Commonly co-occurring stressors in industrialized estuaries disrupt larval development in blue mussels

Recent, substantial declines in blue mussel (Mytilus edulis) beds in northern European estuaries signal an urgent need to unravel how chronic contaminant exposure, climate-driven warming and salinity shifts jointly affect the development and survival of vulnerable early life stages. Here, we investigated the interactive effects of a representative, environmentally relevant pollutant mixture, water temperature and salinity on mussel embryo-larval development. We used a full factorial design with a pollutant mixture concentration range of 0.015–15 μg/L (organic compounds) and 20–20,000 μg/L (dissolved metals) and in the presence of two different temperatures (15 or 18 °C) and two different salinities (25 or 35 ppt) over a 48-h exposure duration. Under control conditions (15 °C, 35 ppt, no pollution), larvae exhibited high normal development with few malformations. Increasing temperature alone enhanced normal development by 15.9% (p ≤ 0.01), whereas reduced salinity alone decreased normal development by 28.7% (p ≤ 0.01). Combined elevated temperature and reduced salinity, did not significantly alter normal development from the control but induced shell malformations in the D-stage (p ≤ 0.05). At 15 °C and 35 ppt, pollutant mixtures produced a strong concentration–dependent decline in normal development and an increase in malformations, with high concentrations arresting most larvae in the trochophore stage. Detrimental pollutant effects were intensified under reduced salinity but were modestly alleviated by elevated temperature. These findings demonstrate that climate change has the potential to alter environmental contaminant effects on mussel larval development in dynamic, contaminated estuaries, which if replicated in wild mussels, could severely compromise recruitment.