factors that influence methylmercury flux rates from wetland sediments

ent 368 (2006) 306–319www.elsevier.com/locate/scitotenv

Science of the Total Environm

Factors that influence methylmercury flux ratesfrom wetland sediments

Jonathan Holmes ⁎, David Lean

Biology Department, University of Ottawa, 150 Louis Pasteur, P.O. Box 450, Station A, Ottawa, ON, Canada K1N 6N5

Received 15 October 2004; received in revised form 11 November 2005; accepted 29 November 2005Available online 10 January 2006

Abstract

Sediments are thought to be an important source of methylmercury (MeHg) to the water column of wetlands. We measuredsediment MeHg pore water concentrations as a function of depth in four wetlands to determine the concentration gradient and usedit determine sediment–water flux of MeHg. Fluxes of MeHg ranged from −1.60 to 10.02 ng m−2 day−1 and were shown to be afunction of 1) redox conditions at the sediment–water interface, 2) oxygen gradient above the sediment surface, 3) watertemperature, and 4) pore water and water column-dissolved sulphide. MeHg water column concentration in each of the fourwetlands was positively correlated with MeHg concentrations present in surface sediment and pore water, and with the calculatedsediment–water MeHg flux rate.

In addition to MeHg, ethylmercury (EtHg) was detected in the sediment in all four wetlands, but not in the pore water or thewater column. EtHg levels in sediment exceeded MeHg concentrations in two of the wetlands. This demonstrates that Hgethylation is a significant part of the Hg cycle in some aquatic environments.© 2005 Elsevier B.V. All rights reserved.

Keywords: Methylmercury; Temperate wetlands; Pore water; Diffusive flux; Sulfide; Ethylmercury; Redox potential

1. Introduction

Bioaccumulation of Hg in aquatic food chains ismostly as MeHg. Too often researchers have onlymeasured total mercury and no clear patterns have beenseen between dissolved total mercury and levels inorganisms. In addition, MeHg is by far the most toxicform of mercury. Consequently, the factors controllingHg methylation is at the centre of the puzzle of why ouraquatic food chains are mercury contaminated. MeHgconcentration in water is the determining factors for Hgconcentration in aquatic organisms (Morel et al., 1998),but we are not certain how much of the dissolved MeHg

⁎ Corresponding author. Tel.: +1 613 739 3900.E-mail address: [email protected] (J. Holmes).

0048-9697/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.scitotenv.2005.11.027

is indeed bioavailable. It is widely held that thesediment–water interface is the dominant site for Hgmethylation (Gilmour et al., 1992; Ramlal et al., 1993;Krabbenhoft et al., 1998). As such, factors that influencerates of Hg methylation and the exchange of MeHg fromsediments to the water column are the most relevant tothe potential concentrations of MeHg found in biota.The rates of Hg methylation in sediments have oftenbeen found to coincide with conditions for sulphatereduction. This includes high levels of organic matter(Callister and Winfrey, 1986; Choi and Bartha, 1994;Pak and Bartha, 1998a,b; Hadjispyrou et al., 1998),appropriate reducing conditions (Korthals and Winfrey,1987; Regnell et al., 1996), and an adequate sulphateconcentration (Gilmour et al., 1992; Brandfireun et al.,1999). Collectively, observations are consistent with the

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307J. Holmes, D. Lean / Science of the Total Environment 368 (2006) 306–319

general thought that Hg methylation is predominantlylinked to the activities of sulphate-reducing bacteria(Compeau and Bartha, 1985; King et al., 2000).

In previous research, sediment pore water MeHgconcentrations have been used to estimate MeHgfluxes from the sediments to the water column inorder to assess the relative contribution of sediments tolake water (Hines et al., 2004) and coastal water (Gillet al., 1999; Covelli et al., 1999). Spatial and seasonalvariability in pore water concentrations and sedimentflux rates of MeHg observed in some of these studieshave been related to factors such as temperature,dissolved oxygen concentration and sulphate-reducingactivity. While in lake production of MeHg was shownto be the most significant source of MeHg in somelakes (e.g. Sellers et al., 2001), the value wasdetermined by a difference in the mass balance whenMeHg photodegradation was included. Perhaps othersystems may not have conditions conducive to in situformation of MeHg and external sources may play amuch larger role.

Wetlands are known to be important sources ofMeHg to downstream lakes and streams (St. Louis et al.,1994; Rudd, 1995). St. Louis et al. (1994) found thatcatchments containing peatlands yielded MeHg con-centrations 4 to 15 times greater than from uplandcatchments with no peatlands. The levels of MeHg inlakes and rivers have been shown to be correlated to thepercentage of wetland within the drainage basin (Mierleand Ingram, 1991; Driscoll et al., 1994; St. Louis et al.,1996).

While considerable work has focused on examiningthe factors affecting MeHg concentration in water andfish from lakes and rivers, relatively little attention hasbeen devoted to studying the factors influencingMeHg levels in temperate wetland waters. In thisstudy, we compare sediment pore water profiles ofMeHg and estimate fluxes of MeHg in four differentwetlands sampled in the summer of 2002. The fourmarshes are similar in pH and dissolved organiccarbon and differ in reducing conditions, nutrientlevels and type of lithosphere. The data were thenused to examine the relationship between MeHg fluxfrom sediments and abiotic parameters present in porewater (H2S, SO4), water column (temperature, Eh,DO2, SO4, Fe, Mn), and sediment (THg, organic andcarbonate content).

Most literature reports have focused on MeHg inenvironmental and biological samples but ethyl mercury(EtHg) has been reported in sediments from the St. ClairRiver (Jernelov and Wennegren, 1980), a mining area inSlovenia (Hintelmann et al., 1995) and the Florida

Everglades (Cai et al., 1997a). The lack of EtHg in datais attributed to analytical methods that were developedmainly to measure MeHg. In this study, the methodol-ogy implemented (Cai et al., 1996, 1997b), enablesmeasurement of MeHg and EtHg in environmental andbiological samples. This study is unique, in that it is thefirst Canadian study in which EtHg (as well as MeHg) isconsidered.

2. Methods

2.1. Study sites

Cooper's Marsh (45°6′39ʺN, 74°31′6ʺW) is ahighly eutrophied wetland located on the shore ofLake St. Francis, 15 km east of Cornwall, Ontario, onthe north shore of the St. Lawrence River. The 218 hamarsh was once a lowland area used for farming in the1780s but the area was flooded in the mid-1800s as aresult of control structures built to raise water levels toaid navigation along the St. Lawrence River. Morerecently, water levels were further increased by asystem of dykes and dams now controlled by theRaisin River Conservation Authority. The wetlandreceives drainage with high phosphorus and nitrogenconcentrations from a local golf course and intensefarming activity.

Baie St. Francois (46°6′18ʺN, 72°56′3ʺW) is aeutrophic wetland located on the South shore of Lake St.Pierre, a fluvial lake of the St. Lawrence River east ofMontreal, Quebec. The wetland receives runoff fromagricultural activity and mercury from natural sourcesvia the Yamaska River.

The wetland referred to as H15C1 (44°49′16ʺN,76°5′27ʺW) is a small mesotrophic wetland that feedsdirectly into Otter Lake, located approximately 15 kmsouth of Smiths Falls, Ontario.

Stillwoods wetland (45°4′9ʺN, 75°49′45ʺW) iswithin the Marlborough Forest Conservation Area, 5km west of North Gower, Ontario. It is a small wetlandsurrounded by mixed conifers and hardwood bush thatfeeds into Steven's Creek and ultimately the RideauRiver system.

Vegetation within all four wetlands is similar. Tallreed and rushes such as Typha and Phragmites are thedominant form of emergent vegetation, but grasses andsedges (Cyperus and Carex) are also present. Floatingsurface vegetation consisted mainly of water lily(Nymphaea and Numphar) and duckweed (Lemnaceae).

All sampling (water, pore water, and sediment) ineach of the wetlands was initiated from a non-metalliccanoe in June of 2002.

308 J. Holmes, D. Lean / Science of the Total Environment 368 (2006) 306–319

2.2. Mercury water sampling

The “clean hands, dirty hands” technique (St. Louiset al., 1994) was followed during sampling. Samples forMeHg analysis were collected in pre-acid washed 1 Lhigh density polyethylene (HDPE) bottles and preservedwith 5 mL ACS grade hydrochloric acid and stored in acooler for transport back to the laboratory. Organomer-cury concentrations were determined by capillary gaschromatography coupled with atomic fluorescencespectrometry (GC–AFS) as described by Cai et al.(1996). This method involves adsorbent pre-concentra-tion of the organomercurials from water samples ontosulfhydryl-cotton fibers followed by elution with acidicKBr and CuSO4 and extraction into dichloromethane.The method detection limit (MDL) was estimated as0.02 ng L−1 (three standard deviations of the loweststandard (n=12) divided by the slope of the calibration).Analysis of field and procedural blanks consisting of de-ionized water revealed no MeHg contamination duringsampling, extraction and analysis. Analytical recoveryspikes of de-ionized water (0.25 ng L− 1 MeHg)averaged 94±7% recoveries (n=20).

Total mercury (THg) samples were collected in 50mL high density polypropylene Falcon tubes that hasbeen pre-rinsed with Milli-Q de-ionized water andindividually sealed in a Ziploc bag. Samples werepreserved with 500 μL of bromine chloride reagent inthe field, stored double-bagged in Ziploc bags andtransported back to the lab in a cooler. Total mercurywas analyzed in water samples using dual gold trap pre-concentration and Cold Vapor Atomic Fluorescencespectroscopy (CVAFS). The analysis was conductedusing a Tekran 2600 system control module equippedwith a Tekran 2610 liquid handling module and Tekranmodel 2620 autosampler, following the modified U.S.EPA Method 1631 guideline for mercury analysis(Revision 1.20, July 2001). Analysis of field andprocedural blanks consisting of de-ionized waterrevealed no THg contamination during sampling,extraction and analysis. The MDL was estimated as0.25 ng L-1.

2.3. Ancillary constituents

Dissolved oxygen (DO2), temperature and redoxpotential were measured 30 cm below the water surfaceand at the sediment–water interface. pH was measuredin surface water.

Water samples for dissolved organic carbon (DOC)analysis were collected in 50-mL polyproplylene Falcontubes and filtered upon returning to the lab through 0.7-

μm Wheaton GF filter paper. DOC was determinedusing a carbon analyzer (O.I. Analytical Model 1010).The estimated MDL (n=21) was 0.7 mg L−1.

Water samples for the determination of totaldissolved Fe and Mn were collected in parallel in 60-mL HDPE Nalgene bottles with polypropylene screwclosures. Samples were filtered on site through a 0.45um Sterivex filter using a 50-mL polypropylene syringe.Samples were preserved with 0.2 mL of ultra high puritynitric acid and refrigerated upon returning to the labuntil analysis could be conducted. Analysis wasperformed using a Vista-Pro radial simultaneousinductively coupled plasma atomic emission spectro-photometer (ICP–AES). The estimated MDL (n=14)was 50 and 10 μg L−1 for Fe and Mn, respectively.Average recoveries for Fe from reference materialSLRS-4 (river water for trace metals, National ResearchCouncil-Canada) was 96±9%, respectively (n=16). Mnconcentration was below the detection limit. Averagerecoveries for Fe and Mn from reference materialTDMA 53.2 (National Water Research Institute, Envi-ronment Canada) were 108±8.2 and 108.4±7.0,respectively (n=9).

Water samples for dissolved sulphate analysis werecollected and filtered in the same manner as that used forcations, with the exception that samples were notpreserved with nitric acid but frozen until analysis.Sulphate concentrations were determined with theLachat Instruments QuickChem® 8000 Flow InjectionAnalyzer using QuickChem® method 10-116-10-1-C(MDL=0.5 mg L−1).

2.4. Sediment pore water sampling

Depth profiles for MeHg, sulphate and sulphide insediment pore water were measured in the four wetlandsin June 2002 using in situ dialysis membrane devices(peepers) of the type described by Carignan et al. (1985)and Carignan and Lean (1991). Each peeper wasconstructed of polycarbonate (Lexan) and consisted ofa 65×20×2.1 cm3 plate in which two rows of 38 cellseach with a volume of 4 mL were distributed down thelength of the peeper cell plate, and a 0.5 cm cover plate.The bottom edges of peepers were beveled at 45° tominimize disturbance when lowered into the sediments.Peeper cell and cover plates were cleaned by soaking in5–10% nitric acid at room temperature for 1 week andthen rinsed with de-ionized water. Each peeper cell wasfilled with de-ionized water and covered with a 0.22-μmhydrophilic polysulphone membrane (HT-200, Gelman)that was held in place over each cell by a 0.5-cmpolycarbonate cover plate that was screwed into the cell


plate using plastic screws. Each cell was then inspectedto ensure that no air bubbles or leaks were present. It iscritical to remove O2 from the peeper cells and frombubbles present in the plastic to avoid its slow releaseinto the cells that can alter the shape of the profiles ofredox-sensitive species such as sulphides (Carignan etal., 1994; Mason et al., 1998). Oxygen was purged fromfully assembled peepers by placing them inside a sealedPlexiglass chamber containing a N2 atmosphere. Theywere initially purged overnight with O2-free nitrogen(O2b0.5 ppm) and then for 15 min a day for 12 daysthereafter.

Peepers were transported to each wetland inside theN2-filled chambers and deployed as quickly as possible(b1 min) to minimize exposure time to the atmosphere.Two to three peepers were deployed per wetland andplaced about 20-cm apart. Peepers were retrieved fromsediments 15 days later and sampled in the field for theanalysis of pore water sulphide, sulphate, and MeHg.The number of peeper cells exposed above thesediment–water interface was recorded. Within 2 minof peeper retrieval, samples (1.5 mL) for sulphidedetermination were collected by piercing the peepermembrane with N2-purged syringes (polypropylene)and injected through Teflon septa into N2-purged amberglass vials (2 mL) containing Cline's reagent: 0.0054 Mp-phenylenediamine and 0.005 FeCl3, 40 mL each(Cline, 1969). Blanks were prepared in the field byinjecting 1 mL of Milli-Q water into vials containingCline's reagents. Samples and blanks were stored on icein a cooler. Sulphide concentration was determined onthe same day as sample collection with a Varian-Cary100 UV–Visible Spectrophotometer at 670 nm(MDL=0.5 μg L−1). No sulphide was detected inanalytical or field blanks.

Samples (1.0 mL) for sulphate determination werecollected from every second peeper cell by piercing thecell membrane with an Eppendorf pipette fitted with anacid cleaned plastic tip and transferring to a 4-mL HDPENalgene bottle containing 25 μL of concentrated HCl.Samples were refrigerated at 4 °C. Analysis wasperformed using the turbidity SO4 assay method asdescribed by Rodier (1975). Sample turbidity was readat 650 nm with a Varian-Cary UV–Visiblespectrophotometer.

In order to retrieve an adequate volume for MeHganalysis in pore water, water from peepers wascombined and pooled for every two consecutive peepercells at each depth. Pore water was transferred to a black60-mL HDPE Nalgene bottles to which 125 μL ofconcentrated HCl was added. Samples were stored in onice in a cooler. Upon return to the lab, samples were

stored at 4 °C until extraction and analysis could beperformed. MeHg was determined by GC–AFS asdescribed by Cai et al. (1996). The MDL for pore waterwas 0.4 ng L−1.

2.5. Sediment sampling

Sediment samples were obtained for the analysis ofMeHg, EtHg, THg, percent water, organic and carbon-ate content. Sediment cores were collected using 1.5-mLLexan (polycarbonate) coring tubes of 7.6 cm diameter.Core samples were taken to a depth of 20 cm whenpossible and sectioned on site at 2-cm intervals into 125-mL low-density polyethylene specimen cups. Coreswere transported to the lab on ice in a cooler andsubsequently frozen until analysis could be performed.

Organomercury species in sediment were extractedand analyzed by GC–AFS as described by Cai et al.(1997a,b). Recoveries from certified reference materialBCR 580 (Commission of the European Communities,Belgium) averaged 91±6% (n=5). No certified refer-ence materials currently exist for EtHg in sediments.Recoveries of MeHg and EtHg from sediment sampleswere assessed by spiking replicates with 0.75 ng L−1 ofEtHg and MeHg standard. MeHg and EtHg recoveriesaveraged 89±7% and 77±13% (n=10), respectively.

Percent water content was determined in sedimentcores by measuring weight loss of dried sediment after24 h at 100 °C. Sediment organic content was estimatedby measuring weight loss on ignition (LOI at 550 °C for1 h). Total carbonate in sediment was subsequentlyestimated by the additional weight loss after heating at1000 °C for 2 h. In each case, samples were stored in adesiccator as they were cooling before weighing.

2.6. Estimation of diffusive flux (J) of MeHg fromsediment pore water

Diffusive flux of MeHg at the sediment–waterinterface, in the absence of bioturbation and bioirriga-tion, is modeled on a modification of Fick's first law ofmolecular diffusion:

J ¼ w D d/

h 2

� �Cw−Cpw

� �Dx

where wD is the diffusion coefficient of MeHg in waterwithout the presence of sediment, Cw and Cpw is themaximum concentration of MeHg in overlying waterand in pore water, respectively, Δx is the distance (cm)separating Cw and Cpw, ϕ is the sediment porosity, and θis sediment tortuosity. Tortuosity is not readily measuredbut has been shown to be related to porosity, a parameter

Table 1Mean pH, below surface and bottom water oxygen, Eh and temperature measurements from wetlands

Location pH O2 (mg L−1) Eh (mV) Temperature (°C)

Watercolumn

Sediment/waterinterface

Watercolumn


Watercolumn


Cooper's Marsh 7.18 2.8 b0.5 95 −110 17.5 16.0H15C1 7.45 2.0 b0.5 45 −88 17.5 17.0Stillwoods 7.14 3.4 b0.5 −20 −81 17.0 17.0Baie St. Francois 7.90 10.5 b0.5 14 −7 24.0 20.0

Eh values are not corrected to standard hydrogen electrode.


that is easily measured as percent water content. For allflux calculations, tortuosity was calculated using thefollowing empirical relationship proposed by Boudreau(1996):

h2 ¼ 1−lnð/2Þ:

Estimating the diffusion of MeHg from pore watersediment requires knowledge of the major forms ofMeHg present and the respective diffusion coefficientsfor each MeHg species. At present this is not possibledue to the paucity of information regarding MeHgspeciation in solution. Heirn (1996) determined awDMeHg of 4×10

−8 cm2 s−1 at 20 °C using benthic fluxchambers. This value is some three orders of magnitudelower than values measured or calculated for inorganicHg species. Values reported from the literature for wDHg2+ range from 5 to 13×10−6 cm2 s−1 (Gobeil andCossa, 1993; Li and Gregory, 1974; Gill et al., 1999;Langer et al., 2001).

Rates of diffusion are related to charge and molecularsize. Gill et al. (1999) estimated a maximal and minimalflux estimate for MeHg by assuming that MeHg existsas a neutral chloride species (MeHgCl, wD=1.3×10−5

cm2 s−1) or in association with large organic macro-molecules (5000 Da, wD=2.0×10−6 cm2 s−1), respec-tively. Recent work (Zhang et al., 2004) has indicatedthat low molecular weight thiols are present insediment–interstitial waters at the 10−7 to 10−9 molarlevels. They estimated that complexation of these thiols

Table 2Mean wetland water column concentration of MeHg, THg, Fe, Mn, DOC an

Location MeHg (%) MeHg (ng L−1) THg (ng L−1)

Cooper's Marsh 5.3 0.078 (b0.01) 1.5 (b0.01)H15C1 63.6 0.702 (0.03) 1.10 (0.12)Stillwoods 30.0 0.296 (0.14) 1.00 (0.07)Baie St. Francois 4.6 0.116 (b0.01) 2.6 (0.14)

Standard deviations are given in parentheses (n=2).n.d.=below detection limits.

with MeHg may significantly effect MeHg speciation.In this study we used the maximal diffusion coefficientfor MeHgCl in water (25 °C) 1.3×10−5 cm2 s−1

reported by Gill et al. (1999). This may cause ourcalculated flux estimates to be slightly high, but therelative values of MeHg flux from pore water sedimentsbetween the four wetlands studied are comparableprovided that similar MeHg complexes are involved.Further work is required to determine the MeHg speciesin sediments and their appropriate diffusion coefficients.Temperature correction to the diffusion coefficient at 25°C were made using the relationship (Lehman, 1979):DT1=DT2(1+0.048ΔT) where ΔT is the temperaturedifference (°C).

3. Results

3.1. Water column characteristics

Wetland water characteristics are shown in Tables 1and 2. pH conditions in all four wetlands were nearcircumneutral, with Baie St. Francois possessing thehighest pH (7.90, Table 1). Water column temperaturesand at the sediment surface were similar, with theexception of Baie St. Francois where water column andsediment surface temperature were 7 and 3 °C warmer,respectively, than the other three wetlands. Concentra-tions of dissolved oxygen (DO2) were under-saturatedwithin wetland water columns, except in Baie St.Francois which had a relatively saturated DO2 water

d SO4

Fe (μg L−1) Mn (μg L−1) DOC (mg L−1) SO4 (mg L−1)

140 (10) n.d. 15.5 (0.3) 11.2 (0.2)310 (5) 470 (20) 12.5 (0.1) 7.7 (0.2)1510 (290) 8010 (130) 13.0 (0.2) 4.6 (0.1)210 (6) 240 (7) 10.5 (0.5) 12.5 (0.3)


column concentration. Oxygen levels were anoxic (b0.5mg L−1 O2) at the sediment surface of all four wetlands.Similar reducing conditions were found at the sedimentsurface of Cooper's Marsh, H15C1, and Stillwoods(−110, −88, and −81 mV, respectively), but not in BaieSt. Francois (−7 mV).

Water column MeHg concentration (Table 2) washighest in wetland H15C1 followed by Stillwoods, BaieSt. Francois, and Cooper's Marsh (0.702, 0.296, 0.116,and 0.078 ng L−1, respectively). MeHg water columnconcentrations correlated well with concentrations in thesediment as well as the MeHg flux from sediments(r=0.97, p=0.02 and r=0.94, p=0.03, respectively).EtHg was not detected in the water column of any of thewetlands.

Total Hg in the four wetlands was similar, but thepercentage of total Hg that is MeHg varied greatly(Table 2) with values of 4.6% in Baie St. Francois, 5.3%in Cooper's Marsh to 30.0% in Stillwoods and 63.6% inH15C1.

Dissolved Fe and Mn were highly correlated r=0.99,p=0.03), within all four wetlands. This is a commonobservation as oxidized forms of Fe and Mn are readilyreduced under similar anoxic conditions. Dissolved Fe,and Mn were greatest in the wetland Stillwoods,followed by H15C1, Baie St. Francois, and Cooper'sMarsh.

Wetland water DOC concentrations were similar inall four wetlands with the highest concentrations foundin Cooper's Marsh (15.5 mg L− 1), followed byStillwoods (13 mg L−1), H15C1 (12.5 mg L−1) andBaie St. Francois (10.5 mg L−1). Sulphate concentrationin the water column of Baie St. Francois was 4.8 to 7.9mg L−1 higher than the other wetlands.

MeHg

0 2 4 6

Dep

th (

cm)

-40

-30

-20

-10

0

10

Fig. 1. Pore water depth profiles for MeHg in Cooper's Marsh (⋄), H15C

3.2. Pore water characteristics

Pore water MeHg concentrations increased to max-imum levels of 11.8, 6.3, and 1.9 ng L−1 for H15C1,Stillwoods and Baie St. Francois, respectively, in thefirst 5 cm below the water–sediment interface (Fig. 1).These values are roughly 20 times that in the overlyingwater. The percent sediment–water content in wetlandH15C1, Stillwoods, Baie St. Francois and Cooper'sMarsh was 84%, 80%, 75% and 98%, respectively.

A large pore water MeHg increase was found inStillwoods at 3 cm depth (6.3 ng L−1) but the largestpeak was found deeper in the sediment core at 12 cm(7.9 ng L−1). In contrast, the pore water maximumMeHg concentration in Cooper's Marsh (5.9 ng L−1)was observed above the water–sediment interface. Thisvalue is 74 times higher than that measured in the watercolumn (0.078 ng L−1). Consequently, a negative porewater MeHg flux was estimated in Cooper's Marsh(−1.60 ng m−2 day−1), whereas fluxes in the Baie St.Francois, Stillwoods and wetland H15C1 were positive,1.37, 1.74 and 10.02 ng m−2 day−1, respectively. MeHgflux rate in the Stillwoods wetland was similar to thatestimated for Baie St. Francois, yet levels found in thewater column of Stillwoods are roughly double thatfound in Baie St. Francois.

Pore water MeHg as a percentage of the total MeHgcontent of the pore water+sediment, ranged 0.1% to2.6%, and generally increased with sediment depth (Fig.2). While Cooper's Marsh had the lowest concentrationof MeHg in sediment, the percent MeHg in pore water ishighest compared to the other three wetlands. PercentMeHg in H15C1, Stillwoods, and Baie St. Francoiswere similar.

(ng L-1)

8 10 12 14

1 (○), Stillwoods (▴) and Baie St. Francois (n), during June 2002.

% MeHg in pore water

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Sed

imen

t D

epth

(cm

)

-20

-15

-10

-5

0

Fig. 2. MeHg content in pore water as a percentage of MeHg in the sediment, in Cooper's Marsh (⋄), H15C1 (○), Stillwoods (▴) and Baie St.Francois (n), during June 2002.


Sulphide pore water concentrations were generallylow in the water column but increased with depth belowthe water–sediment interface, typically from 1–2 to ca.70 μg L−1 (Fig. 3). The exception was Cooper's Marshwhere values were much higher above the interface (250to 350 μg L−1) than below (200 to 270 μg L−1). Porewater sulphide concentrations in Cooper's Marsh weregenerally 2 to 10 times higher than in the other threewetlands.

Wetland pore water sulphate concentrationsshowed sharp maxima within the first 0 to 6 cmbelow the water–sediment interface, with the excep-tion of Baie St. Francois (Fig. 4). The pore watersulphate concentration depth profile in Baie St.Francois was relatively uniform above and belowthe water–sediment interface. Because every two

Sulfide

0 100 2

Dep

th (

cm)

-40

-30

-20

-10

0

10

20

Fig. 3. Pore water depth profiles for sulphide in Cooper's Marsh (⋄), H15C

peeper cells were combined for MeHg sampling toprovide adequate volume for analysis, resolution ofpore water MeHg concentrations with depth is lessthan that for pore water sulphate which was measuredin single cells. Nevertheless, a comparison of thepore water MeHg depth profile (Fig. 1) and the porewater sulphate depth profile (Fig. 4) reveals that porewater sulphate maxima generally coincided with porewater MeHg maxima below the water–sedimentinterface.

3.3. Sediment characteristics

Sediment surface THg concentrations (first 2 cm)in the four wetlands ranged from 66 to 320 ng g−1

(Table 3) and did not vary significantly with depth (data

(ug L-1)

00 300 400

1 (○), Stillwoods (▴) and Baie St. Francois (n), during June 2002.

Sulfate (mg L-1)

0 5 10 15 20

Dep

th (

cm)

-40

-30

-20

-10

0

10

Fig. 4. Pore water depth profiles for sulphate in Cooper's Marsh (⋄), H15C1 (○), Stillwoods (▴) and Baie St. Francois (n), during June 2002.


not shown). Depth profiles of MeHg concentrations insediment for each of the four wetlands showed definitemaxima just below the water–sediment interface anddecreased with depth (Fig. 5). MeHg concentration inthe first 2 cm of sediment (Table 3) was highest inwetland H15C1 (12.8 ng g−1) followed by Stillwoods(3.4 ng g−1), Baie St. Francois (1.6 ng g−1), andCooper's Marsh (0.08 ng g−1). The percentage MeHgof total Hg in sediment (Table 3) was also highest inH15C1 (5.7%) followed by Stillwoods (4.7%), Baie St.Francois (2.4%), and Cooper's Marsh (0.3%).

Although EtHg was not detected in the water columnor sediment pore waters of any of the four wetlands, itwas detected in sediments. Concentrations of EtHg insediment primarily decreased with depth in Cooper'sMarsh and Stillwoods (Fig. 6). In contrast, the EtHgconcentration depth profile in Baie St. Francois andH15C1 sediment was much more uniform. SedimentEtHg concentration in the first 2 cm (Table 3) washighest in Stillwoods (3.7 ng g−1) followed by Cooper's

Table 3Wetland surface sediment characteristics at 0–2 cm depth

Location %MeHg ofTHg

%EtHg ofTHg

MeHg (ng−1 dryweight)

Cooper's Marsh 0.3 0.5 0.8 (0.1)H15C1 5.7 0.4 12 (2.3)Stillwoods 4.7 5.1 3.4 (0.2)Baie St. Francois 2.4 0.5 1.6 (0.3)

Percent MeHg and EtHg of total Hg in sediment, mean MeHg, EtHg and THcontent.Standard deviations are shown in parentheses (n=2).a Percent loss on ignition.

Marsh (1.6 ng g−1), H15C1 (0.9 ng g− 1), and Baie St.Francois (0.3 ng g−1).

Although wetland H15C1 had the highest combinedsediment concentration of EtHg and MeHg, the highestcombined percentage organomercury content of total Hgin sediment was found in Stillwoods (9.8%) thenfollowed by H15C1 (6.1%), Baie St. Francois (2.9%)and Cooper's Marsh (0.8%).

Organic and carbonate content in the first 2 cm ofsediment of each of the four wetlands is listed in Table 3.Organic and carbonate sediment content in all fourwetlands did not vary significantly with depth (data notshown). Cooper's Marsh and Stillwoods had the highestorganic content (90% and 80%, respectively) andcarbonate content (8.2% and 4.0%, respectively). EtHgsediment concentrations were noticeably higher in thesetwo wetlands (1.6 ng g−1 in Cooper's Marsh and 3.7 ngg−1 in Stillwoods) and exceeded levels of MeHg. Bycomparison, H15C1 and Baie St. Francois had loworganic content (33% and 9%, respectively), and EtHg

EtHg (ng−1 dryweight)

THg (ng−1 dryweight)

%LOI a %CO3

1.6 (0.3) 319 (57.6) 90.0 (3.5) 8.2 (4.4)0.9 (0.3) 223.9 (39.6) 33.0 (8.8) 2.5 (0.3)3.7 (0.5) 72.0 (15.6) 80.0 (9.5) 4.0 (0.6)0.3 (0.3) 66.1 (22.5) 9.3 (2.1) 1.0 (0.1)

g sediment concentrations and percent loss on ignition and carbonate

MeHg (ng g-1 dry weight)

0 2 4 6 8 10 12 14

Dep

th (

cm)

-20

-15

-10

-5

0

Fig. 5. Total MeHg in sediment core sections in Cooper's Marsh (⋄), H15C1 (○), Stillwoods (▴) and Baie St. Francois (n), during June 2002.


sediment concentrations in these two wetlands wererelatively low (0.9 and 0.3 ng g−1, respectively), and didnot exceed MeHg concentrations in sediment.

4. Discussion

4.1. Wetland water column, pore water and sedimentcharacteristics

Pore water and sediment MeHg profiles in thewetlands of this study were generally consistent withthose that have been observed in other systems(Brandfireun et al., 1999; Hines et al., 2004). The porewater and sediment MeHg profiles of all four wetlandsshowed elevated concentrations close to or at the

EtHg (ng g

0 1

Dep

th (

cm)

-25

-20

-15

-10

-5

0

Fig. 6. Sediment profiles for EtHg in Cooper's Marsh (⋄), H15C1 (○

sediment surface and generally decreased at greaterdepth. Minor peaks were also observed at greater depth,particularly in Stillwoods. These peaks may represent insitu generation of MeHg possibly by SRB. Manyspecies of SRB are known to methylate Hg and positiverelationships have been found between MeHg formationand sulphate-reducing activity (Benoit et al., 2001; Kinget al., 2001). In addition, mercury methylation justbelow the sediment surface has been shown tocorrespond with rRNA peaks of some groups of SRB(Devereux et al., 1996).

The flux rates of MeHg from sediment to overlyingwater calculated in each of the four wetlands (−1.60 to10.02 ng m−2 day−1) are lower than those reported forother systems (e.g. Gill et al., 1999; Covelli et al., 1999;

-1 dry weight)

2 3 4

), Stillwoods (▴) and Baie St. Francois (n) during June 2002.


Langer et al., 2001); however, it should be noted thatdifferent authors have used different molecular diffusioncoefficients and molecular masses for the diffusingspecies and so only broad comparisons can be made.Also, other factors that affect the movement of MeHgsuch as bioturbation and bioirrigation have been neitherconsidered nor measured.

Of the four wetlands, Baie St. Francois possessedconditions the most unfavorable for sulphate reductionand Hg methylation. This was reflected in the lowconcentrations of pore water MeHg and dissolvedsulphide (Figs. 1 and 3, respectively), low sedimentMeHg concentration (Fig. 4) and high surface sedimentEh (Table 1) relative to the other wetlands. Estimatedflux rates were similar in Baie St. Francois andStillwoods, but a higher concentration and percentageof MeHg of THg (Table 2) was present in the watercolumn of Stillwoods. In Baie St. Francois, diffusion ofMeHg from sediment to the overlying water columnmay have been impeded by the higher dissolved O2

gradient and temperature above the sediments. Althoughsimilar dissolved O2 concentrations were present at thesediment surface of all four wetlands (0.5 mg L−1),however, the concentration in the water column of BaieSt. Francois was much higher and above saturation (10.5mg L−1). At other locations in Baie St. Francois, pH anddissolved O2 was measured as high as 9.8 and 17 mgL−1, respectively. In addition, water temperature in BaieSt. Francois was 7 °C higher compared to the otherwetlands and possessed a relatively high pH. Altogetherthese conditions indicate that primary production ratesin Baie St. Francois exceeded respiration rates.Demethylation is known to be mediated by SRB,nitrate-reducing bacteria and methanogenic species inanoxic systems (Spangler et al., 1973a,b; Oremland etal., 1995), but the rates of methylation largely exceedthose of demethylation (Compeau and Bartha, 1984).Because rates of demethylation are known to be fargreater in aerobic environments (Pak and Bartha, 1998a,b), and metabolic rates generally increase with temper-ature, MeHg diffusing from Baie St. Francois sedimentswould be expected to be far less persistent within thewater column than in Stillwoods as well as the other twowetlands due to a greater rate of demethylation.

MeHg concentrations and methylation rates havebeen shown to be highest in surface sediment (Compeauand Bartha, 1985; Gilmour and Riedel, 1995; Benoit etal., 1999; King et al., 2001). Similarly in this study,MeHg concentrations in pore water were highest justbelow the sediment–water interface, with the exceptionof Cooper's Marsh. Here, the maximum pore waterMeHg concentration was above the sediments (5 cm)

and consequently a negative pore water flux wascalculated. Maximum MeHg concentration was foundto extend up to 12 cm above the sediment surface atanother location. In addition, and in contrast to the otherthree wetlands, sulphide levels in Cooper's Marsh weremuch greater above the sediment–water interface thanbelow. This shows that sulphate reduction and subse-quent Hg methylation was more active above the water–sediment interface. In lakes, methylation activity andsulphate reduction has been shown to peak in the watercolumn just below the oxycline (Eckley, 2003).

Water column MeHg concentrations in H15C1,Stillwoods, and Baie St. Francois were related tomaximum pore water levels a few centimeters belowthe sediment–water interface. However, in Cooper'sMarsh, the maximal MeHg concentration just above thesediment–water interface did not appear to havetranslated via diffusion to the MeHg level found higherin the water column. As both water temperature anddissolved oxygen concentration measured in Cooper'sMarsh were similar to those found in Stillwoods andH15C1, the relatively low MeHg concentration ob-served in the water column of Cooper's Marsh cannot beexplained by potentially high rate of demethylation. Atseveral locations in Cooper's Marsh, dissolved O2 wasgreatly undersaturated (b0.5 mg L−1) only a fewcentimeters below the water surface. The reason forthe low water column MeHg concentration in Cooper'sMarsh relative to MeHg levels found just above thesediments is not known but may be caused by sulphide-mitigated dimethylation. In sulphide systems, MeHgcan be converted to dimethylmercury (DMeHg) (Craigand Moreton, 1983; Wallschlager et al., 1995; Baldi etal., 1995). Relative to MeHg and elemental Hg, DMeHgis highly volatile and has been shown to be readilypurged from soils (Wallschlager et al., 1995) and somuch of it may leave an aquatic system to theatmosphere. Alternatively, DMeHg may be lost bydecomposition to methane and ionic Hg (Baldi et al.,1995). The pore water sulphide levels above the water–sediment interface in Cooper's Marsh are approximately9 to 94 times greater that in any of the other wetlands.Hence, the high sulphide gradient above the interface inCooper's Marsh may act as a barrier to MeHg diffusionto overlying waters by forming DMeHg, which wouldrapidly decompose and/or entirely volatilize from thesystem. This would explain the low MeHg concentra-tion present in the overlying water column (0.08 ngL−1). Because of its reactive and volatile nature,DMeHg can also be easily lost during sample proces-sing. Water sample preservation by acidification, as wasused in this study, would rapidly decompose any


DMeHg that did not volatilize during sample collectionand preservation, to MeHg. DMeHg was therefore notdetected in this work.

While the concentration of MeHg found in porewater and sediment appeared to be related in Baie St.Francois, Stillwoods and wetland H15C1, Cooper'sMarsh was an exception. Cooper's Marsh sediment hadthe least amount of MeHg compared to the other threewetlands (Fig. 5), yet it did not have the least amount ofMeHg in pore water (Fig. 1). This was unexpectedbecause the concentration of dissolved sulphide inCooper's Marsh water in the first 2 cm of sediment wasapproximately 5 to 32 times higher than in the otherthree wetlands. In sulphidic waters, Hg generallyprecipitates out as HgS(s) which is unavailable formethylation (Anderson et al., 1990), or forms dissolvedcomplexes with sulphide and polysulphides. Recentwork has shown that small neutral species like HgS(aq)are available for methylation in sulphidic environments(Benoit et al., 1999) and that polysulphides promote thedissolution of HgS(s) (Jay et al., 2000). The presence ofpolysulphides is also expected to decrease Hg methyl-ation due to the formation of large charged Hg–polysulphide complexes which are considered unableto cross lipid bi-layers due to their size and chargednature. Methylation rates in Desulfovibrio desulfuricansND132 equilibrated with cinnabar did not increase inthe presence of polysulphides (Jay et al., 2002), despitesubstantial increases in total dissolved Hg concentra-tion. Given that polysulphide formation is prevalent inwaters of high sulphide concentration, the presence ofpolysulphides in Cooper's Marsh was likely muchgreater than in the other wetlands. In this case, it wouldbe expected that higher MeHg in pore water relative tosediments was not a result of increased HgS(aq)availability brought about by polysulphide mediatedmetacinnabar dissolution. The possibility of chargedand/or large Hg species as well as small neutral species(e.g. H2S(aq)), for uptake and methylation by SRBshould not be dismissed. In a concurrent study, Hgspeciation calculations in Baie St. Francois sedimentpore water showed that HgS5OH

− and not HgS(aq) wasthe predominant Hg species present (Goulet et al., inpress). In contrast, Zhang et al. (2004) calculated thatinorganic mercury speciation in wetland pore waters isdominated by HgS(aq). Therefore, since Cooper's Marshsulphide levels were relatively high, both polysulphidemediated dissolution of metacinnabar and subsequentavailability of HgS(aq) and possibly polysulphide-Hgspecies for SRB methylation, could potentially be anexplanation for elevated pore water MeHg that weobserved in Cooper's Marsh, relative to the other three

wetlands. Further study of pore water sulphide specia-tion and the potential availability of Hg-sulphide speciesfor methylation are required to validate theseassumptions.

Cooper's Marsh possessed the lowest dissolvedwater concentrations of Fe and Mn, 142 and b10.0 μgL−1, respectively, and the highest sulphide level. It isknown that in anoxic sediments, sulphides produced bymicrobial sulphate reduction are rapidly precipitated outas Fe and Mn sulphides (Canfield, 1989). Given thatsulphide levels in Cooper's Marsh were high above andbelow the sediment–water interface relative to the otherthree wetlands, low Fe and Mn concentrations likelyresulted from precipitation with sulphide. High Fe(II)concentrations have been shown to reduce net Hgmethylation (Mehrotra et al., 2003). Likewise, methyl-ation rates have been shown to be suppressed underiron-reducing conditions (Warner et al., 2003). Hence,the elevated pore water MeHg concentration in Coop-er's Marsh relative to the small concentration present inthe sediment can be considered to be a function of lowFe concentrations.

In contrast, greater combined Fe and Mn concentra-tions present in the water column of H15C1, Stillwoods,and Baie St. Francois were attributable to lower porewater sulphide concentrations and greater iron-reducingactivity. The lack of detectable sulphide in the watercolumn of these three wetlands, allowed the diffusion ofreducible Fe and Mn into the water column.

In the three wetlands' conditions for sulphatereduction (Cooper's Marsh, Stillwoods, and H15C1),sulphate concentrations showed sharp maxima withinthe first 5 cm below the water–sediment interface andwere in close proximity to MeHg pore water peaks. Thedetection of sulphate below the anoxic interface couldbe the result of benthic organisms whose burrowingactions disturb the oxic–anoxic boundary causing somere-oxidation of sulphide in surficial sediments. Theburrowing activity the oligochaete Lumbriculus varie-gatus has been shown to oxidize concentrations of acidvolatile sulphides in surficial sediments in a density-dependent manner (Peterson et al., 1996). In our study,benthic organisms were not visible in any of the coresthat were collected from the four wetlands.

Although the MeHg pore water maxima were found afew centimeters below the sediment surface and theconcentration generally decreased at greater sedimentdepths, the percentage of MeHg in pore water tended toincrease with depth (Fig. 2). This was likely the result ofa depletion of MeHg-scavenging Fe, Mn, and othermineral oxides with increasing sediment depth. Desau-zier et al., 1997 has shown that iron oxides efficiently


scavenge MeHg. Redox potential generally decreasesbelow the oxic–anoxic boundary layer, typically foundjust a few centimeters below the sediment surface inwetlands. Below this boundary, increased microbial andchemical reduction of Fe and Mn oxyhydroxides tosoluble Fe2+ and Mn2+ would be expected to result inless oxide and clay binding sites for MeHg. DecreasingEh below the redox boundary and subsequent increasesin Fe2+ in pore water depth profiles have been observedin lake sediments (Friese et al., 1998; Blodau et al.,1998). The much larger percentage of MeHg in porewater observed in Cooper's Marsh suggests thatpotential oxide MeHg binding sites were much morelimited relative to the other three wetlands. High porewater sulphide levels present in Cooper's Marsh mayhave minimized the formation of Fe and Mn oxides bythe precipitation of Fe2+ and Mn 2+ as FeS and MnS.

4.2. EtHg in sediments

EtHg was detected in the sediments of all fourwetlands but was not present in the pore water or watercolumn. Although previous reports of EtHg in sedi-ments are shown to be a consequence of industrialactivities (Hintelmann et al., 1995; Jernelov andWennegren, 1980) the presence of EtHg recentlyfound in Florida Everglades' sediment (Cai et al.,1997b) and in this study cannot be of direct anthropo-genic origin. The presence of EtHg is likely widespreadand has not been detected due to analytical methodsused. The absence of EtHg in pore water and in thewater column of all four wetlands shows that EtHg isstrongly retained by sediments and/or is rapidlydegraded in the aqueous environment by microbialand/or abiotic mechanisms.

Sediment EtHg concentrations found in our studywere similar to sediment EtHg concentrations found inthe Florida Everglades that range from 0 to 5 ng g−1

(Cai et al., 1997b). The highest concentration of EtHg insediments (which exceeded MeHg concentration) wasfound in Stillwoods and Cooper's Marsh that alsopossessed high organic content relative to the other twowetlands studied.

Humic substances have been shown to be capable ofreacting with Hg2+ to form MeHg (Naganese et al.,1982), but abiotic methylation in anoxic sediments isreportedly one-tenth of the MeHg formed by microbialmethylation (Berman and Bartha, 1986). Recent labo-ratory work by Celo et al. (2006-this issue) claims thatabiotic formation can under some conditions is at leastequal to biotic. The ability of humic substances to formEtHg, however, has not been investigated and therefore

cannot be ruled out. Our results indicate that EtHgconcentration in sediments is a function of the level oforganic carbon in sediment matrices under appropriatereducing conditions. No relationship was found betweenEtHg concentration and organic content of Everglades'surface sediments; however reducing conditions andother factors supportive of EtHg not yet known mayhave differed between sampling locations, thus obscur-ing such an observation. Factors that influence levels ofEtHg in sediments merit further study.

The conversion of inorganic Hg to MeHg throughbiotic processes has been widely accepted. The majorityof studies measuring organomercurials do not use ananalytical method capable of detecting EtHg, thereforethe formation of EtHg by biotic means should not bedismissed. Higher MeHg production and potentialmethylation rates are commonly found in the oxic–sulphidic top boundary layer of river and lake sediments(Benoit et al., 1999; Gilmour and Riedel, 1995; Compeauand Bartha, 1985; King et al., 2001), and has beenattributed to factors such as higher organic matterconcentrations and bacterial activity and mildly reducingconditions. In our study, the highest EtHg sedimentconcentrations generally coincided with the highestMeHg concentrations found just below the sedimentsurface. Higher sediment EtHg concentrations thanMeHg concentrations found in Cooper's Marsh andStillwoods, as well as several sampling sites in theEverglades (Cai et al., 1997a), show that ethylation can begreater than methylation in some aquatic environments.

5. Conclusions

Overall, the comparisons made between the fourwetlands of this study reveal MeHg transport rates (fromsediments to the water column) and water columnMeHg concentrations to be a function of sedimentsurface reducing conditions, the oxygen concentrationof the overlying water column, water temperature andpore water sulphide concentrations. The rate of MeHgdiffusion is related to the sulphide concentration and itsgradient above and below the water–sediment interface,which is likely limited by sediment characteristics suchas iron and manganese buffering capacity for dissolvedsulphides. Because wetlands act as natural sites of Hgmethylation and are important sources of MeHg todownstream river and lake ecosystems, the responsiblemanagement of wetlands is essential. Any alterations tomarsh conditions that cause a decrease in Eh and oxygenlevels can be expected to be followed by increases inMeHg in sediment and pore water, greater diffusion tothe water column and potentially result in elevated Hg


concentrations in fish. Attention should be directed towetlands that are presently (or potentially in the future),impacted by anthropogenic activities that create suchchanges (i.e. eutrophication, flooding).

Acknowledgments

This work was supported by the National Sciencesand Engineering Research Council (NSERC), theCollaborative Mercury Research Network (COMERN)and an NSERC Strategic Grant to DL. We would like tothank Melissa Sparling and Ram Ananth for their helpwith sample collection and analysis.

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