Mercury speciation and retention in a salt marsh undergoing long-term fertilization

Experimental plots in Great Sippewissett Marsh (Falmouth, MA USA) have been undergoing long-term (> 48 years) fertilization through the application of commercial sewage sludge-based fertilizer. The experimental treatment focuses on the effect of added nitrogen on the salt marsh plots, but also supplies mercury (Hg) and other metals. This experiment provides a unique opportunity to test hypotheses regarding the Hg-related response of coastal marine ecosystems to eutrophication as well as assess the efficacy of salt marshes as sinks for increased loadings of Hg to the coastal zone. Hg inventories in sediments of control plots were similar to loadings from atmospheric deposition and inventories in the fertilized plots closely reflected the estimated loadings of Hg contained in the added fertilizer. In both the control and fertilized plots, distribution of Hg appeared somewhat different than the history of loadings, implying some level of Hg mobility. The relative abundance of monomethylmercury (CH3Hg+) within the plots varied with the amount of fertilizer applied with the highest percentage of Hg as CH3Hg+ found in the control plots, and the lowest percentages of CH3Hg+ and S were measured in plots fertilized at the highest dose. The results from this marsh suggest that eutrophication indirectly lowers CH3Hg+ production in this particular ecosystem, but perhaps not as a result of the sequestration of Hg(II) with S.