Methylmercury Cycling in High Arctic Wetland Ponds: Controls on Sedimentary Production (original) (raw)
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Methylmercury Cycling in High Arctic Wetland Ponds: Controls on production
The sources of methylmercury (MeHg; the toxic form of mercury that is biomagnified through foodwebs) to Arctic freshwater organisms have not been clearly identified. We used a mass balance approach to quantify MeHg production in two wetland ponds in the Lake Hazen region of northern Ellesmere Island, NU, in the Canadian High Arctic and to evaluate the importance of these systems as sources of MeHg to Arctic foodwebs. We show that internal production (1.8−40 ng MeHg m −2 d −1 ) is a much larger source of MeHg than external inputs from direct atmospheric deposition (0.029−0.051 ng MeHg m −2 d −1 ), as expected. Furthermore, MeHg cycling in these systems is dominated by Hg(II) methylation and MeHg photodemethylation (2.0−33 ng MeHg m −2 d −1 ), which is a sink for a large proportion of the MeHg produced by Hg(II) methylation in these ponds. We also show that MeHg production in the two study ponds is comparable to what has previously been measured in numerous more southerly systems known to be important MeHg sources, such as temperate wetlands and lakes, demonstrating that wetland ponds in the High Arctic are important sources of MeHg to local aquatic foodwebs.
Scientific Reports, 2018
Mercury (Hg) is a contaminant of major concern in Arctic marine ecosystems. Decades of Hg observations in marine biota from across the Canadian Arctic show generally higher concentrations in the west than in the east. Various hypotheses have attributed this longitudinal biotic Hg gradient to regional differences in atmospheric or terrestrial inputs of inorganic Hg, but it is methylmercury (MeHg) that accumulates and biomagnifies in marine biota. Here, we present high-resolution vertical profiles of total Hg and MeHg in seawater along a transect from the Canada Basin, across the Canadian Arctic Archipelago (CAA) and Baffin Bay, and into the Labrador Sea. Total Hg concentrations are lower in the western Arctic, opposing the biotic Hg distributions. In contrast, MeHg exhibits a distinctive subsurface maximum at shallow depths of 100-300 m, with its peak concentration decreasing eastwards. As this subsurface MeHg maximum lies within the habitat of zooplankton and other lower trophic-level biota, biological uptake of subsurface MeHg and subsequent biomagnification readily explains the biotic Hg concentration gradient. Understanding the risk of MeHg to the Arctic marine ecosystem and Indigenous Peoples will thus require an elucidation of the processes that generate and maintain this subsurface MeHg maximum. Monitoring data collected during the past four decades have shown Hg concentrations in Canadian Arctic marine mammals (e.g., beluga whales, ringed seals, polar bears) to be highly elevated, frequently exceeding toxicity thresholds 1,2. This has raised major concerns over the health of marine mammals and Indigenous Peoples whose traditional diets include marine mammal tissues. Mercury concentrations in marine biota are generally higher in the Beaufort Sea and western Canadian Arctic Archipelago (CAA) than in the eastern CAA and Baffin Bay 1-3. This longitudinal gradient is not limited to apex predators 4,5 , but extends to organisms at lower trophic levels such as zooplankton (e.g., Themisto spp., Calanus spp.) 6 (Fig. 1a). Whereas regional variations in top predator Hg concentrations may be linked to feeding behavior and dietary preference 5 , observed spatial patterns persist after adjustments are made to account for trophic position 3. Extensive efforts have been made to identify factors that control the spatial trends in marine biota and to develop appropriate mitigation strategies to reduce biotic Hg concentrations. Most hypotheses attribute higher marine biotic Hg concentrations in the western Canadian Arctic to elevated inputs of inorganic Hg to these regions. These inputs include (1) atmospheric deposition of anthropogenic Hg from Asian sources 3 , which is enhanced locally by atmospheric mercury depletion events (AMDEs) during polar sunrise 7 ; (2) riverine Hg input from the Mackenzie River 8,9 , which may be enhanced by tundra uptake of atmospheric elemental Hg 10 [and permafrost thawing 11 ; and (3) a naturally high geological background of Hg 12. These inorganic Hg-based hypotheses do not account for the fact that it is methylmercury (MeHg), not inorganic Hg, that accumulates and biomagnifies in marine biota 2. The discovery of a subsurface MeHg enrichment in global oceans 13-16 suggests that seawater MeHg may play a more important role in determining marine biotic Hg concentrations 17,18 , especially in regions such as the Beaufort Sea 15 and the central Arctic Ocean 16 where the maximum MeHg concentration was observed at shallow depths.
Methylated Mercury Species in Canadian High Arctic Marine Surface Waters and Snowpacks
Environmental Science & Technology, 2007
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 ∼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 result in 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 Clconcentrations in snowpacks and concentrations of THg. We hypothesize that as Clconcentrations 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 Cland 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 relatively little to marine pools of both Hg(II) and MMHg at snowmelt.
Methylated Mercury Species in Marine Waters of the Canadian High and Sub Arctic
Environmental Science & Technology, 2008
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 at the 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 midand 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
Environmental …, 2012
Environmental context. Mercury, in its methylated form, is a neurotoxin that biomagnifies in marine and terrestrial foodwebs leading to elevated levels in fish and fish-eating mammals worldwide, including at numerous Arctic locations. Elevated mercury concentrations in Arctic country foods present a significant exposure risk to Arctic people. We present a detailed review of the fate of mercury in Arctic terrestrial and marine ecosystems, taking into account the extreme seasonality of Arctic ecosystems and the unique processes associated with sea ice and Arctic hydrology.
Mercury in the sediments of freshwater lakes in Ny-Ålesund, Arctic
Environmental Monitoring and Assessment, 2020
Mercury and its speciation in aquatic ecosystems have been assessed globally. Even though previous studies were limited to Arctic freshwater lakes, they are highly significant in the context of the changing climate. The present study is based on sediment samples collected from three Arctic freshwater lakes over a period of 4 years (2015-2018). The samples were analysed for total mercury (THg), methyl mercury (MHg), and various mercury fractions. The observed mean THg and MHg concentrations were 22.23 ng/g and 0.41 ng/g respectively; these values were comparable with those for other Arctic freshwater lakes. The mercury content significantly varied among the years as well as among the lakes. Changes in snowdrift and meltwater inputs, which are the major sources of water for the lakes, may have influenced the sediment mercury content along with geographical location and increased productivity. The results of MHg indicated the susceptibility of lake sediments to methylation. The major fractions observed were the organo-chelated form of mercury, followed by the elemental and water-soluble forms. These results indicate the availability of mercury for methylation. Hence, it is necessary to conduct more studies on the influence of climate change, mercury release through permafrost melting, and atmospheric deposition.
The Science of the total environment, 2015
In temperate regions of Canada, mercury (Hg) concentrations in biota and the magnitude of Hg biomagnification through food webs vary between neighboring lakes and are related to water chemistry variables and physical lake features. However, few studies have examined factors affecting the variable Hg concentrations in landlocked Arctic char (Salvelinus alpinus) or the biomagnification of Hg through their food webs. We estimated the food web structure of six high Arctic lakes near Resolute Bay, Nunavut, Canada, using stable carbon (δ(13)C) and nitrogen (δ(15)N) isotopes and measured Hg (total Hg (THg) in char, the only fish species, and methylmercury (MeHg) in chironomids and zooplankton) concentrations in biota collected in 2010 and 2011. Across lakes, δ(13)C showed that benthic carbon (chironomids) was the dominant food source for char. Regression models of log Hg versus δ(15)N (of char and benthic invertebrates) showed positive and significant slopes, indicting Hg biomagnification ...
Environmental Chemistry, 2010
Environmental context. Mercury is a global contaminant that has entered Arctic food webs in sufficient quantity to put at risk the health of top predators and humans that consume them. Recent research has discovered a photochemical process unique to the Arctic that leads to mercury deposition on frozen surfaces after polar sunrise, but the connection between mercury deposition and entry into food webs remains tenuous and poorly understood. We propose here that the Arctic Ocean's sensitivity to the global mercury cycle depends far more on neglected post-deposition processes that lead to methylation within the ice-ocean system, and the vulnerability of these processes to changes occurring in the cryosphere.