Sediment organic carbon and temperature effects on methylmercury concentration: A mesocosm experiment.

The fate and mobility of mercury, and its bioaccumulation primarily as methylmercury (MeHg), in marine ecosystems are influenced by climate related environmental factors, including increased temperature and carbon loading. To investigate the interactions between sediment organic carbon and temperature MeHg bioaccumulation, mesocosm experiments were conducted examining relationships between sediment, water column and biota (sediment-dwelling amphipod and juvenile oyster) MeHg concentration. Experimental treatments consisted of a two by two design of high and low temperature (15 & 25 °C) and high and low sediment organic carbon (4-5% and 13% LOI, pre-experiment). Sediment organic carbon had significant individual effects on MeHg concentration in water and biota, with higher carbon associated with lower MeHg. Temperature individual effects were significant for sediment, water, and only amphipod MeHg concentration, with higher temperature treatments indicating higher MeHg concentration. There were significant temperature × carbon interactions observed for sediment, dissolved, and oyster MeHg concentration. Sediment carbon reduction had greater influence than temperature on increasing MeHg concentrations in both the water column and biota. MeHg concentrations in the bulk sediment were not correlated with MeHg in the water column or in the biota, indicating that even when sediments are the only source of MeHg, bulk sediment measurements do not provide a good proxy for bioaccumulation and that the concentration in bulk sediments is not the primary determinant of MeHg entry into the food web.

Organic carbon content drives methylmercury levels in the water column and in estuarine food webs across latitudes in the Northeast United States.

Estuaries are dynamic ecosystems which vary widely in loading of the contaminant methylmercury (MeHg), and in environmental factors which control MeHg exposure to the estuarine foodweb. Inputs of organic carbon and rates of primary production are important influences on MeHg loading and bioaccumulation, and are predicted to increase with changes in climate and land use pressures. To further understand these influences on MeHg levels in estuarine biota, we used a field study approach in sites across different temperature regions, and with varying organic carbon levels. In paired comparisons of sites with high vs. low organic carbon, fish had lower MeHg bioaccumulation factors (normalized to water concentrations) in high carbon sites, particularly subsites with large coastal wetlands and large variability in dissolved organic carbon levels in the water column. Across sites, MeHg level in the water column was strongly tied to dissolved organic carbon, and was the major driver of MeHg concentrations in fish and invertebrates. Higher primary productivity (chlorophyll-a) was associated with increased MeHg partitioning to suspended particulates, but not to the biota. These findings suggest that increased inputs of MeHg and loss of wetlands associated with climate change and anthropogenic land use pressure will increase MeHg concentrations in estuarine food webs.