Methylated Mercury Species in Canadian High Arctic Marine Surface Waters and Snowpacks

Abstract

We sampled seawater and snowpacks in the Canadian high Arctic for methylated species of mercury (Hg). We discovered that, although seawater sampled under the sea ice had very low concentrations of total Hg (THg, all forms of Hg in a sample; on average 0.14-0.24 ng L(-1)), 30-45% of the THg was in the monomethyl Hg (MMHg) form (on average 0.057-0.095 ng L(-1)), making seawater itself a direct source of MMHg for biomagnification through marine food webs. Seawater under the ice also contained high concentrations of gaseous elemental Hg (GEM; 129 +/- 36 pg L(-1)), suggesting that open water regions such as polynyas and ice leads were a net source of approximately 130 +/- 30 ng Hg m(-2) day(-1) to the atmosphere. We also found 11.1 +/- 4.1 pg L(-1) of dimethyl Hg (DMHg) in seawater and calculated that there could be a significant flux of DMHg to the atmosphere from open water regions. This flux could then resultin MMHg deposition into nearby snowpacks via oxidation of DMHg to MMHg in the atmosphere. In fact, we found high concentrations of MMHg in a few snowpacks near regions of open water. Interestingly, we discovered a significant log-log relationship between Cl- concentrations in snowpacks and concentrations of THg. We hypothesize that as Cl- concentrations in snowpacks increase, inorganic Hg(II) occurs principally as less reducible chloro complexes and, hence, remains in an oxidized state. As a result, snowpacks that receive both marine aerosol deposition of Cl- and deposition of Hg(II) via springtime atmospheric Hg depletion events, for example, may contain significant loads of Hg(II). Overall, though, the median wet/dry loads of Hg in the snowpacks we sampled in the high Arctic (5.2 mg THg ha(-1) and 0.03 mg MMHg ha(-1)) were far below wet-only annual THg loadings throughout southern Canada and most of the U.S. (22-200 mg ha(-1)). Therefore, most Arctic snowpacks contribute

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

“…[145,146] Another possible source of MeHg is the atmospheric photodegradation of volatile DMHg evaded from seawater and lakes. [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.…”

“…The effect of this net deposited Hg from snow melt on THg and MeHg concentrations in receiving waters (rivers, lakes, oceans) is the subject of ongoing research. Recent studies [18][19][20] focussed on the fate in marine systems of Hg from snowpacks during and following snowmelt determined that melt runoff was not a substantial contributor to the THg or MeHg budgets of Arctic seawater. In freshwater, however, significant increases in THg fluxes to sediments during the 20th century indicate that deposited atmospheric Hg has had an effect on Hg levels in lake sediments and, by extension, on freshwater Hg budgets in the Arctic.…”

“…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.…”

“…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. [18,19] By examining the predominant form of Hg as it enters the Arctic and then passes through aquatic systems (marine and fresh water) to eventually accumulate in top predators, it is clear that two transformations play crucial roles. First, the relatively benign Hg 0 circulating globally in air or water must be oxidised to Hg II and second, Hg II must be methylated and accumulated in top predators (Fig.…”