Methylated Mercury Species in Marine Waters of the Canadian High and Sub Arctic

Abstract

Distribution of total mercury (THg), gaseous elemental Hg(0) (GEM), monomethyl Hg (MMHg), and dimethyl Hg (DMHg) was examined in marine waters of the Canadian Arctic Archipelago (CAA), Hudson Strait, and Hudson Bay. Concentrations of THg were low throughout the water column in all regions sampled (mean +/- standard deviation; 0.40 +/- 0.47 ng L(-1)). Concentrations of MMHg were also generally low atthe surface (23.8 +/- 9.9 pg L(-1)); however at mid- and bottom depths, MMHg was present at concentrations sufficient to initiate bioaccumulation of MMHg through Arctic marine foodwebs (maximum 178 pg L(-1); 70.3 +/- 37.3 pg L(-1)). In addition, at mid- and bottom depths, the % of THg that was MMHg was high (maximum 66%; 28 +/- 16%), suggesting that active methylation of inorganic Hg(II) occurs in deep Arctic marine waters. Interestingly, there was a constant, near 1:1, ratio between concentrations of MMHg and DMHg at all sites and depths, suggesting that methylated Hg species are in equilibrium with each other and/or are produced by similar processes throughout the water column. Our results also demonstrate that oceanographic processes, such as water regeneration and vertical mixing, affect Hg distribution in marine waters. Vertical mixing, for example, likely transported MMHg and DMHg upward from production zones at some sites, resulting in elevated concentrations of these species in surface waters (up to 68.0 pg L(-1)) where primary production and thus uptake of MMHg by biota is potentially highest. Finally, calculated instantaneous ocean-atmosphere fluxes of gaseous Hg species demonstrated that Arctic marine waters are a substantial source of DMHg and GEM to the atmosphere (27.3 +/- 47.8 and 130 +/- 138 ng m(-2) day(-1), respectively) during the ice-free season.

The fate of mercury in Arctic terrestrial and aquatic ecosystems, a review

“…[86] Little is known about this process in the Arctic; however, the Arctic Ocean does possess pervasive strong nutrient maxima below the polar mixed layer. [100] MeHg, both in terms of concentration and as a percentage of THg, has been found to be highest in the middle and bottom depths of Hudson Bay and in waters of the Canadian Arctic Archipelago, [101] This suggests the in situ production of MeHg previously reported [44,86,99] may also occur in subsurface Arctic seawaters.…”

“…In the global ocean, THg is usually found in the range of 0.02 to 0.5 ng L À1 [1,3,44,104] with Arctic seas or out flowing water from the Arctic possibly at the low end of this range. [1,19,101] A recent paper reported an estimate that the top 200 m of the Arctic Ocean had a THg inventory of 620 to 945 t (best estimate 945 t), which is far larger than estimated annual fluxes into or out of the surface Arctic Ocean, and this implies Hg residence times of 5 to 10 years. [36] Of the large inventory of Hg held in the surface Arctic Ocean, Hg II plays a central role due to its reactivity.…”

“…6) are poorly quantified in the Arctic, but recent work has shown them to be potentially important in polar surface waters. [18,19,99,101] Clearly, the processes controlling the production of MeHg and its entry into the bottom of the food web are fundamental to understanding Hg trends at all trophic levels of the marine food web. Like Hg 0 , DMHg evades readily into the atmosphere where rapid photolytic decomposition to MeHg followed by deposition has been proposed as the source of MeHg in snow and melt water ponds in coastal regions.…”

“…[16,18,19,147,148] Production of DMHg by pure cultures of Antarctic marine bacteria, [149] and by macroalgae isolated from an Arctic fjord, [150] has been demonstrated. Results from recent work on Arctic seawater [101] and many studies in the Atlantic and Pacific Oceans [43,86,106] suggest that DMHg can also be produced in deep marine waters. Supersaturation of Arctic surface seawater over a wide area by DMHg [19,29] and the presence of elevated GEM in the marine boundary layer in areas rich in sea ice [26] point to the potential importance of this process in the Arctic MeHg cycle.…”

“…[245] In contrast, it is now well established that the Hg deposited during AMDEs can be readily re-emitted from the snowpack during winter conditions following AMDEs [7,16,25,80,101,187,241,[246][247][248] and during snow metamorphism and melt. [20,79] Establishing a link between AMDEs and enhanced Hg levels in Arctic biota is complicated partly by a poor understanding of the net outcome of the AMDE deposition pathway, and partly by the complex Hg biogeochemistry of aquatic marine and fresh water ecosystems that contain inorganic Hg II from a variety of sources (of which AMDEs are but one) and methylate the inorganic Hg II into MeHg.…”