methylmercury cycling in high arctic wetland ponds...

Methylmercury Cycling in High Arctic Wetland Ponds: Controls onSedimentary ProductionIgor 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 beendemonstrated to biomagnify in Arctic freshwater foodwebs to levels that maybe of concern to Inuit peoples subsisting on freshwater fish, for example. Thekey process initiating the bioaccumulation and biomagnification of MeHg infoodwebs is the methylation of inorganic Hg(II) to form MeHg, and ultimatelyhow much MeHg enters foodwebs is controlled by the production andavailability of MeHg in a particular water body. We used isotopically enrichedHg stable isotope tracers in sediment core incubations to measure potentialrates of Hg(II) methylation and investigate the controls on MeHg productionin High Arctic wetland ponds in the Lake Hazen region of northern EllesmereIsland (Nunavut, Canada). We show here that MeHg concentrations insediments are primarily controlled by the sediment methylation potential andthe quantity of Hg(II) available for methylation, but not by sedimentdemethylation potential. Furthermore, MeHg concentrations in pond waters are controlled by MeHg production in sediments,overall anaerobic microbial activity, and photodemethylation in the water column.

■ INTRODUCTIONMercury (Hg) is a global pollutant with serious implications forhuman health.1 Of the different chemical forms of Hg thatoccur in the environment, methylmercury (MeHg) is the mosttoxic1 and the principal Hg species found in higher trophic levelorganisms because it readily biomagnifies through foodwebs.2

Therefore, the production of MeHg by the methylation ofHg(II) is the key process initially constraining the accumulationof Hg through aquatic foodwebs. Hg(II) methylation isprimarily mediated by anaerobic bacteria in aquatic environ-ments such as wetland and lake sediments (e.g., ref 3) or lakehypolimnia4 but can also occur in oxic marine waters.5

However, the biogeochemical controls on Hg(II) methylationand net MeHg production are poorly understood (see ref 6 fora review), despite the clear relevance of this information formitigating 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 activityof methylating microorganisms:7

= ·MeHg production (bioavailable Hg(II))

(microbial activity)

In turn, microbial activity governs the rate constants ofmethylation (km) and demethylation (kd), such that netmethylation can be expressed as

= · − ·k knet MeHg production [Hg(II)] [MeHg]m d

However, many factors can influence either, and often both,of these controlling parameters. For example, MeHg

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 microorganismssometimes dominate the microbial community at lower pH.9

Redox conditions, sulfate, dissolved organic matter (DOM) andtemperature are other important factors that control MeHgproduction.6 In sediments from sites contaminated with Hg,methylation appeared to be limited by the availability of organicmatter to methylating microorganisms,10 but in boreal peatlandpore-waters, the % of total Hg in the MeHg form (%MeHg, asurrogate measure of methylation activity) was negativelycorrelated with dissolved organic carbon (DOC) and positivelycorrelated with sulfate concentrations.11 Since sulfate-reducingbacteria are thought to be responsible for the majority of themicrobially produced MeHg,6 methylation is promoted whenredox conditions and growth substrate availability favor sulfatereduction. Furthermore, low sulfate concentrations can limitsulfate reduction and by extension Hg(II) methylation.12

However, at high sulfate concentrations, higher rates of sulfatereduction and increased sulfide concentrations decrease thebioavailability of Hg(II) due to the precipitation of HgS and/orthe formation of charged Hg−S complexes, which are lessbioavailable than neutral Hg−S complexes, thus inhibitingMeHg production.13,14 Other metals, such as iron, which can

Received: February 13, 2012Revised: May 23, 2012Accepted: July 10, 2012Published: July 16, 2012

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© 2012 American Chemical Society 10523 dx.doi.org/10.1021/es300577e | Environ. Sci. Technol. 2012, 46, 10523−10531

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also compete with Hg to bind sulfide, may moderate the extentto which sulfide controls Hg(II) bioavailability.MeHg contamination in aquatic foodwebs is of particular

concern in the Arctic, where Inuit peoples depend ontraditional country foods such as Arctic char (Salvelinusalpinus), which can have elevated MeHg concentrations, forsustenance.15,16 In a companion paper, we recently showed thatwetland ponds in the High Arctic can be important sources ofMeHg to local freshwater foodwebs, but that mass-balancederived estimates of MeHg production can vary significantlybetween sites with similar general characteristics (see ref 17,this issue), suggesting that certain biogeochemical factors play acrucial role in controlling net Hg(II) methylation and byextension MeHg bioaccumulation in these systems. Theobjective of this study was to quantify and compare potentialrates of Hg(II) methylation in intact sediment cores from eightwetland ponds to identify the biogeochemical factorscontrolling MeHg production.

■ MATERIALS AND METHODS

Site Description. The Lake Hazen region, located onnorthern Ellesmere Island within Quttinirpaaq National Park(Figure 1), experiences anomalously warm summer conditionsfor its latitude due to its location on the lee side of the GrantLand Mountains. For example, mean July air temperaturerecorded at Lake Hazen camp (81°49′ N, 71°20′ W) was 6 °C,with average daily minimum/maximum temperatures of 2 and10 °C, respectively. The summer melt period extends for 8−10weeks, resulting in a greater diversity and abundance ofvegetation compared to surrounding areas18 even though theregion is considered a polar desert, receiving only ∼95 mm ofprecipitation annually.19 Wetland complexes in the vicinity ofthe Lake Hazen camp exhibit characteristics consistent withboth marshes and shallow water wetlands and were generallycharacterized by a central pond surrounded by a wet sedgemeadow plant community composed of water sedge (Carexaquatilis), cotton grass (Eriophorum spp.), two-flowered rush

Figure 1. Location of sampling sites (lakes, ponds and streams, see Supplementary Tables S2 and S3 for site characteristics) in the Lake Hazenregion, northern Ellesmere Island, NU, in the Canadian High Arctic.

Environmental Science & Technology Article

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(Juncus biglumis), alpine foxtail (Alopecurus alpinus), and arcticwillow (Salix arctica). Vegetation within the pond includedMare’s tail (Hippuris vulgaris), floating buttercup (Ranunculushyperboreus), and semaphore grass (Pleuropogon sabinei), as wellas aquatic mosses (e.g., Calliergon giganteum). Ponds weregenerally shallow (maximum depth usually <1 m) and thereforemost freeze to the bottom during the winter and are thusfishless or only provide seasonal habitat for fish at times whensome of the ponds are hydrologically connected to Lake Hazen,either directly or via streams. The limnology of ponds in theLake Hazen region has previously been described,20 andancillary data on the sampled ponds and streams are given inTable 1.Survey of Ponds, Lakes, and Streams. Unfiltered surface

water samples for MeHg and THg analyses were collected inacid-cleaned Teflon bottles from a total of 15 ponds, two lakes,and four streams in July 2005. A small number of these siteswere resampled in July of 2007 and 2008. Additional THg andMeHg samples were collected from a subset of sites and filteredusing a Nalgene MF75 series filter-unit equipped with a 0.45μm cellulose nitrate membrane precleaned with a 1% HClsolution.Methylation/Demethylation Assays in Intact Sedi-

ment Cores. Gross potential rates of Hg(II) methylationand MeHg demethylation in sediments were quantified using a

method modified from (refs 3 and 12) by injecting enriched Hgstable-isotope tracers, instead of radiotracers, into intactsediment cores at the onset of core incubations. Potentialrates were quantified at eight different sites (Ponds 1, 2, 3, 7, 9,12, 14 and Skeleton Lake), on one or two independent datesbetween June 23 and July 22, 2007 (except for Pond 2 wheremeasurements were also performed on July 8, 2005), for a totalof 10 site−time combinations. At each site, triplicate cores werecollected in 5 cm diameter clear Lexan core tubes equippedwith injection ports (small holes sealed with a rubber/siliconeseptum) distributed vertically at 1-cm intervals. Stock solutionsof 198Hg(II) and Me199Hg spike were diluted in filtered pondwater and allowed to equilibrate for 1 h prior to injection.Methylation and/or demethylation of the spike duringequilibration, although not measured, is expected to havebeen negligible. The oxic conditions of the solution are notconducive to methylation, and furthermore, separate experi-ments showed that MeHg was stable in filtered pond water forup to 1 week when kept in the dark.17 Immediately aftersediment collection, the spike solution was injected into coresat 1-cm intervals, starting 1 cm above the sediment−waterinterface down to a depth of 4 cm. The addition of 77 ng of198Hg(II) and 0.25 ng Me199Hg into each 1 cm intervalincreased the ambient THg and MeHg concentrations in thesediments by 0.5−60%, but usually by less than 20%. Cores

Table 1. Summary of Water Chemistry Parameters from Sampled Ponds and Streamsa

site date temp (°C) pH Alk (mg L−1) EC (μS cm−1) TDS (mg L−1) DOC (mg L−1)PC

(μg L−1)Chl. a

(μg L−1)CO2

(% sat.)

Pond 1 2005 avg 13.9 540 342 13.5 816 0.93 45Pond 1 06/28/07 13.3 9.06 91.0 261 12.9 558 1.12 28Pond 1 07/11/07 14.4 9.14 90.7 427 290 13.9 527 0.89 28Pond 1 07/18.07 11.5 9.14 92.8 418 316 15.4 569 0.69 22Pond 1B 07/07/07 86.4 849 14.5 548 0.57 66Pond 2 07/08/05 14.2 160 91 8.2 1229 1.70 19Pond 2 07/08/07 11.4 8.75 59.7 157 99 7.9 501 0.43 47Pond 3 07/13/07 13.8 8.83 151.2 399 275 18.1 451 0.96 50Pond 4 07/14/07 12.8 8.43 96.3 372 231 4.7 153 <0.15 94Pond 7 07/9/07 14.0 8.42 235.6 1386 1278 40.6 2615 1.80 179Pond 9 07/12/07 14.9 8.45 220.6 876 566 36.0 565 0.25 326Pond 12 07/16/07 13.4 8.58 96.6 774 555 13.6 303 0.21 69Pond 14 07/05/07 11.1 8.49 108.4 533 330 5.9 412 0.50 128Skeleton Lake 07/19/07 11.6 8.57 101.9 384 237 5.7 157 0.21 92

site dateNH4

+

(μg L−1)NO3

−

(μg L−1)TDN

(μg L−1)PN

(μg L−1)TP

(μg L−1)TDP

(μg L−1)Cl−

(mg L−1)SO4

2‑

(mg L−1)pCH4(μatm)

Pond 1 2005 avg 28 4 878 73.5 14 10 1.9 138.2 304Pond 1 06/28 70 3 1110 51.2 16 14 1.1 98.9 204Pond 1 07/11 80 3 1190 28.0 32 12 1.2 107.6 145Pond 1 07/18 39 2 1410 37.9 35 12 1.3 112.8 125Pond 1B 07/07 32 3 1030 23.6 37 9 2.6 510.3 ndPond 2 07/08/05 10 2 540 83.5 17 16 0.59 12.3 453Pond 2 07/08 65 3 722 23.6 10 8 1.2 16.3 55Pond 3 07/13 39 3 1360 27.3 18 12 6.0 47.5 163Pond 4 07/14 35 4 322 8.1 5 3 0.7 86.7 94Pond 7 07/9 53 2 1970 88.6 17 12 22.0 629.6 n.d.Pond 9 07/12 47 2 2150 25.7 14 12 7.6 241.4 547Pond 12 07/16 109 4 960 15.4 12 11 0.2 24.2 38Pond 14 07/05 27 2 440 32.4 14 5 1.7 175.9 ndSkeleton Lake 07/19 31 7 349 9.8 5 4 0.4 37.0 36aParameters include temperature, pH, alkalinity (Alk), conductivity (EC), total dissolved solids (TDS), dissolved organic carbon (DOC), particulatecarbon (PC), chlorophyll a (Chl. a), carbon dioxide saturation (CO2), ammonium (NH4

+), nitrate/nitrite (NO3−), total dissolved nitrogen (TDN),

particulate nitrogen (PN), total phosphorus (TP), total dissolved phosphorus (TDP), chloride (Cl−), sulfate (SO42‑) and partial pressure of

dissolved methane (pCH4) (n.d. = not detectable).



were incubated in the field, inside a cooler, filled with site waterto maintain in situ temperature, for 4 h prior to extrudingsediments at 2-cm intervals. Sediment sections were collectedin whirlpack bags and frozen within 1 h of core extrusion usinga generator-powered small field freezer. The amount of 198Hg-(II) injected into each section was quantified post-incubationusing mass spectrometry (see Sample Analysis below).Methylation rates (km, % d−1) were then calculated as theproportion of added 198Hg(II) methylated to Me198Hg, dividedby incubation time. Demethylation rates (kd, d−1) werecalculated using first-order decay kinetics:

= −⎛⎝⎜⎜

⎞⎠⎟⎟k

t1

lnMeHg

MeHgt

d0

where MeHg0 and MeHgt refer to the concentrations at thestart and end, respectively, of the incubation with a duration oftime = t (MeHg0 is estimated from the theoretical amount ofMe199Hg injected into the sediment). These rates areconsidered potential rates as the added tracers may have adifferent chemical speciation and/or sediment−water partition-ing and may therefore be more reactive than ambient Hg(II)and MeHg. However, measuring potential rates is useful tocompare Hg(II) methylation activity between sites andinvestigate factors that might control methylation.Sample Analysis. All analyses were performed at the

University of Alberta Biogeochemistry Analytical ServiceLaboratory (BASL). Sediment samples were freeze-dried priorto analysis. All sediment and water samples were distilled andanalyzed for MeHg using isotope-dilution gas chromatographyinductively coupled plasma mass spectroscopy (GC-ICP-MS).21,22 Water samples collected in 2005 were analyzed forMeHg using cold vapor atomic fluorescence (CVAFS)detection instead of ICP-MS detection.23 THg concentrationsin water samples were quantified by BrCl oxidation, SnCl2reduction, gold trap amalgamation, and CVAFS detection.24

THg in sediment samples was quantified similarly, with theaddition of an acid digestion step prior to analysis as previouslydescribed for plant tissues25 and detection of Hg isotope ratiosby ICP-MS.26 Additional details about QA/QC and thestandard THg and MeHg analytical protocols employed areprovided in the Supporting Information.

■ RESULTS AND DISCUSSIONSurvey of Ponds, Lakes, and Streams. Concentration of

MeHg in unfiltered pond waters collected in July 2005 from theLake Hazen region ranged from 0.04 to 1.5 ng L−1, with amedian of 0.21 ng L−1 (Figure 2, Supplementary Table S4).THg concentrations ranged from 0.4 to 3.7 ng L−1 (median =1.4 ng L−1), and %MeHg ranged from 4% to 53%, with amedian value of 19%. Sites that were resampled in July of 2007and 2008 generally had concentrations similar to the 2005values, with the exception of Ponds 1 and 2, which had lowerMeHg concentrations in 2008 due to the flooding of these twowetlands with Lake Hazen water containing very lowconcentrations of MeHg.17 Comparison of Hg species infiltered and unfiltered water samples revealed that ∼65% ofMeHg and ∼76% of THg was in the filtered (dissolved)fraction. Similar to the findings reported by St. Louis et al.27

smaller ponds (<6500 m2), relative to larger ponds and lakes,tended to have higher MeHg median concentrations (0.32 vs0.08 ng L−1, p = 0.027, Mann−Whitney U-test, α = 0.05) and %MeHg (21% vs 9%, p = 0.043). MeHg and %MeHg in streams

were also lower than values measured in wetland ponds.Typically, oxic freshwaters in temperate ecosystems have %MeHg values of ∼10%,7 and therefore the high %MeHg valuesreported here can be interpreted as an indication thatsubstantial Hg(II) methylation is occurring in wetland pondsin the Lake Hazen region. Some high MeHg concentrationshave also been reported in other Arctic wetlands fromEllesmere, Cornwallis, and Devon Islands,28,29 although mostfreshwater systems, including ponds and wetlands, in theCanadian Arctic Archipelago are ultraoligotrophic and thereforehave low MeHg concentrations (see ref 29 for example).

Water Chemistry. Ponds in the Lake Hazen region arecharacterized by alkaline pH values, high DOC concentrations,and low suspended chlorophyll concentrations (Table 1) asproductivity is dominated by submerged macrophytes andmosses and emergant plants rather than planktonic algae. Pondwaters tend to be undersaturated in CO2 with respect toatmospheric equilibrium concentrations, meaning that dis-solved CO2 was being drawn down to support in-pond primaryproduction. However, dissolved CH4 concentrations were high,

Figure 2. Concentrations of MeHg (A), THg (B), and proportion ofTHg in the MeHg form (%MeHg, C) in unfiltered waters from ponds,lakes, and streams surveyed in 2005, 2007, and 2008 in the LakeHazen region.



indicative of the high levels of anaerobic respiration taking placein the sediments of these systems.Methylation/Demethylation Rates in Sediments. Gross

rates of Hg(II) methylation (km, d−1) were measured in three

sediment cores collected at separate random locationsthroughout each pond; in total, methylation assays werepreformed for 10 unique pond/date combinations. Because allsites shared similar general characteristics, such as climate,vegetation, geology, and atmospheric inputs of Hg, comparisonof methylation potentials measured at these sites can provideinsights into factors controlling MeHg production. The pondsinvestigated ranged in surface area from 400 to 19000 m2, andkm varied almost as much within each pond (coefficient ofvariation, CV = 40% on average) as it did among differentponds (CV = 58%), demonstrating that there is significantspatial heterogeneity within each pond with respect to Hg(II)methylation (Supplementary Table S5). Furthermore, thedifference in variability within and among ponds was notstatistically significant (1-way ANOVA, p = 0.43, α = 0.05). Forexample, in Skeleton Lake, km measured in one of the cores wasabout five times higher than in the other two cores, probablydue to the presence of a microbial mat overlaying sediments inthat particular core. A high degree of spatial heterogeneity inporewater MeHg concentrations and the presence ofmethylation hotspots have also been reported in temperateboreal wetlands.11 Furthermore, km was significantly greater inthe upper layer of sediments than deeper down (km = 7.1 ±6.0% d−1 for 0−2 cm layer vs 2.9 ± 2.1% d−1 for 2−4 cm layer,paired t test p = 0.0003, N = 29, α = 0.05) (Figure 3A),consistent with the principle that methylation activity is greatestnear the sediment−water interface (e.g., ref 6), where redoxconditions are favorable and fresh organic matter is moreplentiful. Demethylation rates (kd), on the contrary, varied littleamong sites but, like km, were also significantly greater near thesediment−water interface (kd = 2.3 ± 2.8 vs 1.7 ± 2.7 d−1 forthe 0−2 cm and 2−4 cm sediment sections, respectively; pairedt test p = 0.02, N = 29, α = 0.05) (Figure 3b). However,demethylation rates are difficult to quantify accurately becausethe loss of MeHg during the incubation was small compared tothe background MeHg spike concentrations, and it should benoted that ∼1/3 of measured kd were just below the detectionlimit for quantifying demethylation (estimated as 3 × the spikeinjection precision of 5%).Measured values of km were relatively high compared to

values obtained in other ecosystems, demonstrating thatmicrobial communities in High Arctic wetlands are efficientat methylating Hg(II), at least during the summer season whenour measurements were taken. For example, previouslypublished methylation potentials measured in intact sedimentcores averaged (±SD) 2 ± 2% and 0.2 ± 0.2% d−1 inproductive and unproductive freshwater systems in Sweden,10

2% d−1 (range 0−12% d−1) at various sites in the FloridaEverglades,13 and ranged from ∼1.5 to 10% d−1 in Alaskantundra lakes.30 However, km values as high as 36.5% d−1 havebeen reported for some wetlands of the San Francisco BayDelta.31 However it should be noted that differences inmethodology, such as the amount of Hg(II) tracer added, spikeequilibration procedures, incubation conditions, and thethickness of sediment horizons investigated, can makecomparison of km values across studies difficult to interpretand probably account in part for the difference in measured kmvalues between ecosystems.

Using a multiple regression model, we determined thatambient MeHg concentrations in the 0−2 cm layer ofsediments were correlated with both km (partial r2 = 0.17)and sediment Hg(II) concentrations (calculated as THg −MeHg, partial r2 = 0.34), demonstrating that methylation rates,equal to the product of km and Hg(II), control MeHgconcentrations in sediments (overall model: p = 0.00002, adjR2 = 0.47, Table 2). This is consistent with previous findingsthat in situ methylation (i.e., km) controls sediment MeHgconcentrations, as opposed to demethylation or externalinputs.10,13,31 However, our regression model explains only47% of the variability in sediment MeHg concentrations,suggesting that additional factors, such as bioavailability ofHg(II), are also important in controlling MeHg concentrationsin sediments. For example, km values in Pond 7 were close tothe overall median, indicating a higher potential for Hg(II)methylation than might have been predicted from the low %MeHg measured in these sediments (Supplementary Table S5).This suggests that Hg(II) in Pond 7 sediments was not asreadily available for methylation, possibly because it is eitherbound to organic matter (Pond 7 sediments had a very highorganic matter content as determined by % Loss on Ignition; %LOI, Supplementary Table S5) or reduced sulfur ligands. Pond7 had the highest water column sulfate concentrations of all theponds surveyed, and it has previously been observed that high

Figure 3. Methylation (A) and demethylation (B) potentials in pondsediments; note the different units used for km and kd. The horizontaldashed lines represent the overall median value.



sulfate concentrations can inhibit Hg(II) methylation bystimulating sulfate reduction in sediments and thus theproduction of reduced sulfur that can bind Hg(II) to forminsoluble and/or non-bioavailable HgS complexes.14 Hg(II)also appeared to limit methylation in Pond 14, where km and %MeHg were close to average, suggesting that the potential forMeHg production was not low, but that the low sedimentMeHg concentrations were the result of low sediment Hg(II)concentrations and availability. Across all sites, %MeHg wascorrelated with km (partial R2 = 0.63, p < 0.00001, Table 2),consistent with a number of other studies conducted in eitherfreshwater or marine sediments10,13,32 and resulting in %MeHgsometimes being used as a proxy for methylation activity.11,32

Demethylation potential (kd) was negatively correlated tosediment MeHg, as might be expected, but only when %MeHg,instead of MeHg concentration, was used as the dependentvariable in the multiple regression analysis, and even then kdonly accounted for a small portion of the variability in sediment%MeHg (partial R2 = 0.04, Table 2). This suggests thatdemethylation, while still an important MeHg sink, may not bean important factor in explaining among-core and among-sitevariability in ambient MeHg concentrations.The organic matter content of the sediments (measured as %

LOI), which provides substrate for microbial respiration andaffects Hg speciation/binding, did not correlate with eitherambient MeHg concentration in sediments, km or kd. This isperhaps surprising because it has previously been postulatedthat primary production and the availability of organic mattercontrols MeHg production in estuaries,32 wetlands,31 andfreshwater/brackish sediments.10 For example, Windham-Myers et al.31 demonstrated that the experimental removal ofemergent wetland vegetation led to decreased MeHgproduction in sediments. However, %LOI is probably not anideal proxy for labile carbon, which is more likely to controlmethylation than bulk organic carbon. Furthermore, additive

models, such as multiple linear regression, do not capturepotential interactive effects of multiple variables, possiblyresulting in the complex role of organic matter beingoverlooked in this case. It should be noted that the studiesthat reported organic matter controls on rates of Hg(II)methylation10,31,32 all included brackish environments (estua-ries and salt marshes), where controls on MeHg productionmay be different than in true freshwater ecosystems. Forexample, it has been shown that the addition of organic carbonto peatland mesocosms did not on its own stimulate MeHgproduction because in carbon-rich peatlands, methylation is notlimited by organic matter (electron donor) but by theavailability of electron acceptors, such as sulfate.33

Finally, ambient MeHg concentration in the 2−4 cmsediment horizons did not correlate with any of the aboveparameters but could be predicted from ambient MeHgconcentrations measured in the 0−2 cm sediment sections (p= 0.0002, R2 = 0.41), again demonstrating that MeHg isprincipally produced near the sediment−water interface andthat newly produced Me198Hg measured in the 2−4 cmsediment layer is likely in part explained by down-core diffusionfrom the 0−2 cm layer where the 198Hg(II) was originallymethylated.

Factors Controlling MeHg in Pond Water. Todetermine factors controlling MeHg concentration in pondwater, multiple-regression models (Table 2) were constructedincorporating various water chemistry and biogeochemicalparameters (Table 1). Concentrations of both dissolved MeHg(MeHgD) and total MeHg (MeHgT; measured in unfilteredwater) in pond water were positively and significantlycorrelated to dissolved Hg(II) concentrations in the watercolumn (calculated as THg − MeHg), suggesting again thatMeHg production is in part limited by Hg(II) availability inthese systems. This correlation might also suggests that bothHg(II) and MeHg are mobilized proportionately to the water

Table 2. Regression Model Parametersa

dependent (modeled) variable independent (predictive) variables standard coefficient p-value partial R2 overall adjusted R2 regression p-value

sediment MeHg (0−2 cm) km (0−2 cm) 0.496 0.0017 0.17 0.469 0.0001sediment Hg(II) (0−2 cm) 0.642 0.00012 0.34

sediment %MeHg (0−2 cm) km (0−2 cm) 0.799 <0.00001 0.63 0.650 <0.00001kd (0−2 cm) −0.205 0.078 0.04

sediment MeHg (0−4 cm) MeHg (0−2 cm) 0.644 0.0002 0.42 0.415 0.0002water MeHgD Hg(II)D 0.133 0.00086 0.77 0.769 0.00086water MeHgD Hg(II)T 0.787 0.0069 0.62 0.619 0.0069water MeHgT Hg(II)D 0.843 0.0022 0.68 0.675 0.0022water MeHgT Hg(II)T 0.713 0.021 0.51 0.509 0.021water %MeHgD MeHg (0−2 cm) 0.597 0.0009 0.45 0.966 0.001

pCH4 0.594 0.0008 0.22NH4

+:NO3− 0.374 0.0071 0.10

PC −0.372 0.0049 0.07UV-A exposure −0.239 0.028 0.15

water %MeHgT MeHg (0−2 cm) 0.408 0.103 0.32 0.659 0.047pCH4 0.486 0.060 0.15NH4

+:NO3− 0.586 0.038 0.30

PC −0.406 0.105 0.06UV-A exposureb 0.00

aPartial R2 values refer to the variability explained by each independent variable in the model, while the overall adjusted R2 is the variability explainedby the model as a whole. Multiple regressions were performed in a stepwise manner with an entry/removal threshold of p = 0.15. Results wereusually consistent whether forwards or backwards stepwise regression was used. (MeHgD = dissolved MeHg in the water column; MeHgT = totalMeHg in the water column; Hg(II)D = dissolved Hg(II) in the water column; Hg(II)T = total mercury in all forms in the water column; Hg(II)calculated as THg-MeHg). bRemoved from model by stepwise regression.



column from a common source, such as the sediments. Tocontrol for the influence of Hg(II), %MeHg instead of MeHgconcentration was used as the dependent variable in theregression models discussed below.%MeHgD in pond water was positively correlated to

sediment MeHg in the 0−2 cm horizon, the ratio ofammonium to nitrate (NH4

+:NO3−), and partial pressure of

dissolved methane (pCH4) and negatively correlated toparticulate carbon and average exposure to UV-A radiation,and together these variables explained a very large portion ofthe variance in %MeHgD (adj R2 = 0.96, p = 0.001). SedimentMeHg concentrations account for almost half of the variabilityin %MeHgD in the water column (partial r2 = 0.45).Considering that sediment MeHg concentration is only aproxy for the flux of MeHg across the sediment−water interfaceand that transport rates and processes (turbulent mixing vsdiffusion) are also likely to vary across ponds, the degree towhich sediment MeHg concentrations explain %MeHgD valuesin the water column supports the hypothesis that sediments arethe most important source of MeHg in these High Arcticwetland ponds. However, in temperate/boreal lakes, Hg(II)methylation can also take place in the water column,4 inperiphyton biofilms,34 and in moss mats,35,36 and these sourcesmay also have contributed to the MeHg pool in pond water. Itis also possible that at other times of the year, sources such assnowmelt runoff27,28 may also be important contributors to theMeHg pool in these ponds. Non-sediment MeHg sources werenot measured here but should be investigated in the future.Both NH4

+:NO3− and pCH4 can be interpreted as proxies to

gauge the relative importance of anaerobic (i.e., denitrificationand methanogenesis) versus aerobic (nitrification and methaneoxidation) microbial processes. These results suggest thatHg(II) methylation is greater in wetlands sustaining greaterlevels of anaerobic microbial respiration with relatively limitedpotential for oxidation. Microbial MeHg demethylation is alsothought to be favored over methylation under aerobic andoxidative conditions,6 and this is supported by the observationthat ponds with the lowest aqueous MeHg concentrations(Ponds 7 and 14) also had undetectable concentrations ofdissolved CH4 (Table 1), suggesting that conditions favorableto methane oxidation are also conducive to demethylation.Therefore, pond waters with higher NH4

+:NO3− ratios and

higher pCH4 resulting from the dominance of low redox(anaerobic) biogeochemical processes are likely to be sites witha higher ratio of methylation to demethylation. An inverserelationship between concentrations of nitrate and MeHg hasalso been observed in hypolimnetic waters of a temperatelake.37 Because nitrate is thermodynamically favored oversulfate as an electron acceptor during microbial respiration oforganic matter, higher nitrate concentrations can lead todecreased activity of sulfate-reducing bacteria and by extensiondecreased rates of Hg(II) methylation. It has also beenhypothesized that nitrate regulates the release of MeHg fromsediments by controlling the oxidation state of iron andmanganese. MeHg can adsorb to iron/manganese oxyhydr-oxides in the sediments; however, in the absence of nitrate,those complexes can be reduced, releasing any adsorbed MeHgto overlying waters.37 The correlation of %MeHg with pCH4also suggests that methanogenic microorganisms are possiblycontributing to the water column pool of MeHg, as they haverecently been discovered to be able to methylate Hg(II).38

UV-A radiation is primarily responsible for the photo-demethylation of MeHg,39 explaining the observed negative

correlation between UV-A and MeHgD. Therefore, MeHg indeeper ponds with higher concentrations of UV-absorbingDOC will have a longer lifetime in the water column, due to themore efficient attenuation of UV-A with depth, resulting indecreased exposure to UV-A. Finally, the negative correlation of%MeHgD with particulate carbon is explained by the fact thatboth MeHg and Hg(II) can adsorb onto particles and bind toorganic matter. Therefore, particulate carbon can decreaseMeHg in pond water by (i) limiting the amount of Hg(II)available for methylation and (ii) scavenging MeHg from thewater column as it deposits to the sediments. Better model fitswere obtained with MeHgD than MeHgT (adj R2 = 0.66, p =0.047), suggesting that sediments contribute to the water-column pool of MeHg primarily via the diffusion of MeHgDacross the sediment−water interface, rather than by the exportof particulate-bound MeHg during sediment resuspension.It is interesting to note that neither DOC nor sulfate

concentrations correlated with MeHg concentrations in thewater column. The availability of labile carbon has beenreported to control MeHg production (see discussion above),and DOC concentrations are often positively correlated withMeHg concentrations in freshwaters (e.g., ref 40). However,DOC may also have a mitigating effect by complexing Hg(II)and decreasing its bioavailability to methylating microorgan-isms,41,42 explaining why porewater %MeHg was negativelycorrelated with DOC in a boreal peatland.11 Sulfate-reducingbacteria are thought to play an important role in themethylation of Hg(II),6 and it has been repeatedly demon-strated that the addition of sulfate to sediments and overlyinglake waters12 as well as wetlands33 stimulates MeHgproduction. However, there are also examples in the literaturewhere km was either not correlated10 or inversely correlatedwith sulfate concentrations.13 Clearly, the relationship betweensulfate concentration and Hg(II) methylation is complex andnonlinear. At low sediment porewater sulfate concentrations(up to 200−500 μM), sulfate and methylation rates arepositively correlated,2 but at higher sulfate concentrations andsulfate reduction rates, the resulting high sulfide concentrationscauses Hg(II) speciation to switch from a dominance ofneutrally charged Hg−S complexes (e.g., HgS0) with high cellmembrane permeability to non-bioavailable charged Hg−Scomplexes,14 leading to reductions in methylation rates.13 Thelack of a relationship between MeHg and sulfate concentrationsin this study likely reflects the fact that sulfate concentrations(130−1200 μM) were close to optimal for Hg(II) methylation,except in Pond 7 (>6500 μM) as discussed above, meaning thatsulfate concentrations were probably neither limiting sulfatereduction nor present in large enough quantities to result inexcess sulfide formation.Finally, the effect of other potentially important factors

controlling MeHg production, such as temperature and pH,6

could not be tested because of the narrow temperature and pHrange observed among wetland ponds in which we measuredpotential methylation rates. For example, pH ranged from 8.4to 9.1 in the study ponds, and mean surface water temperaturesfrom June 27−July 16, 2007 ranged from 10.9 to 11.6 °C, withthe exception Skeleton Lake (7.7 °C, HOBO pendanttemperature and light loggers, model UA-002, Onset ComputerCompany), which had the lowest water column MeHgconcentrations of the sites included in this analysis. However,previous MeHg measurements made in lakes and ponds ofnorthern Ellesmere Island revealed higher concentrations insmaller, warmer ponds compared to larger, colder ponds and



lakes,27 suggesting that net MeHg production is greater athigher water temperatures, consistent with previous findings.6

Higher temperatures generally stimulate microbial activity,resulting in increased Hg(II) methylation during the summerseason;43 however, demethylation rates may also be higher athigher temperatures,44 meaning that the effect of temperature isunclear and may be site-dependent.Ecological Significance. We demonstrate here that MeHg

concentrations in High Arctic pond waters are controlled by theproduction of MeHg in sedimentsitself controlled by acombination of factors including methylation potential, THgconcentrations in sediments, and the anaerobic microbialdecomposition of organic matteras well as photodemethyla-tion in the water column. This conceptual framework isconsistent with the results from MeHg mass-balance studiesperformed in two ponds from the Lake Hazen region17 and inAlaskan tundra lakes,30 both of which also demonstrated thatthe most important source of MeHg to these systems is internalproduction rather than external inputs and that photo-demethylation is the principal sink for MeHg. Moreimportantly, mass-balance estimates of MeHg production inthese High Arctic wetland ponds are comparable to temperateand boreal freshwater systems,17 at least during the shortsummer season, which coincides with the time that Arcticaquatic organisms are most active. MeHg produced in thesesystems can become bioaccumulated and biomagnified in localand downstream aquatic foodwebs because these wetlandsprovide habitat for zooplankton and numerous shorebird andwaterfowl species that have been observed to feed on aquaticinvertebrates from these ponds, as well as juvenile Arctic charwhen some of these ponds become connected to Lake Hazen atthe end of summer during the seasonal peak in lake waterlevels.17 Furthermore, MeHg production is likely to increase inresponse to climate change. The open-water season is predictedto lengthen, increasing the period of peak methylation activity,and increased permafrost degradation may increase both theabundance of wetland ponds and the release of Hg(II) frompermafrost.45 The Arctic landscape is dotted with a very largenumber of wetland ponds, and therefore these systems areimportant biogeochemical reactors controlling the cycling ofcarbon, nutrients and contaminants, such as Hg, in thecircumpolar region.

■ ASSOCIATED CONTENT

*S Supporting InformationAdditional analytical details, discussion on the role of iron inHg(II) methylation, and 5 data tables. This material is availablefree of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

Present Addresses‡Department of Earth and Environmental Sciences, Universityof Waterloo.§Aquatic Ecosystem Protection Research Division, Environ-ment Canada.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We are grateful to Sara Berkel, Megan Hawkins, Caroline Lee,Jennifer Graydon, and Kahled Al-Badani for assistance duringsample preparation, processing and analysis. We also thankCharlene Nielsen for preparing Figure 1. This research wasfunded through a Natural Science and Engineering ResearchCouncil (NSERC) Discovery Grant to V.St.L., and theNorthern Scientific Training Program (Aboriginal Affairs andNorthern Development Canada), Circumpolar/Boreal AlbertaResearch Grant (Canadian Circumpolar Institute), NSERCPost-Graduate Doctoral Scholarship and Alberta IngenuityStudentship to I.L. Logistical support was provided by the PolarContinental Shelf Program (Natural Resources Canada).

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