury as the only element exceeding effect-based thresholds; therefore, results are interpreted primarily in the context of mercury exposure, while contributions from other metals and mixture effects cannot be excluded. Mercury was quantified in gills, hepatopancreas and gonads of crabs collected at a contaminated and a reference site. Mercury concentrations were consistently higher at the contaminated site across all tissues, reaching ≈0.8 mg kg^-1 in gonads. Gonadal mercury at the contaminated site decreased significantly between 2021 and 2022, indicating an early reduction in metal bioavailability following Z. noltii transplantation. Biochemical responses were organ-specific. Crabs from the contaminated site exhibited elevated antioxidant enzyme activities (catalase and glutathione-S-transferase) in the hepatopancreas and increased lipid peroxidation in gills, reflecting oxidative stress. Energy reserve mobilisation varied among tissues, reflecting higher metabolic costs under contaminant exposure. In contrast, gonads showed stable oxidative and energetic profiles over time. These findings provide quantitative evidence that gonadal mercury dynamics and associated biomarkers in C. maenas can track early-stage ecosystem recovery. Although Z. noltii promotes phytostabilization and reduce mercury bioavailability, the limited mercury decrease after two years indicates that ecotoxicological recovery remains at an initial stage.