Article pubs.acs.org/est
Methylmercury Cycling in High Arctic Wetland Ponds: Controls on Sedimentary Production Igor Lehnherr,*,†,‡ Vincent L. St. Louis,† and Jane L. Kirk†,§ †
Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada S Supporting Information *
ABSTRACT: Methylmercury (MeHg) is a potent neurotoxin that has been demonstrated to biomagnify in Arctic freshwater foodwebs to levels that may be of concern to Inuit peoples subsisting on freshwater fish, for example. The key process initiating the bioaccumulation and biomagnification of MeHg in foodwebs is the methylation of inorganic Hg(II) to form MeHg, and ultimately how much MeHg enters foodwebs is controlled by the production and availability of MeHg in a particular water body. We used isotopically enriched Hg stable isotope tracers in sediment core incubations to measure potential rates of Hg(II) methylation and investigate the controls on MeHg production in High Arctic wetland ponds in the Lake Hazen region of northern Ellesmere Island (Nunavut, Canada). We show here that MeHg concentrations in sediments are primarily controlled by the sediment methylation potential and the quantity of Hg(II) available for methylation, but not by sediment demethylation potential. Furthermore, MeHg concentrations in pond waters are controlled by MeHg production in sediments, overall anaerobic microbial activity, and photodemethylation in the water column.
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INTRODUCTION Mercury (Hg) is a global pollutant with serious implications for human health.1 Of the different chemical forms of Hg that occur in the environment, methylmercury (MeHg) is the most toxic1 and the principal Hg species found in higher trophic level organisms because it readily biomagnifies through foodwebs.2 Therefore, the production of MeHg by the methylation of Hg(II) is the key process initially constraining the accumulation of Hg through aquatic foodwebs. Hg(II) methylation is primarily mediated by anaerobic bacteria in aquatic environments such as wetland and lake sediments (e.g., ref 3) or lake hypolimnia4 but can also occur in oxic marine waters.5 However, the biogeochemical controls on Hg(II) methylation and net MeHg production are poorly understood (see ref 6 for a review), despite the clear relevance of this information for mitigating the issue of Hg contamination in aquatic foodwebs. Conceptually, methylation rates can be thought of as being a function of the quantity of bioavailable Hg(II) and the activity of methylating microorganisms:7
production has been reported to be higher at lower pH values, either because Hg(II) bioavailability increases as pH decreases8 or because certain types of Hg(II) methylating microorganisms sometimes dominate the microbial community at lower pH.9 Redox conditions, sulfate, dissolved organic matter (DOM) and temperature are other important factors that control MeHg production.6 In sediments from sites contaminated with Hg, methylation appeared to be limited by the availability of organic matter to methylating microorganisms,10 but in boreal peatland pore-waters, the % of total Hg in the MeHg form (%MeHg, a surrogate measure of methylation activity) was negatively correlated with dissolved organic carbon (DOC) and positively correlated with sulfate concentrations.11 Since sulfate-reducing bacteria are thought to be responsible for the majority of the microbially produced MeHg,6 methylation is promoted when redox conditions and growth substrate availability favor sulfate reduction. Furthermore, low sulfate concentrations can limit sulfate reduction and by extension Hg(II) methylation.12 However, at high sulfate concentrations, higher rates of sulfate reduction and increased sulfide concentrations decrease the bioavailability of Hg(II) due to the precipitation of HgS and/or the formation of charged Hg−S complexes, which are less bioavailable than neutral Hg−S complexes, thus inhibiting MeHg production.13,14 Other metals, such as iron, which can
MeHg production = (bioavailable Hg(II)) · (microbial activity)
In turn, microbial activity governs the rate constants of methylation (km) and demethylation (kd), such that net methylation can be expressed as
Received: Revised: Accepted: Published:
net MeHg production = k m·[Hg(II)] − kd·[MeHg]
However, many factors can influence either, and often both, of these controlling parameters. For example, MeHg © 2012 American Chemical Society
10523
February 13, 2012 May 23, 2012 July 10, 2012 July 16, 2012 dx.doi.org/10.1021/es300577e | Environ. Sci. Technol. 2012, 46, 10523−10531
Environmental Science & Technology
Article
Figure 1. Location of sampling sites (lakes, ponds and streams, see Supplementary Tables S2 and S3 for site characteristics) in the Lake Hazen region, northern Ellesmere Island, NU, in the Canadian High Arctic.
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MATERIALS AND METHODS Site Description. The Lake Hazen region, located on northern Ellesmere Island within Quttinirpaaq National Park (Figure 1), experiences anomalously warm summer conditions for its latitude due to its location on the lee side of the Grant Land Mountains. For example, mean July air temperature recorded at Lake Hazen camp (81°49′ N, 71°20′ W) was 6 °C, with average daily minimum/maximum temperatures of 2 and 10 °C, respectively. The summer melt period extends for 8−10 weeks, resulting in a greater diversity and abundance of vegetation compared to surrounding areas18 even though the region is considered a polar desert, receiving only ∼95 mm of precipitation annually.19 Wetland complexes in the vicinity of the Lake Hazen camp exhibit characteristics consistent with both marshes and shallow water wetlands and were generally characterized by a central pond surrounded by a wet sedge meadow plant community composed of water sedge (Carex aquatilis), cotton grass (Eriophorum spp.), two-flowered rush
also compete with Hg to bind sulfide, may moderate the extent to which sulfide controls Hg(II) bioavailability. MeHg contamination in aquatic foodwebs is of particular concern in the Arctic, where Inuit peoples depend on traditional country foods such as Arctic char (Salvelinus alpinus), which can have elevated MeHg concentrations, for sustenance.15,16 In a companion paper, we recently showed that wetland ponds in the High Arctic can be important sources of MeHg to local freshwater foodwebs, but that mass-balance derived estimates of MeHg production can vary significantly between sites with similar general characteristics (see ref 17, this issue), suggesting that certain biogeochemical factors play a crucial role in controlling net Hg(II) methylation and by extension MeHg bioaccumulation in these systems. The objective of this study was to quantify and compare potential rates of Hg(II) methylation in intact sediment cores from eight wetland ponds to identify the biogeochemical factors controlling MeHg production. 10524
dx.doi.org/10.1021/es300577e | Environ. Sci. Technol. 2012, 46, 10523−10531
Environmental Science & Technology
Article
Table 1. Summary of Water Chemistry Parameters from Sampled Ponds and Streamsa site
date
temp (°C)
pH
Alk (mg L−1)
Pond 1 Pond 1 Pond 1 Pond 1 Pond 1B Pond 2 Pond 2 Pond 3 Pond 4 Pond 7 Pond 9 Pond 12 Pond 14 Skeleton Lake
2005 avg 06/28/07 07/11/07 07/18.07 07/07/07 07/08/05 07/08/07 07/13/07 07/14/07 07/9/07 07/12/07 07/16/07 07/05/07 07/19/07
13.9 13.3 14.4 11.5
9.06 9.14 9.14
91.0 90.7 92.8 86.4
site
date
14.2 11.4 13.8 12.8 14.0 14.9 13.4 11.1 11.6 NH4+ (μg L−1)
Pond 1 Pond 1 Pond 1 Pond 1 Pond 1B Pond 2 Pond 2 Pond 3 Pond 4 Pond 7 Pond 9 Pond 12 Pond 14 Skeleton Lake
2005 avg 06/28 07/11 07/18 07/07 07/08/05 07/08 07/13 07/14 07/9 07/12 07/16 07/05 07/19
28 70 80 39 32 10 65 39 35 53 47 109 27 31
8.75 8.83 8.43 8.42 8.45 8.58 8.49 8.57 NO3− (μg L−1) 4 3 3 2 3 2 3 3 4 2 2 4 2 7
EC (μS cm−1)
TDS (mg L−1)
540
13.5 12.9 13.9 15.4 14.5 8.2 7.9 18.1 4.7 40.6 36.0 13.6 5.9 5.7 TDP (μg L−1) 10 14 12 12 9 16 8 12 3 12 12 11 5 4
TDN (μg L−1)
PN (μg L−1)
342 261 290 316 849 91 99 275 231 1278 566 555 330 237 TP (μg L−1)
878 1110 1190 1410 1030 540 722 1360 322 1970 2150 960 440 349
73.5 51.2 28.0 37.9 23.6 83.5 23.6 27.3 8.1 88.6 25.7 15.4 32.4 9.8
14 16 32 35 37 17 10 18 5 17 14 12 14 5
427 418 160 157 399 372 1386 876 774 533 384
59.7 151.2 96.3 235.6 220.6 96.6 108.4 101.9
PC (μg L−1)
Chl. a (μg L−1)
CO2 (% sat.)
816 558 527 569 548 1229 501 451 153 2615 565 303 412 157 Cl− (mg L−1)
0.93 1.12 0.89 0.69 0.57 1.70 0.43 0.96
