Evidence for Regulation of Monomethyl Mercury by Nitrate in a Seasonally Stratified, Eutrophic Lake

Abstract

The accumulation of monomethyl mercury (CH3Hg+) in aquatic ecosystems is a redox sensitive process that is accelerated under sulfate-reducing conditions. While nitrate (NO3-) reduction is energetically favored over sulfate reduction, the influence of NO3 on the accumulation of CH3Hg+ has not been reported in the literature. We examined temporal and vertical patterns in redox constituents and CH3Hg+ concentrations in the hypolimnion of a dimictic lake, Onondaga Lake, prior to and following increases in NO3- inputs. Detailed water-column profiles and a long-term record revealed marked decreases in the accumulation of CH3Hg+ in the anoxic hypolimnion coinciding with long-term decreases in the deposition of organic matter coupled with recent increases in NO3-concentrations. CH3Hg+ concentrations in the hypolimnion were substantially abated when No3 was present above the sediment-water interface. A decrease in the peak hypolimnetic mass of CH3Hg+ and shortening of the period of elevated CH3Hg+ concentrations resulted in more than a 50% decline in the accumulated CH3Hg+. N03- regulation of CH3Hg+ accumulation may be a widespread phenomenon in oxygen-limited freshwater and terrestrial environments, and could have an important notpreviously recognized, effect on the biogeochemistry of mercury.

Recent advances in the study of mercury methylation in aquatic systems

“…In an eutrophic and hypolimnetic reservoir in Idaho sediments that were rich in organic matter, low in dissolved oxygen, under redox conditions, and with increased nutrient content were suggested to lead to elevated methylation rates compared with other systems (Gray and Hines 2009). This is in contrast to other recent studies that have observed negative correlations of nitrate and MeHg concentrations in the water column of (Todorova et al 2009). A mesocosm study of estuarine sediments that were amended with various levels of N and P loadings to surface water showed that increased nutrients led to increased phytoplankton biomass productivity with stimulated microbial activity in sediments with Hg tracers that were considered highly bioavailable.…”

“…In oligotrophic systems, nitrogen availability was positively correlated with MeHg concentration and %MeHg, suggesting that in some systems methylation may be stimulated by N availability (Braaten et al 2014). Eutrophic conditions are also believed to influence methylation (Todorova et al 2009), though this may be due to the enhancement of methylation in the presence of organic matter. In an eutrophic and hypolimnetic reservoir in Idaho sediments that were rich in organic matter, low in dissolved oxygen, under redox conditions, and with increased nutrient content were suggested to lead to elevated methylation rates compared with other systems (Gray and Hines 2009).…”

Linkage between community diversity of sulfate-reducing microorganisms and methylmercury concentration in paddy soil

“…Most Hg in soil is detected as ionic Hg 2+ species and shows relatively low toxicity, however, it still poses some environmental risk due to potential for Hg methylation followed by bioamplification in soilplant systems (Meng et al 2010;Liu et al 2012). Earlier studies have examined microbial methylation of Hg in estuarine, wetland, and freshwater sediments (Gilmour et al 1992; Heyes et al 2006;Duran et al 2008;Todorova et al 2009), but little attention to date has been paid to the linkage between specific types of microorganism and Hg methylation in soil environments. Since some studies reported high MeHg contents in both of soil and rice in the Wanshan Hg mine area of Guizhou province in southwest China (Feng et al 2008;Zhang et al 2010), concerns have been raised about the fate of Hg and potential production of MeHg in the local soil-rice system.…”

Simulated watershed mercury and nitrate flux responses to multiple land cover conversion scenarios

“…Several small catchment studies suggest diverse catchmentscale response (loadings and/or concentrations) of total mercury (HgT), MeHg, and N r (as NO À 3 , NO 3 -N, or NO 2 -NO 3 ) to a variety of anthropogenically altered landscapes, for example, focusing on how NO À 3 and MeHg are processed in a lake [22], in standing water behind beaver dams [23], and in surface waters in catchments affected by forest fires [24,25]. Important questions remain, however, particularly in watersheds larger and with greater land cover diversity than that of the standard small catchment study, i.e., the spatial scale and land cover types at which most decision-making processes take place.…”