The impact of saltwater intrusion on mercury cycling in coastal plain wetlands

Coastal wetlands are considered hotspots and the primary source of mercury (Hg) to adjacent waters. Because of the global sea level rise and hurricanes low-lying coastal areas are experiencing saltwater intrusion causing alteration in Hg biogeochemistry. However, limited studies have focused on saltwater effects on Hg in coastal wetlands. To explore potential salinity impact on Hg levels and bioaccumulation in food web, we collected water, sediment, and invertebrate samples for multiple times over two years along with in situ water quality measurements in the natural forest-marsh salinity gradient. Sampling sites consist of a freshwater wetland (FW; salinity < 0.5 ppt), partially degraded wetland (PDW; salinity < 5 ppt), and saltmarsh (SM; salinity < 18 ppt) and are located near Winyah Bay (South Carolina) and Albemarle - Pamlico Sound (North Carolina). We found that while elevated salinities ultimately reduced overall mercury levels by reducing dissolved organic carbon levels, dissolved oxygen and sulfate/chloride ratio are important determinants for methylmercury (MeHg) production in water column after the hurricanes in coastal wetlands of North Carolina. Similarly, mean sediment mercury levels were decreasing in the order of FW, PDW, and SM with increasing salinities in South Carolina. However, mean biota mercury levels did not show difference in FW and PDW but significantly lower in SM. Overall, we found a strong empirical evidence that salinity controls MeHg levels in sediment and its bioaccumulation in food web in coastal wetlands. My results suggest that increasing salinity levels can significantly reduce mercury levels in sediment and attenuate its accumulation in biota in coastal plain wetlands