Methylmercury and Total Mercury in Plant Litter Decomposing in Upland Forests and Flooded Landscapes

Methylmercury and Total Mercury inPlant Litter Decomposing in UplandForests and Flooded LandscapesB R I T T D . H A L L * , † A N DV I N C E N T L . S T . L O U I S

Department of Biological Sciences, University of Alberta,Edmonton, Alberta T6E 2E9, Canada

The overall objectives of this study were to examine theeffects of flooding on the decomposition and mercury (Hg)content of tissues from plants common to boreal uplandforests at the Experimental Lakes Area in northwesternOntario. We used litterbags to study changes in total Hg(THg), methyl Hg (MeHg), carbon (C), and nitrogen (N) in 12different plant tissues (birch, alder, blueberry, andLabrador tea leaves, bunchberry plants, jack pine needles,Sphagnum spp., Polytrichum spp., and Pleurozium spp.bryophytes, lichen, and fresh and extensively decomposedwood) placed on unflooded boreal forest soils and inexperimentally created reservoirs over an ∼800 day period.Rates of decomposition (as indicated by differences inthe percentage of C and N mass left in the tissues overtime) were slower in plant tissues placed on unflooded soilscompared to the same tissues that were inundated inreservoirs. Depending on tissue type and initial THgconcentrations, decomposing litter on both unflooded andflooded soils was either a source or a sink for THg.Tissues where initial THg concentrations were greaterthan 30 ng g-1 represented a source of THg to the surroundingenvironment, whereas tissues that had initial concentrationsof less than 30 ng g-1 gained THg mass. Initial rates ofchange in THg were more rapid in plant tissues placed inreservoirs compared to the same plant tissue placed onunflooded soils, but there were no differences in final THgmasses after ∼800 days. Plant tissues placed in reservoirsexhibited large increases in MeHg mass, whereasMeHg mass decreased in the same plants placed onunflooded soils. This is the first study examining THg andMeHg cycling in decomposing plants in upland borealforests and reservoirs.

IntroductionMercury (Hg) is a highly volatile metal that is easily trans-ported from anthropogenic sources to remote areas (1).Contamination of aquatic ecosystems with Hg creates healthconcerns because consumption of fish is the primarymeans by which humans are exposed to the neurotoxic,methylated form of Hg (methyl Hg; MeHg (2)). Therefore, asolid understanding of how Hg moves from the atmospherethrough watersheds is crucial to understanding the con-tamination of fisheries.

Oxidized inorganic Hg (HgII) can enter watersheds in anumber of ways. HgII is water-soluble and is transferredfrom the atmosphere to watersheds directly in wet precipi-tation (3). Reactive gaseous and/or particulate HgII may alsobe deposited to the forest canopy as dry deposition andwashed off during precipitation events as throughfall (4). Inthe boreal ecoregion of Canada, litterfall inputs of HgII towatersheds were twice as high as HgII inputs in wet depositionin open regions (5). Although it is not fully understood howthe HgII in foliage gets there, it is currently assumed that alarge portion of it originates through stomatal uptake ofatmospheric elemental Hg0 (6, 7). During subsequent se-nescence and litter decomposition, HgII may be released(8), which can in turn be transferred through watersheds inrunoff to aquatic systems (9-11). Once in anaerobic zonesof wetlands (12, 13), lake sediments (14), or even saturatedupland soils (15, 16), HgII can be methylated to MeHg,primarily by sulfate reducing bacteria (17). MeHg thenbioaccumulates through aquatic food webs (18).

Although wet deposition can be an important source ofHgII to watersheds far from anthropogenic sources, atmo-spheric deposition of MeHg to our study site (the Experi-mental Lakes Area (ELA) in northwestern Ontario) is relativelyinsignificant (3, 5). However, boreal plants contain MeHg,which is possibly obtained through uptake from soil waters(7, 19) or by methylation of HgII inside plant tissues (20).Litterfall can therefore be an important MeHg input to forestfloors following senescence (5, 19). However, in well-drainedupland soils, the deposited MeHg may be destroyed bymicrobial demethylation and evaded back to the atmosphereas reduced Hg0.

Generally, upland forest soils act as sinks for atmosphericinputs of total Hg (THg; all forms of Hg) and MeHg (13)because the majority of THg and MeHg binds to organic andmineral soil particles. However, when terrestrial and wetlandareas are inundated in the creation of reservoirs for hydro-electricity production or other reasons, the decompositionof flooded organic carbon in anoxic zones fuels HgIImethylation, and the resulting flooded landscape becomesa source of both MeHg and HgII to reservoirs and downstreamenvironments (21-24). Previous studies at the ELA haveshown that increased rates of bioaccumulation of MeHg infish in reservoirs results from increased production of MeHgstimulated by the decomposition of organic matter inreservoirs, not from the leaching of MeHg already stored inthe decomposing organic matter (8, 24).

One of the objectives of this study was to examine theleaching of THg from, and the production of MeHg in,decomposing tissues from plants common to boreal uplandforests at the ELA. We used litterbags to study THg, MeHg,carbon (C), and nitrogen (N) cycling in 12 different planttissues placed in three unperturbed boreal forest sites overan ∼800 day period. An equally important focus of this studywas to explore the contribution of different decomposingplant tissues to THg leaching and MeHg production inreservoirs. This study was part of the FLooded UplandDynamics EXperiment (FLUDEX), a whole ecosystem ma-nipulation designed to examine the biogeochemical cyclingof Hg and C in three reservoirs created over upland borealforests that varied in amounts of organic C stored in soilsand vegetation (25, 26). Because litterfall represents such animportant flux of Hg to watersheds, understanding THgleaching and MeHg production in decomposing litter on soilsand in reservoirs will help identify sources of MeHg con-tamination of fish in lakes.

* Corresponding author phone: (608)262-3979; fax: (608)262-0454; e-mail: [email protected].

† Current address: Environmental Chemistry and TechnologyProgram, University of Wisconsin-Madison, Madison, WI53706-1484.

Environ. Sci. Technol. 2004, 38, 5010-5021

5010 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 19, 2004 10.1021/es049800q CCC: $27.50 2004 American Chemical SocietyPublished on Web 08/21/2004

MethodsSite Descriptions. The ELA consists of a number of boreallakes set aside by the Canadian Federal and Ontario Provincialgovernments for limnological and ecological research. TheELA field station is located ∼50 km southeast of Kenora,Ontario, on the Precambrian Shield (27). The ELA experiencesa cold temperate continental climate with mean July andJanuary temperatures of 18.5 and -17.3 °C, respectively, from1969 to 2001. Mean annual wet deposition for this 32-yearperiod was 699 mm, with ∼25% falling as snow. Upland areasat the ELA ranged from open lichen-covered granite/gneissrocks to shallow nutrient-poor acidic soils supporting jackpine (Pinus banksiana), black spruce (Picea mariana), andpaper birch (Betula papyrifera) forest communities.

Three experimental reservoirs (with maximum depth of2 m and average depth of 1 m) were constructed by buildingdikes along low-lying contours of the sites followed byflooding with water pumped from a nearby oligotrophic lake.Prior to flooding, the reservoirs had varying amounts oforganic C stored in the plants and soils. Reservoirs (calledLow C, Medium C, and High C [total organic C stored ) 30900, 34 900, and 45 900 kg C ha-1, respectively]) were floodedannually in May or June from 1999 to 2001 with low-C(dissolved organic C [DOC] concentrations ∼400µmol L-1),low-Hg water (THg and MeHg concentrations ∼1.0 and 0.03ng L-1, respectively (25)). The reservoirs were emptied eachSeptember or October to simulate drawdown in the shallowzones of northern hydroelectric reservoirs in the winter dueto increased power demand. For more details on the rationalbehind site selection for the reservoirs as well as reservoirconstruction and operation, please see refs 25 and 26.

Litterbag Construction. Twelve plant tissues were usedin this study (Table 1). Six plant tissues were common to allthree upland sites (birch, alder [Alnus crispa], and blueberry[Vaccinium myrtilloides and V. angustifolium] leaves, bunch-berry [Cornus canadensis] plants, jack pine needles, andwood). The other plants were specific to individual sites(Labrador tea leaves [Ledum groenlandicum], bryophytemosses [Sphagnum spp., Polytrichum spp., and Pleuroziumspp.], lichen [Cladina spp.], and old wood; Table 1). In thespring of 1999, birch, alder, blueberry, and Labrador tea leavesand jack pine needles were picked from trees and shrubs.Vinyl gloves were worn when handling all material, and planttissues were stored in clean plastic Ziploc bags. Bunchberrysamples consisted of both the aboveground leaves and stems.Sphagnum spp. were collected from wetland patches in

upland forests, and the capitulum (growing tip) was removedfrom each plant to ensure there was no regrowth in situ.Upland dwelling bryophytes (Pleurozium spp. and Polytri-chum spp.) were collected from large patches of moss locatedin a nearby old growth forest and trimmed to exclude therhizoids. Lichen (Cladina spp.) was collected from exposedbedrock on a ridge top and trimmed to exclude soil.

All samples were dried at ambient temperature for twoweeks on labmat in a clean laboratory at the ELA to avoidcontamination of plant samples. Large amounts of planttissue were retained at the beginning of the study to measureinitial concentrations of C, N, THg, and MeHg. Approximately5 g dry weight (d.w.) of each plant tissue was placed in a 10× 10 cm litterbag constructed of acid washed 400 µm Nitexmesh sewn together using heavy nylon thread. Litterbagswere also filled with wooden blocks (∼13 g) cut from a singlepiece of fir lumber containing no sapwood and knots and ∼5g pieces of extensively decomposed wood (‘old wood’)collected from fallen decomposing trees in the upland sites.

Litterbag Placement. To compare decomposition ratesand changes in THg and MeHg content in plants on unfloodedsoils and in the reservoirs, five sets of triplicate litterbagscontaining each plant tissue type were placed in the threereservoirs and on top of existing litter in the forest im-mediately adjacent to each reservoir. We also wanted tocompare rates of decomposition at different depths in thereservoirs, so five sets of triplicate litterbags were placed inshallow (<25 cm) and deeper (>1 m) zones of each reservoir.

Our forest and reservoir sites differed in areal coverageand species composition of plant communities, so litterbagsplaced in each site contained plant tissues characteristic ofthe site. The High C site was a wet jack pinedominated forest,with an understory of wetland plants. Litterbags containingthe six common plant tissues (birch, alder, and blueberryleaves, bunchberry plants, jack pine needles, and wood),Sphagnum spp. moss, and Labrador tea leaves were placedin a hollow in the wet jack pine dominated forest communityoutside of the flooded area and at two locations in the 0.74ha High C reservoir. The Medium C site was a dense jackpine forest with birch and alder, with an understory ofblueberry shrubs and various bryophytes and herbs. Litter-bags containing the six common plant tissues as well as oldwood were placed in this homogeneous forest as well as intwo locations in the 0.50 ha Medium C reservoir. The LowC site had shallow soils supporting sparse stands of jack pineand birch with a blueberry shrub dominated understory andareas of thin glacial till with lichens, bryophytes, blueberryshrubs, and exposed bedrock. Litterbags containing the sixcommon tissues as well as bryophytes (Pleurozium spp. andPolytrichum spp.) and lichen were placed in the jack pine,birch, and blueberry dominated forest stand and in twolocations in the 0.63 ha Low C reservoir.

Litterbag Collection and Processing. Three litterbags ofeach plant tissue type were retrieved from all unflooded andflooded sites in autumn 1999 (the first summer that thereservoirs were flooded) and in spring and autumn of both2000 and 2001. Any foreign debris on the outside of eachlitterbag was removed by gloved hand. Intact litterbags werethen allowed to dry on labmat in a clean laboratory at theELA at ambient temperature for two weeks. Once dry, planttissues were removed from the litterbags, weighed, and freeze-dried. Freeze-dried tissues from each litterbag were groundusing an acid-cleaned coffee grinder, with the exception ofwood blocks, which were shaved using a clean stainless steelrasp. A portion of each dried sample was analyzed forconcentrations of C, N, THg, and MeHg.

Analytical Methods. Carbon and Nitrogen. C and Nconcentration in plant tissues from each litterbag wereanalyzed using an Exeter Analytical Model 440 elementalanalyzer at the University of Alberta Limnology Laboratory

TABLE 1. Types of Plant Tissues Placed in Litterbags onUnflooded Soils and Reservoirs

unflooded forestsand reservoirs

plant tissue high C medium C low C

living treesbirch leaves (Betula papyrifera) X X Xpine needles (Pinus banksiana) X X Xwood blocks (Abies spp.) X X X

herbs and shrubsalder leaves (Alnus crispa) X X Xblueberry leaves (Vaccinium spp.) X X Xbunchberry plants (Cornus X X Xcanadensis)Labrador tea leaves (Ledum Xgroenlandicum)

bryophytesSphagnum spp. XPolytrichum spp. XPleurozium spp. X

lichen (Cladina spp.) Xold wood X

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(Edmonton, AB) and a Carlo Erba EA1108 elemental analyzerat the University of Waterloo Environmental GeochemistryLaboratory (Waterloo, ON). Concentrations of samples ana-lyzed at both labs were always within 20% of each other. Themass of C or N in each litterbag was calculated by multi-plying the C or N concentration by the final air-driedweight of tissue in the litterbag. The percent mass of C or Nremaining in each litterbag at the time of sampling wasstandardized to the initial C or N mass placed into eachlitterbag (see below). C:N ratios were also calculated for eachsample.

Mercury. All Hg analysis was performed in the Universityof Alberta Low-level Hg Analytical Laboratory. For THg andMeHg analyses, equal amounts of a particular freeze-driedtissue from the three triplicate litterbags sampled from onesite were pooled together and analyzed to provide one THgor MeHg concentration per tissue per site per sampling date.Due to the high cost associated with Hg analysis, THg andMeHg were only analyzed for three out of five sampling dates(each autumn sampling date). Samples from all threelocations (deep, shallow, and unflooded) were analyzed forTHg, whereas only the deep and unflooded samples wereanalyzed for MeHg (see Table S1 in Supporting Information).

THg analyses were performed on 50-200 mg of ground,freeze-dried plant tissue digested in 7 mL of 7:3 (vol:vol)HNO3:H2SO4 in sealed Teflon digestion bombs. Samples wereinitially digested at 125 °C for 2 h, after which 1 mL of BrCland 19 mL of distilled, deionized water were added. Sampleswere then heated overnight at 60 °C. 50 µL-3 mL of digestedsample was placed into glass bubblers with ∼150 mL ofMilli-Q water, 1 mL of SnCl2 (to reduce all HgII to Hg0), and200 µL of NH2OH*HCl (to neutralize the oxidation to HgII).Samples were purged for 10 min using ultrahigh purity (UHP)N2. During the purging process, Hg0 was transferred to trapscontaining gold-coated glass beads (gold traps) and dried inthe N2 stream for 10 min. Mercury on gold traps was thenthermally desorbed to the analytical system, and THg wasdetected using cold vapor atomic fluorescence spectropho-tometry (CVAFS) as described in ref 28 with detection at0.1-0.3 ng g-1 at a blank level of 0.3-0.4 ng L-1. Standardreference material (NRC-TORT) and standards were analyzedin tandem with all samples, and concentrations were alwayswithin 10% of certified values. Spike recoveries were alwayswithin 90-110% of original spike addition. 10% of THgsamples were analyzed in duplicate.

For MeHg analysis, ∼300 mg of processed plant tissueswas digested overnight at 60 °C in Teflon bottles containing5 mL of 25% KOH in methanol as described in refs 29 and30. However, some important changes were made to thismethod. These previously reported methods involved theaddition of a subsample of the digested sample directly toMeHg bubblers, but when we did this the bubblers wereoverwhelmed by the large volume of KOH/methanol solutionthat needed to be added to detect the sometimes lowconcentrations of MeHg in plant tissues. Instead, a 1 mLaliquot of the digest was centrifuged, and 500-800 µL ofsupernatant was distilled at 135 °C in 95-100 mL of Milli-Qwater, 1 mL of 50% H2SO4, 1 mL of 25% CuSO4, and 0.5 mLof 20% KCl until 85% of the sample was transferred to thereceiving jar. The distillate was then placed in glass bubblerswith 500 µL of acetate buffer and 100 µL of Na tetraethylborate and purged onto Carbotraps using UHP N2. Mercuryspecies were thermally desorbed from Carbotraps, separatedusing a gas chromatography column, reduced using apyrolytic column, and detected by CVAFS (31-33) (detectionlimits of 0.1-0.3 ng g-1 at a blank level of 0.05 ng L-1). Spikerecovery for MeHg analysis was between 80% and 120% oforiginal added spike and concentrations in standard referencematerial (NRC-TORT) and analytical standards were alwayswithin 10% of certified values. 10% of MeHg samples were

analyzed in duplicate. As a comparison, samples were alsoanalyzed using a direct distillation of plant tissues in 2:1:2(vol:vol:vol) in H2SO4:KCl:CuSO4. Samples that underwentinitial digestion in KOH/methanol prior to distillation resultedin higher recoveries and better replicates. As a further check,samples of ground Pleurozium spp. and bunchberry plantstaken at the beginning of the study were sent to Flett ResearchLtd. (Winnipeg, MB) and analyzed using similar methods toours (initial digest of 5-10 mg of tissue in 300 µL of KOH/methanol, followed by ethylation and analysis on CVAFS).Average MeHg concentrations in Pleurozium spp. andbunchberry plants analyzed in our lab (1.43 ( 0.41 and 0.30( 0.02 ng g-1, respectively) were very similar to those analyzedby Flett Research (1.60 ( 0.22 and 0.35 ( 0.02 ng g-1,respectively).

The mass of THg and MeHg in each litterbag wascalculated by multiplying the THg or MeHg concentrationby the final air-dried weight of tissue in the litterbags, givingus three THg masses for each tissue, from each site, on eachdate. The percent mass of THg or MeHg remaining in eachlitterbag at the time of sampling was standardized to theinitial THg or MeHg mass in each litterbag. Data are presentedas an average for unflooded or reservoir sites ( one standarderror; however, because the samples were pooled, standarderrors on THg and MeHg masses reflect changes in tissuemass.

Statistical Analysis. We were primarily interested indetermining differences between C, N, THg, and MeHg massin plant tissues placed on unflooded soils and those placedin reservoirs, as opposed to differences in masses among thethree different forest sites or three different reservoirs. Aswell, detailed statistical analysis could not be performed onTHg and MeHg data from individual forests and reservoirsbecause plant tissue was pooled prior to analysis. Therefore,we tested combined data from all three reservoirs againstcombined data from all three unflooded sites. We also wantedto determine differences in C and N mass in plants placedin shallow (<25 cm) and deep (>1 m) locations. Therefore,within each sample date, we tested for differences in C andN mass among plant tissue placed in deep and shallowlocations in the reservoirs and on unflooded soils. Similarly,we tested for differences in THg mass in plant tissue placedamong deep and shallow locations in the reservoir and planttissue placed on unflooded soils, for each fall sampling date.We tested for differences between MeHg mass in plantstissues placed on unflooded soils and MeHg mass in planttissue placed in deep regions of the reservoirs. ANOVAs andmultiple comparison procedures (SigmaStat Version 3.00)were used for all statistical analysis (34), and p values lessthan 0.05 were considered significant.

Results and DiscussionInitial C and N Concentration and C:N Ratios in PlantTissue. Initial C concentration in birch, blueberry, alder, andLabrador tea leaves, jack pine needles, wood, and old woodwas between 49.8 ( 0.04% C and 56.6 ( 0.1% C (Table 2).The lowest C concentrations were found in bunchberryplants, bryophytes (Sphagnum spp., Pleurozium spp., andPolytrichum spp.), and lichens. The highest C concentrationwas found in the old wood. N concentration in plant tis-sues ranged from 0.03 ( 0.01% N in old wood to 2.8 (0.1% N in the leaves of alder, a known N fixing species. Ratiosof C:N were lowest in birch, blueberry, and alder leavesand bunchberry plants and highest in old wood and wood(Table 2).

Initial MeHg and THg Concentrations in Plant Tissue.Initial THg concentrations in plant tissue ranged from under5 ng g-1 to over 90 ng g-1 (Table 2). THg concentrations werelowest in blueberry and birch leaves, wood, old wood, andbunchberry plants (3.6-9.8 ng g-1). Moderate THg concen-

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trations were found in Labrador tea leaves and lichens (27.1and 33.2 ng g-1, respectively), whereas bryophytes had thehighest THg concentrations (52.6-93.9 ng g-1; Table 2).

Initial MeHg concentrations were lowest in alder, birch,and blueberry leaves, bunchberry plants, jack pine needles,wood, and old wood (0.02 to 0.19 ng g-1; Table 2). Inter-mediate MeHg concentrations were found in bunchberryplants, Polytrichum spp., Sphagnum spp., Labrador tea leaves,and lichens (0.30-0.56 ng g-1), while the feather moss,Pleurozium spp., had the highest initial MeHg concentrations(1.43 ng g-1; Table 2). Initial MeHg concentrations weresimilar to those found in an earlier survey of MeHg in plantsat the ELA (20).

Decomposition of Litter on Unflooded Soils and inReservoirs. Decomposition of Plant Tissues in UnfloodedForests. Loss of tissue mass from all plant tissues placed onunflooded soils, with the exception of wood and old wood,was highest during the first year, but decomposition con-tinued at gradual and constant rates in subsequent years,with final losses of between 20%-80% of the original massafter 800 days (Figures 1A and 2A). Wood and old woodexhibited very little to no tissue mass loss during the studyperiod (Figure 3A).

All plant tissues placed on unflooded soils, with theexception of Sphagnum spp., exhibited a gradual decline inC concentration, losing up to 20% of their original C over thethree-year study. Average C concentration in Sphagnum spp.increased by 5% by the end of the study. By the end of thestudy, N concentration increased by 5% to 40% in all tissuesexcept lichen (which decreased by 15%) and wood (whichincreased by 85%; see Table S2 in Supporting Information).

Trends in C and N mass loss generally reflected changesin tissue mass. Birch, alder, blueberry, and Labrador tea leavesand bunchberry plants on unflooded soils exhibited gradualdeclines in C and N mass, with final losses as high as 80%and 70% of original C and N mass, respectively (Figures 1and 2, B and C). C and N mass in bryophytes and lichensplaced on unflooded soils also declined but at slower ratesthan the more labile tissues (between 10% and 20% of originalC and N mass lost). Wood and old wood showed very littleC mass loss but exhibited up to 30% increase in N mass(Figure 3B and C).

Birch, alder, and blueberry leaves and bunchberry plantsdemonstrated sharp decreases in C:N ratios after the firstsummer but tended to stabilize after the first year (Figures1D and 2D). C:N ratios in jack pine needles, wood, and oldwood had constant declines over time and by the end of thestudy were generally 10%-50% lower than original values(Figures 1D and 3D). Decreases in C:N ratios also occurred

after the first season of study in bryophytes and lichens;however, these decreases were less than those observed inother tissues. Mid-way through the study, C:N ratios inbryophytes were similar to preflood values but had decreasedby 20% by the end of the study (Figure 2D).

Tissue decomposition rates in unflooded forest sites weresimilar to those found in other studies examining plant tissuedecomposition in the boreal forest ecoregion (35-37). Twomain factors interact to affect leaf litter decomposition ratesin terrestrial ecosystems (38). The first is the micro- andmacroclimates (most notably temperature and moistureconditions (39) and light (40)) to which the litter is exposed.The second factor is related to the chemical nature of thelitter or litter quality (38). Globally, the effects of litterchemistry on plant decomposition are secondary to the effectsof climatic conditions (39). However, locally, where decom-posing plants experience similar climates, litter qualityprimarily determines litter decomposition.

Litter quality is often defined by the C:N ratio in planttissues. Terrestrial autotrophs generally have low N con-centration, and C:N ratios in terrestrial plants range from 7.5to 225, with a mean of 36 (41, 42). Initial C:N ratios in ourplant tissues are within published ranges, with a low of 18.4for alder leaves, a shrub capable of N2 fixation, and a highof 226 for wood. Studies have found tissues with large C:Nratios (greater than 25) experience little net mineralizationof N and therefore decompose slowly over time (43). Basedon initial C:N ratios in our plant tissues, we expected to seehigher decomposition rates in birch, alder, and blueberryleaves and bunchberry plants, very little decomposition inwood and old wood, and intermediate rates in the othertissues. Our data supported this hypothesis.

Decomposition of Plant Tissues in Reservoirs. All floodedplant tissues, with the exception of jack pine needles, wood,and old wood, had rates of initial tissue mass loss that were∼2 times greater compared to the same tissues placed in theunflooded forest sites (Figures 1A′ and 2A′). Jack pine needlesexperienced similar tissue mass loss in both unflooded andflooded locations (Figure 1A′). As on unflooded soils, woodand old wood exhibited very little to no mass loss in thereservoirs during the study period (Figure 3A′).

As with tissues placed on unflooded soils, average Cconcentration in all flooded plant tissues decreased, whileN concentration increased (with the exception of bryophytesin which N concentration decreased). Changes in C and Nconcentration of plant tissues placed in the reservoirs wereeither similar to, or greater than, those placed on forest soils(see Table S2 in Supporting Information).

TABLE 2. Tissue Mass, Carbon (C) and Nitrogen (N) Concentrations, C:N Ratios, and Total Mercury (THg) and Methylmercury(MeHg) Concentrations in Tissues in Litterbags Prior to Placement in Natural Forests and Reservoirs

plant tissuetissue mass

(g litterbag-1)C concentration

(% C)N concentration

(% N) C:NTHg

(ng g-1)MeHg

(ng g-1)

living treesbirch leaves 5.12 ( 0.02 49.8 ( 0.04 2.4 ( 0.05 21.1 7.13 0.16pine needles 5.16 ( 0.01 52.1 ( 0.1 1.3 ( 0.01 41.0 14.07 0.09wood blocks 12.81 ( 0.04 52.1 ( 0.4 0.2 ( 0.01 226.4 3.62 0.23

herbs and shrubsalder leaves 5.11 ( 0.01 52.1 ( 0.1 2.8 ( 0.12 18.4 12.84 0.02blueberry leaves 5.06 ( 0.004 51.4 ( 0.1 2.3 ( 0.02 22.3 5.75 0.19bunchberry plants 5.10 ( 0.01 46.1 ( 0.4 2.4 ( 0.02 19.1 9.77 0.30Labrador tea leaves 5.37 ( 0.03 53.3 ( 0.3 1.6 ( 0.03 33.5 27.13 0.45

bryophytesSphagnum spp. 5.07 ( 0.01 46.6 ( 0.3 1.0 ( 0 46.6 52.57 0.55Polytrichum spp. 5.10 ( 0.01 47.5 ( 0.3 1.5 ( 0.07 31.0 93.85 0.40Pleurozium spp. 5.01 ( 0.01 48.4 ( 0.2 1.0 ( 0.02 46.5 64.64 1.43

lichen 5.15 ( 0.01 45.8 ( 0.02 0.1 ( 0.01 76.3 33.17 0.56old wood 5.17 ( 0.04 59.6 ( 0.1 0.03 ( 0.01 198.6 9.68 0.02


C and N mass was not statistically different between tissuesplaced in deep and shallow locations in the reservoirs (pvalues ranged from 0.06 to 0.94 for all sampling dates), so we

averaged the C and N mass in tissues placed in all floodedlocations. Final C and N masses in alder, birch, and blueberryleaves and bunchberry plants placed in the reservoirs were

FIGURE 1. Changes in decomposition and mercury mass in foliage in unflooded and flooded forests. A: Percent of original tissuemass remaining. B: Percent of original C mass remaining. C: Percent of original N mass remaining. D: C/N ratios. E: Percent oforiginal total mercury (THg) remaining. F: Percent of original methylmercury (MeHg) remaining. Hatched areas represent periods ofinundation.


significantly lower than those placed on unflooded soils (pvalues between <0.001 and 0.014), indicating greater de-

composition of flooded tissues compared to those placed onforest soils (Figures 1 and 2, B′ and C′).

FIGURE 2. Changes in decomposition and mercury mass in ground-covering plants in unflooded and flooded forests. A: Percent of originaltissue mass remaining. B: Percent of original C mass remaining. C: Percent of original N mass remaining. D: C/N ratios. E: Percent oforiginal total mercury (THg) remaining. F: Percent of original methylmercury (MeHg) remaining. Hatched areas represent periods ofinundation.


FIGURE 3. Changes in decomposition and mercury mass in wood and old wood in unflooded and flooded forests. A: Percent of originaltissue mass remaining. B: Percent of original C mass remaining. C: Percent of original N mass remaining. D: C/N ratios. E: Percent oforiginal total mercury (THg) remaining. F: Percent of original methylmercury (MeHg) remaining. Hatched areas represent periods ofinundation.


C:N ratios in all submerged tissues, with the exception ofbryophytes, decreased over time, and decreases were slightlygreater in flooded tissues compared to like tissues placed onunflooded soils. C:N ratios in flooded bryophytes increasedby between 20% and 25% by the end of the study (Figure2D′), whereas C:N ratios in like tissues placed on unfloodedsoils decreased.

Litter breakdown occurs more rapidly in freshwatersystems than in terrestrial systems (43), because decay occursmost rapidly in environments where biotic activity is highest(44). Therefore, we expected plant tissues placed in reservoirsto decompose more rapidly than tissues placed on unfloodedforest soils. For like tissues, we observed faster rates of planttissue mass loss, and C and N mass loss, in flooded tissuescompared to those on unflooded soils in the early period ofthe experiment.

There are three distinct phases in the breakdown ofvascular plants in water. The first is rapid, initial loss of solubleand labile organic material through leaching. This is followedby a period of microbially mediated decomposition, whichinvolves the immobilization of elements (especially N andP) into bacteria and bacterial byproducts. The final stage ismechanical and invertebrate fragmentation (43), althoughthe physical barrier presented by the nitex litterbag preventedbreak down of tissues by macroinvertebrates. Once organicmatter is degraded to the small chain fulvic and humic acidsthat define dissolved organic carbon, photochemical deg-radation becomes important in mineralizing carbon (45).Elevated decomposition rates in tissues in reservoirs asindicated by increased leaching of C and N in flooded plantscompared to unflooded plant tissues supports increasedmicrobial activity in our reservoirs.

In aquatic systems, litter quality is considered the primarycontroller of decomposition rates. In our study, decomposi-tion rates followed the order: herbs (bunchberry) > decidu-ous leaves (birch, alder, blueberry, Labrador tea) > jack pineneedles > bryophytes and lichens > wood and old wood.However, environmental factors such as temperature anddissolved nutrient concentrations are also important (43, 46).Generally, litter decomposition is accelerated in warmerwaters that have higher concentrations of dissolved N andP (47). There were no differences in changes in C and N massamong reservoirs, which suggests that environmental factorsaffecting decomposition were similar in all three reservoirs.In fact, there were little differences in water column (48) andsoil (26) temperatures and dissolved N and P concentrationsamong reservoirs within a given year (unpublished data, R.A. Bodaly and A. Majewski, Freshwater Institute, Winnipeg,MB).

Changes in THg in Decomposing Litter on UnfloodedSoils and in Reservoirs. THg Concentrations. Concentrationsof THg in plant tissues placed in both unflooded and floodedforests changed over time (Tables 2 and 3). Generally, overthe course of the experiment, THg concentrations increasedin plant tissues that had initial THg concentrations less than30 ng g-1 (birch, alder, blueberry, and Labrador tea leaves,jack pine needles, and wood). In plant tissues with initialTHg concentrations exceeding 30 ng g-1 (bryophytes andlichens), THg concentrations decreased by the end of theexperiment (Table 3).

Changes in THg Mass. On Unflooded Soils. Overall, THgmass in birch, blueberry, alder, and Labrador tea leaves, jackpine needles, and wood increased with time (see Table S3in Supporting Information). In unflooded forest sites thelargest increases in THg mass were observed in birch andblueberry leaves and wood (Figures 1E-3E). THg mass inbunchberry plants decreased after the first season of studybut increased back to initial values by the end of the study.The mass of THg in bryophytes, lichen, and old wood placedin forests decreased between 6% and 50% over time.

In Reservoirs. Because differences in C and N mass inflooded plants in deep and shallow zones were not statisticallysignificant, THg mass in plants in deep and shallow zonesin all reservoirs were averaged to obtain THg masses thatwere representative of all flooded locations. THg mass inflooded birch, alder, blueberry, and Labrador tea leaves, jackpine needles, wood, and old wood increased sharply afterthe first season of flooding (Figures 1E′-3E′). Generally, ratesof change in THg mass in the first season of flooding weresignificantly greater in tissues in reservoirs than in the sametissues on unflooded soils (p value ) 0.009 for all tissues).However, there were no significant differences in final THgmass in tissues placed in reservoirs compared to those placedon forest soils (p values ) 0.985 and 0.341 for all tissuessampled in September 2000 and September 2001, respec-tively). The mass of THg also increased in flooded alder andLabrador tea leaves, jack pine needles, wood, and old wood;however, THg increases in these tissues tended to occur onlygradually over time (Figures 1E′-3E′). We observed decreasesin THg mass in bunchberry plants, bryophytes, and lichenplaced in the upland reservoirs.

Decreases in THg mass in Sphagnum spp. were supportedby results from a previous study by Heyes et al. (49) thatfound THg mass decreased in Sphagnum fuscum placed inlitterbags in pristine and impounded wetlands. However,they also observed decreases in THg mass in other planttissues, Carex rostrata (grass) stems and Picea mariana(spruce) needles.

What Factors Control Changes in THg Mass in Decompos-ing Plant Tissues? In both unflooded and flooded sites, THgmass increased in plant tissues with initial THg concentra-tions that were less than 30 ng g-1 and decreased in planttissues with initial THg concentrations exceeding 30 ng g-1,suggesting that changes in THg mass depend on initial THgconcentrations (Figure 4A). We did not sample soil concen-trations in the forest sites, but concentrations of THg inorganic soils in reservoirs were measured prior to floodingfrom cores collected using a stainless steel barrel corer linedwith plastic sleeves (25). THg concentrations were muchhigher in organic soils (between 78 and 89 ng g-1 d.w.;unpublished data, K. R. Rolfhus, University of Wisconsin atLa Crosse, La Crosse, WI) than in decomposing plants. Webelieve that the organic soils were a small source of THg todecomposing plants, and transfer may occur during rainevents when precipitation, throughfall, and runoff canfacilitate the movement of THg from soils to plants. However,if THg in plants equalized with THg in soils only, we wouldhave expected to see the largest increases in THg concentra-tions in tissues in the site with the highest organic soil THgconcentrations. Since site to site differences in changes inTHg mass in plants were small (48), we assume thatprecipitation, throughfall, and runoff, in addition to organicsoils, could have been sources of THg to decomposing plants.We conclude that THg concentrations in decomposing planttissues in natural forests and reservoirs equilibrate with THgconcentrations in surrounding environments and that initialTHg concentrations control changes in THg mass in de-composing tissues.

Differences in Changes in THg Mass on Unflooded Soilsand Reservoirs. For plant tissues decomposing both on forestsoils and in reservoirs, changes in THg mass were dictatedby the combined effect of tissue mass loss and equilibriumwith concentrations of THg in surrounding soils and water.Generally, rates of change in THg mass were greater in floodedplants than in the same species placed on unflooded soils.These results are consistent with those of Heyes et al.(49), which found greater THg decreases in Carex stems,spruce needles, and Sphagnum fuscum in impoundedwetlands compared to tissues in nonimpounded wetlands.However, for plant tissues that increased in THg mass, there


were few differences in final THg masses in flooded comparedto unflooded tissues. This suggests that THg mass in floodedtissues reached equilibrium with reservoir water faster thanunflooded tissues did with forest soils but that equilibriumwas reached in both unflooded and flooded sites within the800-day study period. For tissues that showed a decrease inTHg mass (bryophytes and lichens), final THg mass insubmerged tissues were less than final THg mass in tissueson the corresponding unflooded forest soils.

Changes in MeHg in Decomposing Litter on UnfloodedSoils and in Reservoirs. MeHg Concentrations. Over thecourse of the study, MeHg concentrations increased in alltissues placed on unflooded soils with the exception ofLabrador tea leaves, bryophytes, and lichen (Tables 2 and 3).Final MeHg concentrations in unflooded Labrador tea leaves,Sphagnum spp., Pleurozium spp., and lichen decreasedslightly to values that were lower than initial MeHg con-centrations. There was no difference between initial and final

TABLE 3. Average Tissue Mass (g), Total Mercury Concentration (ng THg g-1), and Methylmercury Concentration (ng MeHg g-1)in Plant Tissue in Litterbags Sampled from Unflooded Forests and Reservoirs

tissue mass (g)a THg concentration (ng g-1)b MeHg concentration (ng g-1)c

tissue day unflooded forest reservoir unflooded forest reservoir unflooded forest reservoir

living treesbirch leaves

79 days 3.7 ( 0.2 1.90 ( 0.06 13.27 ( 1.07 39.12 ( 3.39 0.74 ( 0.03 0.97 ( 0.07431 days 2.6 ( 0.2 1.60 ( 0.07 30.71 ( 4.01 49.74 ( 3.17 0.34 ( 0.07 2.77 ( 0.33798 days 2.2 ( 0.3 1.40 ( 0.12 40.91 ( 6.42 67.04 ( 5.49 0.36 ( 0.07 5.93 ( 0.26

pine needles79 days 4.8 ( 0.1 4.37 ( 0.05 16.09 ( 0.07 15.19 ( 0.42 0.31 ( 0.03 0.45 ( 0.07431 days 3.7 ( 0.2 3.77 ( 0.04 21.92 ( 1.42 18.23 ( 0.49 0.35 ( 0.04 0.67 ( 0.04798 days 2.8 ( 0.3 3.30 ( 0.03 35.41 ( 2.98 25.51 ( 0.55 1.20 ( 0.16 1.07 ( 0.17

wood blocks79 days 13.0 ( 0.1 12.99 ( 0.38 3.01 ( 0.09 3.49 ( 0.18 0.17 ( 0.001 0.52 ( 0.11431 days 12.3 ( 0.2 12.27 ( 0.14 7.39 ( 1.21 5.99 ( 0.25 1.52 ( 0.30 0.26 ( 0.01798 days 11.0 ( 0.4 12.18 ( 0.09 8.42 ( 0.87 4.72 ( 0.16 5.80 ( 1.95 2.11 ( 0.96

herbs and shrubsalder leaves

79 days 3.9 ( 0.2 2.37 ( 0.06 13.03 ( 0.74 30.66 ( 2.34 0.99 ( 0.06 0.93 ( 0.04431 days 2.5 ( 0.1 1.96 ( 0.05 27.38 ( 3.11 41.83 ( 1.25 0.41 ( 0.08 2.21 ( 0.33798 days 2.4 ( 0.2 1.80 ( 0.08 33.98 ( 1.76 51.89 ( 1.63 0.62 ( 0.07 3.31 ( 0.41

blueberry leaves79 days 3.7 ( 0.1 2.30 ( 0.05 10.95 ( 0.48 29.26 ( 2.08 1.31 ( 0.42 3.34 ( 0.52431 days 2.7 ( 0.1 2.00 ( 0.04 24.18 ( 0.72 41.97 ( 1.90 0.57 ( 0.05 3.49 ( 0.34798 days 2.6 ( 0.1 1.86 ( 0.07 33.29 ( 2.01 52.38 ( 2.91 0.98 ( 0.17 6.62 ( 0.39

bunchberry plants79 days 2.7 ( 0.2 0.83 ( 0.07 15.57 ( 0.28 84.51 ( 5.37 1.71 ( 0.24 5.44 ( 1.67431 days 1.5 ( 0.2 0.60 ( 0.04 35.92 ( 2.10 67.23 ( 2.58 0.27 ( 0.06 5.32 ( 1.44798 days 0.9 ( 0.1 0.48 ( 0.05 49.58 ( 1.03 80.63 ( 8.09 0.23 ( 0.02 3.04 ( 0.40

Labrador tea leaves79 days 4.4 ( 0.1 3.5 ( 0.1 33.87 41.06 ( 8.02 0.43 1.37431 days 3.3 ( 0.1 3.0 ( 0.03 30.21 41.80 ( 0.34 0.54 1.95798 days 2.6 ( 0.2 3.0 ( 0.1 66.36 57.25 ( 5.69 0.28 6.22

bryophytesSphagnum spp.

79 days 4.4 ( 0.03 4.7 ( 0.1 56.41 32.94 ( 10.38 0.43 1.28431 days 4.3 ( 0.1 4.5 ( 0.03 38.49 30.34 ( 3.20 0.20 1.88798 days 3.9 ( 0.5 4.4 ( 0.2 47.35 28.22 ( 3.83 0.40 1.84

Polytrichum spp.79 days 4.7 ( 0.1 4.0 ( 0.1 50.16 42.53 ( 3.01 0.14 1.97431 days 4.1 ( 0.2 3.9 ( 0.04 60.86 38.63 ( 4.99 0.47 4.05798 days 3.7 ( 0.1 3.6 ( 0.1 74.77 44.71 ( 5.27 0.40 5.69

Pleurozium spp.79 days 4.5 ( 0.01 4.1 ( 0.1 58.72 38.44 ( 2.37 1.88 5.61431 days 5.1 ( 0.5 3.9 ( 0.03 66.38 39.56 ( 5.93 0.60 2.33798 days 4.7 ( 0.1 3.7 ( 0.04 54.00 45.30 ( 10.19 0.92 4.57

lichen79 days 5.2 ( 0.1 3.9 ( 0.1 26.08 29.78 ( 0.45 0.36 0.99431 days 4.5 ( 0.2 3.2 ( 0.3 29.49 22.64 ( 0.03 0.10 0.77798 days 5.3 ( 0.2 2.5 ( 0.4 24.91 23.95 ( 6.34 0.36 1.57

‘old wood’79 days 5.1 ( 0.1 5.3 ( 0.1 7.01 9.30 ( 0.99 0.06 3.34431 days 5.0 ( 0.1 4.9 ( 0.03 7.94 12.54 ( 2.48 0.04 1.40798 days 5.0 ( 0.1 5.0 ( 0.03 9.35 15.26 ( 0.42 0.05 4.51

a Standard errors for tissue mass were calculated from tissue mass in plants from nine litterbags sampled from unflooded or flooded sites onone date. b Standard errors for THg concentrations in birch, alder, and blueberry leaves, pine needles, wood, and bunchberry plants were calculatedfrom six pooled samples from three unflooded or three flooded forests sampled on one date. Standard errors for THg concentrations in Labradortea, bryophytes, and lichens in flooded forests were calculated using two samples from one reservoir sampled on the same date. Standard errorsfor THg concentrations in Labrador tea, bryophytes, and lichens on unflooded soils were not calculated since concentrations for these samplescame from one pooled sample from one unflooded forest. c Standard errors for MeHg concentrations in birch, alder, and blueberry leaves, pineneedles, wood, and bunchberry plants were calculated from one pooled sample taken from three unflooded or three flooded sites sampled onone date. Standard errors for MeHg concentrations in Labrador tea, bryophytes, and lichens in unflooded forest were not calculated becauseconcentrations for these samples came from one pooled sample from a single unflooded and flooded site.


MeHg concentrations in Polytrichum spp. placed on un-flooded soils.

MeHg concentrations increased in all flooded plantstissues (Tables 2 and 3). The greatest increases in MeHgconcentrations were observed in flooded alder leaves,bunchberry plants, and old wood which increased to valuesthat were over 200 times initial MeHg concentrations. Sig-nificant increases in MeHg concentrations were also observedin flooded birch and blueberry leaves (37 and 35 times overinitial MeHg concentrations). Only small (3 to 14 times)increases in MeHg concentrations were observed in Labradortea leaves, jack pine needles, bryophytes, lichen, and wood.

MeHg Mass. On Unflooded Soils. Only alder leaves, jackpine needles, wood, and old wood placed on unflooded soilsexperienced increases in MeHg mass (Figures 1F and 3F; seeTable S3 in Supporting Information). MeHg decreased inbirch and Labrador tea leaves, bunchberry plants, Sphagnumspp., Pleurozium spp., Polytrichum spp., and lichen to finalvalues that were between 31% and 72% of original MeHgmass. There was no change in MeHg mass in blueberry leaves.Increases in MeHg were unexpected in plants lying on dry

soils, since Hg methylation is as anaerobic process. It ispossible that placing plants in litterbags allowed the formationof anaerobic microzones, thus promoting MeHg production.

Decreases in MeHg mass in plant tissues placed onunflooded soils indicate a loss of MeHg from plant tissuesdecomposing in unsaturated, well-drained forest soils.Decomposing plants may therefore represented an input ofMeHg to forest soils, which would then presumably beavailable for transport to aquatic systems. MeHg concentra-tions in unflooded soils were 1.13, 0.20, and 0.52 ng g-2

(unpublished data, K. R. Rolfhus), which were similar to MeHgconcentrations in decomposing plant tissues. It is alsopossible, however, that since MeHg production is suppressedand MeHg degradation enhanced in aerobic environments(50), MeHg in plant tissues on forest soils was destroyed bymicrobial demethylation.

In Reservoirs. All types of flooded plant tissue, with theexception of lichen, exhibited large increases in MeHg massby the end of the study (Figures 1F′-3F′). The mass of MeHgwas significantly higher in the plants placed in reservoirscompared to those placed on unflooded soils (p values )

FIGURE 4. A: The percent of original total mercury (% THg) mass remaining in litterbags at the end of the experiment as a function ofthe initial THg concentrations. The vertical line represents initial THg concentrations of 30 ng g-1. B: The percent of original methylmercury(% MeHg) mass remaining in litterbags at the end of the experiment as a function of the initial MeHg concentration.


0.002, <0.001, <0.001 for October 1999, September 2000,and September 2001, respectively). MeHg mass increases inflooded plants were at least 2 to 3, and as high as 125, timesgreater than MeHg mass increases in the same plants placedon unflooded soils. The increase in MeHg mass was slowestin flooded lichen during the first flooding season (only 31%higher than the original MeHg mass), decreasing to valuesthat were 20% lower than original mass by the end of thestudy. The largest increases in MeHg mass were observed inflooded old wood, where MeHg mass increased by over25000% of original MeHg mass. Lichen was the only planttissue that lost MeHg over the entire study period in bothflooded and unflooded forests. Changes in MeHg mass werebalanced between tissue mass loss and increases in tissueMeHg concentrations.

In our study, general trends indicated that, over time,MeHg mass in blueberry and Labrador tea leaves, jack pineneedles, Pleurozium spp., and Polytrichum spp. decreasedon unflooded soils and increased in reservoirs. These resultsare consistent with those reported by Heyes et al. (49) whofound that MeHg mass increased in plant tissues (spruceneedles, sedge grasses, and Sphagnum spp.) placed inimpounded wetlands compared to dry wetland sites. Changesin MeHg mass in our study were not related to initial MeHgconcentrations but, with the exception of old wood, to thelability of the flooded tissues (Figure 4B).

A number of studies have shown that inundation of planttissue results in increased MeHg concentrations in sur-rounding water and biota (21-25, 49, 51-53). It is possible,although unlikely, that increases in MeHg mass in planttissues placed in saturated or flooded environments weredue to uptake of MeHg from the surrounding water and soils.Increases in MeHg mass more likely reflect increased ratesof methylation in and on plant tissues. Oxygen levels at thesediment-water interface were low (i.e. less than 2 ppm),especially in the first two years after flooding (unpublisheddata, R. A. Bodaly and A. Majewski). It is very likely thatflooded plant tissue experienced anoxic conditions conduciveto enhanced MeHg production. As well, whole ecosystemMeHg input-output budgets in reservoirs (25) and a sealedenclosure experiment examining methylation in jack pineneedles and birch leaves (24) clearly showed that increasesin MeHg were due to increased microbial Hg methylationrates.

Increases in MeHg concentrations were observed in waterand biota sampled from the FLUDEX reservoirs (25), butincreases in MeHg mass in plants were not proportional toincreases in water. Plants experienced up to a 250-foldincrease in MeHg, whereas MeHg increases in water neverexceeded 8-fold (25). This suggests that other processes inthe reservoirs contributed to overall MeHg concentrationsin water. For example, plants with large increases in MeHg(i.e. old wood) may have been less abundant in the reservoirsthan plants that had smaller increases in MeHg (i.e. wood).MeHg concentrations in water were measured at the outflowof each reservoir and represented the integration of all MeHgproduction and degradation occurring in the entire reservoir.As well, proportional increases in MeHg in water may haveoccurred in microzones near the plant surfaces but decreasedonce diluted with water from the entire reservoir. Althoughwe cannot quantify the contribution of plants to the totalMeHg pool in our reservoirs, we believe that decomposingplants and soils were the major contributors to MeHg to ourreservoirs.

We do not have sufficient data to determine if rates ofincreases in MeHg in flooded plants were consistently higherin one FLUDEX reservoir compared to the others. However,differences in MeHg increases in plant tissues may explainthe differences we observed in MeHg concentrations in water.In our reservoirs, much of the leaves and needles below the

water line remained attached to the plants (personalobservation) and were therefore situated in the water column.The position of flooded plant tissue in the water columnpossibly facilitated fluxes of MeHg from areas of activemethylation (plants) to the water column. Plants decompos-ing in reservoirs could be an important source of MeHg toaquatic organisms. Because the majority of MeHg in fishcomes from their diet (54), aquatic invertebrate grazing onflooded, decomposing plant tissues and the bacteria associ-ated with them could be an effective vector for the transferof MeHg to higher trophic organisms.

AcknowledgmentsSpecial thanks to all ELA personnel (especially E. Joyce) whohelped pick plant tissue and assemble litterbags and to P.Humenchuk, C. Matthews, M. Pinsonneault, and M. Puchniakfor processing samples. Drs. R. Flett and D. Kelly worked onmethods development for MeHg analysis in plants, and J.Kirk and S. Wanigarante analyzed samples for THg and MeHg.Employees at the University of Alberta Limnology Laboratoryand the University of Waterloo Environmental GeochemistryLaboratory analyzed samples for carbon and nitrogen.Manitoba Hydro, the Department of Fisheries and Oceans,Hydro Quebec, Circumpolar/Boreal Alberta Research Award,and the University of Alberta provided funding for this study.

Supporting Information AvailableTHg and MeHg mass in samples, C and N concentrations insamples, a table detailing the number of samples analyzedfor each parameter, and THg:C and MeHg:C ratios (FiguresS1 and S2). This material is available free of charge via theInternet at http://pubs.acs.org.

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Received for review February 9, 2004. Revised manuscriptreceived July 8, 2004. Accepted July 15, 2004.

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Methylmercury and Total Mercury in Plant Litter Decomposing in Upland Forests and Flooded Landscapes