Impact of Dissolved Organic Matter on Porewater Hg and MeHg Concentrations in St. Louis River Estuary Sediments

Inorganic mercury in sediments is a potential source of bioaccumulative methylmercury in the aquatic food web; however, in many cases, total mercury concentrations in the sediment alone cannot predict methylmercury production and distribution. Fish that primarily feed in the St. Louis River Estuary, a Great Lakes coastal wetland, have higher concentrations of methylmercury in their tissues than fish that primarily feed upriver or in Lake Superior; however, the exact sources of mercury to methylating microorganisms and methylmercury to fish remain unknown. Solid-phase mercury pools, sediment and porewater chemistry, and sediment–porewater partitioning are assessed here to obtain a more detailed understanding of the role of sediments in methylmercury production. We show that in sediments where concentrations of reduced sulfur groups in dissolved organic matter in the porewater approach or exceed aqueous inorganic sulfide, there are higher concentrations of total mercury in the porewater. Reduced sulfur groups in dissolved organic matter raise the fraction of total mercury in the porewater where it may be more accessible for methylation and uptake to the food web. In the St. Louis River Estuary, conditions which facilitate greater partitioning of total mercury to the porewater correspond to sediments with lower sediment total mercury concentrations, highlighting that absolute total mercury concentration in the sediment alone cannot accurately assess the potential for mercury mobility and bioaccumulation. This study elucidates factors, mainly the presence of reduced sulfur groups in dissolved organic matter in the porewater, which facilitate the partitioning of total mercury to the porewater.