Riverine Discharge Fuels the Production of Methylmercury in a Large Temperate Estuary

Thesignificance of riverine discharge in stimulating methylmercuryproduction in temperate estuarine water is associated with its supplyof mercury and organic matter that helps fuel remineralization andmethylation. Estuaries are an important food source for the world'sgrowingpopulation, yet human health is at risk from elevated exposure tomethylmercury (MeHg) via the consumption of estuarine fish. Moreover,the sources and cycling of MeHg in temperate estuarine ecosystemsare poorly understood. Here, we investigated the seasonal and tidalpatterns of mercury (Hg) forms in Long Island Sound (LIS), in a locationwhere North Atlantic Ocean waters mix with the Connecticut River.We found that seasonal variations in Hg and MeHg in LIS followed theextent of riverine Hg delivery, while tides further exacerbated theremobilization of earlier deposited riverine Hg. The net productionof MeHg near the river plume was significant compared to that in otherlocations and enhanced during high tide, possibly resulting from theenhanced microbial activity and organic carbon remineralization inthe river plume. Statistical models, driven by our novel data, furthersupport the hypothesis that the river-delivered organic matter andinorganic Hg drive net MeHg production in the estuarine water column.Our study sheds light on the significance of water column biogeochemicalprocesses in temperate tidal estuaries in regulating MeHg levels andinspires new questions in our quest to understand MeHg sources anddynamics in coastal oceans.