Fate of mercury in the Arctic marine environments: sources and transport of total mercury and methylmercury in the Arctic shelf, fjord, and sea ice

Abstract

The fate of mercury (Hg) in the Arctic ecosystem has been a crucial issue due to the elevated accumulation of methylmercury (MeHg) in Arctic organisms. However, the sources, sinks, and speciation of Hg in Arctic marine environments remains unclear. In this work, the mass fluxes of Hg and MeHg were investigated in diverse Arctic environments, such as the Kongsfjorden, the East Siberian shelf and slope, and sea ice on the Chukchi Plateau. In the first study on the Kongsfjorden, we identified the major sources and sinks of THg and MeHg in seawater using Spreadsheet-based Ecological Risk Assessment for the Fate of Mercury (SERAFM) model based on measured total Hg (THg) and MeHg concentrations in seawater and exterior sources. These results revealed that major THg sources are tidal inflow and tidewater glacial runoff, while major MeHg sources are tidal outflow and in situ methylation in shallow halocline water, which were consistent with the THg and MeHg distribution trends. Further, we predicted future changes in Hg distribution and found that inorganic Hg(II) concentration in seawater mainly increases in response to the tidewater glacier retreating in the fjord. In the second study, we investigated major inputs and outputs of MeHg in seawater through the mass budget of MeHg in the East Siberian Sea (ESS), using the measured MeHg concentrations and related environmental parameters in seawater and sediment. This result highlights that the benthic diffusion and resuspension exceed other sources (e.g., atmospheric deposition and river water input), and the major sinks of MeHg are identified as dark demethylation and evasion. Extending it into the entire Arctic shelf system, annual MeHg diffusion from shelf sediments was estimated at 23,065 ± 39 mol yr–1, which was the third most important source of MeHg in the shelf water; hence, we concluded that MeHg input from shelf sediments in the Arctic Ocean was more significant than previously estimated. In the third study, we explored the spatial distribution of THg and MeHg in surface sediment of the ESS. The THg concentrations in surface sediment exhibited an increasing trend from inner sites (0.25 ± 0.025 nmol g-1) into outer sites (0.51 ± 0.11 nmol g-1) and from eastern sites (0.23–0.28 nmol g-1) into western sites (0.33–0.37 nmol g-1). Through THg distribution along the latitudinal transect, we confirmed that the transport and deposition of suspended particulate matter through ocean currents determines the spatial distribution of THg in the ESS via hydrodynamic sorting by particle size. On the contrary, THg concentrations in the longitudinal transect appear to be affected by terrestrial input from the multiple rivers on the Russian coast with the higher THg in the western sites. The spatial distribution of MeHg and %MeHg/THg exhibited a distinct pattern compared to THg. It was noticeable that an increase of %MeHg/THg was found at the sties of 50–60 m isobaths, which could be the result of enhanced SRB activity at the transition zone between continuous and discontinuous permafrost. To prove this hypothesis, we analyzed sediment organic matter (OM) composition obtained by FT-ICR-MS, pore water DOM composition by EEM fluorescence, and microbial community structures based on the 16S rRNA and hgcA gene. The high levels of MeHg and %MeHg/THg were tightly related to the high protein and lipid content in sediment, and high humic fraction in the pore water, suggesting that transition isobaths are enriched with microbial activities sediment OM. Further, 16S rRNA and hgcA analysis results indicated that the major putative Hg methylaing bacterial group in the ESS could be Delataproteobacteria, dominated by sulfate (or sulfur) reducing bacteria. However, most hgcA sequences were unaffiliated with known species: novel Hg(II) methylators appear to be abundant in the ESS. Overall, the results newly suggest that the promoted activities of SRB in the surface sediment of the permafrost transition zone actively methylate Hg(II) to MeHg, while they accumulate refractory humic DOM in the pore water. Finally, we observed Hg concentrations in the sea ice, melt pond, snow, and under-ice water on the Chukchi Plateau along with associated environmental variables. First-year ice (FYI) from ice camp 2 exhibited THg maxima in the mid layer, implying evasion of Hg(0) through the deteriorated ice cavity. Meanwhile, multiyear ice (MYI) from the ice camp 1 showed typical THg maxima at the top layer due to the atmospheric deposition of Hg. Further, a positive correlation was found between %MeHg and Chl-a in the middle and bottom layers of FYI and MYI, suggesting that in situ Hg(II) methylation could be the significant source of MeHg in the sea ice. The estimation of enrichment factors in sea ice, melt pond, and snow pack further revealed that additional Hg input (e.g., atmospheric deposition) largely affected Hg distributions in sea ice and snowpack. The yearly Hg influx from sea ice to the Arctic Ocean decreased for MYI (e.g., 18.7 Mg yr-1 in 1984 into 6.7 Mg yr-1 in 2020 for THg) and increased for FYI (e,g., 0.73 Mg yr-1 in 1984 into 2.1 Mg yr-1 in 2020 for THg) over the last 37 years, which was consistent with the sea ice regime shift from MYI to FYI. It was also revealed that the amount of THg input to seawater through sea ice melting is comparable to the sediment diffusion, while the quantities of MeHg input to seawater from sea ice melting are insignificant compared to other sources (such as in situ methylation, coastal erosion, rivers, and sediment diffusion). To summarize, we improved our understanding of Hg dynamics in the Arctic marine environment by estimating diverse fluxes of THg and MeHg in the Arctic shelf, fjord, and sea ice.