Mercury concentrations in oligohaline wetland vegetation and associated soil biogeochemistry

Environ Monit Assess (2011) 181:373–383DOI 10.1007/s10661-010-1835-3

Mercury concentrations in oligohaline wetland vegetationand associated soil biogeochemistry

Jonathan M. Willis · Robert P. Gambrell ·Mark W. Hester

Received: 27 October 2009 / Accepted: 5 December 2010 / Published online: 29 December 2010© Springer Science+Business Media B.V. 2010

Abstract Concentrations of mercury were deter-mined in above- and below-ground tissues of dom-inant plant species, as well as soils, in the wetlandsof Lake Maurepas, Louisiana. Indicators of wet-land soil biogeochemical status, such as soil redoxpotential, pore-water nutrient concentrations, andpore-water total sulfides, were also determined.Total mercury concentrations in plant tissues werewithin the typical range for vegetation not ex-posed to mercury contamination. Similarly, totalmercury concentrations in soils were typical ofuncontaminated wetlands within this geographicregion. Soil methyl mercury levels in this studyare slightly lower than those reported in otherstudies of nearby wetlands. This may reflect theless extensive geographic sampling in this study,or the low water levels in the Lake Maurepas

J. M. Willis · R. P. GambrellDepartment of Oceanography and Coastal Sciences,Louisiana State University, Baton Rouge,LA 70803, USA

J. M. Willis (B)Department of Biology, University of Louisianaat Lafayette, Lafayette, LA 70504, USAe-mail: [email protected]

M. W. HesterCoastal Plant Ecology Laboratory, Departmentof Biology, University of Louisiana at Lafayette,Lafayette, LA 70504, USA

system immediately prior to and during this study,which would have altered soil biogeochemical sta-tus. This is corroborated by measurements of soilredox potential and soil pore-water nitrogen andsulfur constituents conducted during this studythat suggest minimal sulfate reduction was occur-ring in surficial soils. This study indicates that thewetlands surrounding Lake Maurepas are typi-cal of many uncontaminated oligohaline wetlandsin the southeastern U.S. in regard to mercuryconcentrations.

Keywords Mercury · Saururus cernuus ·Pontedaria cordata · Sagitarria lancifolia ·Typha latifolia · Peltandra virginica ·Schoenoplectus americanus

Introduction

The cycling of mercury in aquatic systems, par-ticularly oligohaline wetlands, continues to be anarea of intense research due to the extremely toxicand bioaccumulative nature of some mercuryspecies (e.g., methyl mercury) favored in theseenvironments (Lacerda and Fitzgerald 2001). Formany regions the introduction of mercury intoaquatic environments is believed to occur via at-mospheric deposition of mercury released by thecombustion of fossil fuels; this is currently hy-pothesized for the northern Gulf of Mexico as

374 Environ Monit Assess (2011) 181:373–383

well (National Science and Technology CouncilCommittee 2004). Wetlands are often considereda source of methyl mercury to adjacent waterbodies, as under sufficiently reduced conditionsmethyl mercury formation within these wetlandsis primarily mediated by sulfate-reducing bacte-ria (Compeau and Bartha 1984; Gilmour et al.1992; St. Louis et al. 1994; Guentzel 2009). Wet-land vegetation may also alter mercury cyclingthrough a variety of mechanisms, such as assimi-lation into minimally available below-ground tis-sues, translocation into aboveground componentsand subsequent release during decomposition oratmospheric exchange, and the stimulation of bac-terial productivity through the provision of la-bile carbon root exudates (Windom et al. 1976;Windham et al. 2001).

The importance of vegetation in the cyclingof trace and toxic metals, including mercury, hasbeen described for a number of wetland habitatsand interspecific variation in plant traits can resultin differential cycling of metals (Windham et al.2004). Interestingly, studies across plant speciesindicate that the concentration of mercury tendsto be higher in older rather than younger leaves,suggesting active movement of mercury into olderleaves to reduce toxicity (Capiomont et al. 2000;Windham et al. 2003). However, varying seasonaltissue accumulation of mercury trends have beendetected in multiple species, with implications formercury cycling in associated systems (Capiomontet al. 2000; Windham et al. 2004). In anotherstudy by Windham et al. (2001), physiologicaldifferences between these species, such as thepresence of salt glands on Spartina alternif loraand their absence on Phragmites australis, resultedin greater mercury translocation into, and re-lease from, aboveground tissues by S. alternif lora.Thus, a shift in the relative dominance of thesespecies would likely have implications for mer-cury cycling in these marshes. Breteler et al.(1981) investigated uptake of mercury by S. al-ternif lora in both experimentally contaminatedplots as well as in marshes historically subjectedto elevated mercury introduction from industrialactivity and found higher concentrations of mer-cury in roots than shoots under both control andelevated mercury conditions. Further, it appeared

that recent exposure to mercury (experimentallycontaminated plots) did not result in elevatedmercury levels in S. alternif lora tissues, whereaselevated mercury concentrations were found inS. alternif lora taken from the marsh subjected toindustrial effluents (Breteler et al. 1981). Canaarioet al. (2007) examined mercury accumulation bySarcocornia fruticosa, Halimione portulacoides,and Spartina maritime in several Portuguese saltmarshes and also found a tendency for mercuryconcentrations to be higher in below-ground ma-terial than aboveground material. Windham et al.(2003) also reported that both P. australis andS. alternif lora accumulated greater concentrationsof mercury, as well as chromium, copper, zinc, andlead in below-ground tissues than abovegroundtissues. Interestingly, Canaario et al. (2007) deter-mined that sediment methyl mercury levels weremuch greater in vegetated than unvegetated sedi-ments, likely due to the enhancement of microbialactivity by plant roots. Similarly, Windham-Myerset al. (2009) found that the removal of wetlandmacrophytes reduced sediment methyl mercuryconcentrations, again likely through affecting theadjacent microbial community.

Specific factors that may regulate the concen-tration of methyl mercury in aquatic soils and sed-iments, include the concentration of microbiallyavailable mercury, labile organic matter, soil re-dox status, as well as the abundance and ratioof sulfate and sulfide (Lambertsson and Nilsson2006; Merritt and Amirbahman 2009). To bemethylated, inorganic mercury must be availableto appropriate (i.e., sulfate-reducing) bacteria andthis availability is mediated by a number of water,soil and vegetative constituents, such as bulk or-ganic matter as well as dissolved organic matter,root exudates, and inorganic ligands (Compeauand Bartha 1984; Ravichandran 2004; Lambertssonand Nilsson 2006). Concentrations of sulfateand sulfides can alter mercury methylation byaffecting microbial sulfate reduction via sulfateacting as a limiting reactant and also through limit-ing inorganic mercury availability through sulfideprecipitation (Compeau and Bartha 1984; Benoitet al. 1999, 2003; Han et al. 2008). The collectivemodulating effects of sulfate and sulfide concen-trations on microbial methylation of mercury

Environ Monit Assess (2011) 181:373–383 375

result in the existence of a relatively narrow sul-fate range generally considered optimal for methy-lation (Benoit et al. 2003; Hollweg et al. 2009).Soil redox potential, a consequence largely of theenergetics of microbial metabolism, informs atwhat point sulfate reduction is thermodynamicallyfavorable relative to other alternative electronacceptors (Reddy and DeLaune 2008). Thus, soilredox potentials substantially above those indica-tive of iron reduction and the transition to sul-fate reduction, or soil redox potentials well intothe range of sulfate reduction or methanogenesisshould represent conditions that are generally un-favorable for mercury methylation (Delaune et al.2002).

Several studies of the total and methyl mercuryconcentrations of soils, sediments, and waters ofthe Lake Maurepas wetlands have recently beencompleted (Delaune et al. 2008, 2009; Yu et al.2008). However, no assessments of total mercuryconcentrations in tissues of macrophytes for thisregion are currently available and backgroundlevels of mercury for oligohaline macrophytes areminimally available in the peer-reviewed litera-ture. The research detailed herein is intended toprovide insight into the concentrations of totalmercury in the above- and below-ground tissuesof several wetland plant species common to theSoutheastern United States. This study also pro-vides further information on soil mercury concen-trations and relevant biogeochemical processes.

Materials and methods

Experimental design

A field study was implemented by selecting sixsites throughout the Lake Maurepas wetlandsthat were representative of oligohaline vegeta-tive habitats throughout the Northern Gulf ofMexico and Southeastern U.S. (see Fig. 1 for sitelocations). A subset of these sites, Blind River,Reserve Relief, Tobe Canal, and Turtle Cove,occur along the southern portion of Lake Mau-repas and represent a gradient of salinity andsulfate concentration (Shaffer et al. 2003). Thefinal two sites are located along the northern

Fig. 1 Map of field sites in the Lake Maurepas wetlands

portion of Lake Maurepas and were chosen toprovide further spatial and environmental diver-sification. Four permanent plots were establishedwithin each of these sites, yielding a total of24 plots. All plots were essentially monospecificthroughout the study with the dominant speciesat each site as follows: Amite River: Saururuscernuus, Blind River: Pontederia cordata; JoyceWMA: Typha latifolia; Reserve Canal: Peltan-dra virginica; Tobe Canal: Sagittaria lancifolia;Turtle Cove: Schoenoplectus acutus. Seasonallydetermined metrics in each plot included pore-water pH, pore-water conductivity/salinity, pore-water total sulfides, pore-water nutrients (NO3-NO2-N, NH4-N, SO4-S), soil organic matter, soilredox potential (1 and 10 cm depths), soil totalHg, soil methyl Hg, as well as above- and below-ground plant tissue Hg. Plots were sampled inSpring (March 29–30, 2007), Summer (July 19–20, 2007), Fall (November 15–16, 2007), and Win-ter (February 22–23, 2008). A repeated-measuresrandomized block design with four blocks withineach of the six sites (i.e., each plot treated asa block) yielded a 24-plot experimental design.Thus, data were analyzed as a one-way repeated-measures ANOVA with site as the main variableusing the general linear model procedures of SAS9.1. Residuals of data were tested for meeting


the criteria of parametric analysis, with variancesappearing homogeneous and only mild depar-tures from normal distributions noted. Repeatedmeasures ANOVAs were adjusted for departuresfrom sphericity using the Huynh–Feldt correction(Girden 1992; Von Ende 2001).

Data collection

Soil redox potential was determined at 1 and10 cm depths using three bright, Pt soil redoxelectrodes per depth and a calomel referenceelectrode as described in Patrick et al. (1996).Redox measurements were taken adjacent towhere soil was collected for mercury determi-nations and care was taken not to allow thecalomel reference electrodes to come into con-tact with any component of the mercury sam-pling process. Where available, soil pore-waters(composite sample 15 cm in depth) were collectedusing acid-washed soil sippers (see McKee et al.1988 for description of technique). Immediatelyafter collection of samples, one 3-ml aliquot ofpore-water was expunged into an equal volume ofantioxidant buffer (SAOB reagent, ThermoOrionCorp.) and analyzed for total dissolved sulfideswithin 24 h using an ion-selective electrode (OrionResearch Inc.). A second aliquot for nutrient de-termination was expunged into a sample bottleand immediately placed on ice for transport backto the laboratory. Nutrient analysis was accom-plished following EPA methods 350.1 (ammonia),353.2 (nitrate-nitrite), and 375.4 (sulfate). A thirdaliquot was expunged into a sample bottle foronsite determination of pore-water pH, conduc-tivity, and salinity using hand held meters (OrionResearch Inc; YSI) and approved EPA methods(APHA 2005). Soil cores were collected to adepth of 5 cm using a 7.62-cm diameter thin-wallaluminum soil corer and placed into clean poly-ethylene bags. These soil cores were processedfor the determination of total and methyl mercuryfollowing the methods outlined in Bloom (1989).Additional soil cores were collected to a depthof 15 cm using a 7.62-cm diameter thin-wall alu-minum soil corer and placed into a preweighed,polyethylene bag for the determination of organicmatter upon returning to the lab (Soil Testing andPlant Analysis Council 2000).

For the field study, plant cover was assessedthrough visual estimation (Barbour et al. 1999)in permanent plots. Samples of both above- andbelow-ground biomass were collected into cleanpolyethylene bags and, upon returning to the lab,vigorously cleaned with deionized water and blot-ted dry with KimWipes (total mercury determi-nation). Aboveground tissue for analysis includedlive stems and leaves of different ages to generatean unbiased estimate. These fresh tissue sampleswere then homogenized with stainless steel cuttingtools, digested for a minimum of 12 h in trace-metal-grade sulfuric and nitric acid at 100◦C, ox-idized with bromine chloride for a minimum of12 h, and then analyzed for total mercury con-centration (see EPA method 1631 appendix fordetails). Relative standard deviations of all cali-brations were 15% or less. Quality control assess-ments were performed using N.I.S.T. Estuarinesediment SRM 1646 (92% average recovery) andN.R.C. Dogf ish muscle DORM-2 (88% averagerecovery).

Results

Total and methyl mercury characterization

Significant season and site effects were detected inwetland soil total mercury concentrations (Fig. 2panel a; F = 2.80, P = 0.0479 and F = 3.57, P =0.0071, respectively) as well as the interactionthereof (Fig. 2 panel a; F = 3.04, P = 0.0012).However, all values reported for total soil mercuryfall within a range that would be considered back-ground soil levels for the United States (Eisler2006) and also within the range reported for theLake Maurepas wetlands by Yu et al. (2008).Thus, these statistically significant effects likelyresult from microscale environmental differencesand do not appear to represent point source conta-mination. No significant differences in soil methylmercury concentrations were detected in regardto season, site, or the interaction thereof (Fig. 2panel b). Importantly, soil methyl mercury levelsin this study were well within the normal rangefor an uncontaminated wetland, and actually oc-curred towards the lower end of the expectedrange.


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Fig. 2 Effect of site and season on soil total mercury (panela), soil methyl mercury (panel b), aboveground plant tissuetotal mercury (panel c), below-ground plant tissue total

mercury (panel d), interstitial nitrate-nitrite (panel e), andinterstitial ammonium (panel f). All values are mean ± s.e.

Significant effects were detected in above-ground plant tissue total mercury concentrationsin regard to the interaction of season and site

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Fig. 3 Effect of site and season on interstitial sulfate (panela), interstitial salinity (panel b), interstitial pH (panel c),soil organic matter (panel d), soil redox potential at 1 cm

(panel e), and soil redox potential at 10 cm (panel f). Allvalues are mean ± s.e.

was detected for site, but a significant effect ofseason (Fig. 2 panel d; F = 3.09, P = 0.0051) andsignificant interaction of site and season was de-tected (Fig. 2 panel d; F = 4.67, P = 0.0002). Aswith total soil mercury concentration, all total

below- and above-ground tissue mercury concen-trations fell within a range considered backgroundfor wetland plant species in uncontaminatedareas. The statistically significant differences inbelow-ground plant tissue concentrations may


reflect microscale phenomena not captured by theexperimental design.

Pore-water nutrients

Pore-water nitrate-nitrite-N concentrations weresignificantly higher in the Amite River site than inother sites (Fig. 2 panel e; F = 22.16, P < 0.0001),particularly in the spring sampling, which resultedin a significant interaction (Fig. 2 panel e; F =4.58, P < 0.0001). Pore-water ammonium-N con-centrations were significantly higher in the BlindRiver site than in other sites (Fig. 2 panel f; F =29.27, P < 0.001) and were significantly higher inspring than other sampling periods (Fig. 2 panelf; F = 8.10, P < 0.001). A significant interactionof site and season was detected (Fig. 2 panel f;F = 1.982, P = 0.034), likely resulting from theJoyce, Tobe Canal, and Turtle Cove sites havingelevated pore-water ammonium-N in the spring,but lower levels in other seasons, whereas theBlind River site demonstrated consistently el-evated pore-water concentrations for the studyduration.

General pore-water characteristics

A significant effect of site was detected for pore-water sulfate with higher concentrations at themore saline influenced sites, Tobe Canal and Tur-tle Cove (Fig. 3 panel a; F = 82.002, P < 0.001).A significant interaction of season and site wasdetected for pore-water salinity (Fig. 3 panel b;F = 4.967, P = 0.003), likely a result of the springand summer pore-water salinities being higherthan the pore-water salinities for all sites otherthan the Amite River and Reserve Canal sites. Asignificant effect of site was also detected in regardto pore-water salinity, with Tobe Canal and TurtleCove being more saline than other sites (Fig. 3panel b; F = 73.016, P < 0.001). No significanteffect of season or interaction of season and sitewas detected for pH. However, an overall effectof site was detected, with Turtle Cove pore-waterbeing more acidic than other sites (Fig. 3 panelc; F = 5.173, P = 0.003). Pore-water total sulfideconcentrations were generally low (3.07 ppm) tobelow detection for all sites during each of thefour sampling periods.

General soil characteristics

No significant effect of season or interaction ofseason and site was detected for soil organic mat-ter. An overall effect of site was detected, withTurtle Cove having less organic matter than othersites (Fig. 3 panel d; F = 52.006, P < 0.001). How-ever, all soils contained substantial organic matter(Turtle Cove: 10–20%, all other sites: 30–80%).A significant interaction of season and site wasdetected for both surface (Fig. 3 panel e; F =13.283, P < 0.001) and deep (Fig. 3 panel f; F =34.117, P < 0.001) soil redox potential, with Re-serve Canal, Tobe Canal, and Turtle Cove beingmore reduced in summer than the other field sites.A significant effect of season was also detected,for both surface (Fig. 3 panel e; F = 59.948, P <

0.001) and deep (Fig. 3 panel f; F = 111.160, P <

0.001) soil redox potential, with soils being muchless reduced in winter than the other seasons.

Discussion

This assessment of mercury levels in the LakeMaurepas wetlands generally indicates that forall examined partitions (soil, aboveground planttissues, below-ground plant tissues), total mercuryconcentrations are within a range considered tobe representative of an uncontaminated wetland(Eisler 2006). Concentrations of total soil mercuryreported in this study, 7.97 to 136.72 ng/g, arewithin the range reported for Louisiana sedimentsand soils presented by other researchers, such asO’Rourke et al. (2001) 70 to 120 ng/g, Dupre et al.(1999) below detection to 250 ng/g, Kongchumet al. (2006) 78 to 240 ng/g, Delaune et al. (2008)10.6 to 177 ng/g, Yu et al. (2008) 8.7 to 288.9 ng/g.These levels of mercury likely reflect the lack ofelevated atmospheric deposition of mercury andthe absence of major industrial sources of mercuryin the Lake Maurepas area.

Consistent trends in the mercury concen-trations of vegetative tissues were not readilydiscernable between sites. This indicates thatspecies-specific differences were not apparent inthis vegetative mercury data set since samplingplots at a given site were essentially monotypic,and vegetative samples were collected from the


dominant species in the plots. However, it is im-portant to note that the intent of this study was toprovide overall estimates of vegetative tissue mer-cury concentrations across the Lake Maurepaswetlands, rather than to elucidate species-specificdifferences. To this end, sampling procedures,such as pooling a range of leaf sizes and agesinto single digestions, likely diminished the de-tectability of species effects. Although the speciesincluded in this study share many similar life his-tory traits, such as being clonal forbs, they varygreatly in others, such as root to shoot ratio, bio-mass production, and phenology (Whigham andSimpson 1978; Doumlele 1981; Grace and Wetzel1981; Howard and Mendelssohn 1999; Spaldingand Hester 2007). Thus, a more fine-scale assess-ment simultaneously incorporating evaluations ofplant physiology and metabolism would likelyreveal differential aspects of how these speciesaccumulate mercury. For example, a number ofsecondary metabolites have been characterizedfor S. cernuus (Kubanek et al. 2000; Rajbhandariet al. 2001), and it is possible that some of thesecompounds could influence mercury assimilationand storage in this species.

Total mercury levels in aboveground plant tis-sue determined in this study, 5.29 to 79.2 ng/g,were similar to values detected in other studiesof uncontaminated wetlands, such as Moore et al.(1995) 4 to 160 ng/g, and Rencz et al. (2004) 5 to58 ng/g, as well as Gulf Coast seagrasses Lewis andChancy (2008) mean of 23.1 ng/g. Total mercuryin vegetation for this study were also similar tothat reported for the understory of boreal uplandforest in Canada as reported by Hall and St. Louis(2004) 5 to 58 ng/g, and Mailman and Bodaly(2005) 4 to 52 ng/g, as well as mosses in the arcticLanders et al. (2005) 20 to 112 ng/g. In general,it does not appear that either above- or below-ground vegetation components represent substan-tial total mercury reservoirs in the Lake Maurepaswetlands.

Methyl mercury levels were determined in soilsto a depth of 5 cm, which are frequently the majorsite of mercury methylation in aquatic systems(Korthals and Winfrey 1987), and were also foundto be within a range typical of an uncontaminatedwetland (Eisler 2006). Interestingly, soil methylmercury concentrations determined in this study

(0.1 to 1.1 ng/g) are somewhat lower than thosereported by Yu et al. (2008) 0.1 to 11.4. However,it should be noted that 28 of 35 sites sampled byYu et al. (2008) had soil methyl mercury concen-trations between 0.1 and 1.4 ng/g, suggesting thatthe six sites selected for this study are likely inthe typical range for the majority (80%) of theLake Maurepas wetlands. The apparent variationin the overall range found by Yu et al. (2008)and this study may have also been influencedby differences in hydrology prior to sampling.Water levels were unusually low during portionsof this study, with wetland soils moist, but notsaturated during each sampling event, whereas Yuet al. (2008) reported all soils either saturated orflooded. Hall et al. (2008), investigated surfaceand pore-water total and methyl mercury in sev-eral Louisiana wetlands, including the Blind Riverarea, and found higher levels of methyl mercury infreshwater wetlands compared to adjacent surfacewaters, suggesting that the wetlands may functionas a net source of methyl mercury to these openwater bodies. This level of variation within thesame basin highlights the need for multiple yearinvestigations as well as the need for a thoroughunderstanding of the local environment (e.g., hy-drology) to appropriately frame interpretations.

All sites examined in this study exhibited highlevels of soil organic matter (30% to 80%), exceptTurtle Cove (10% to 20%). This is important asorganic matter can provide binding sites for mer-cury, thus resulting in less biological availability.All wetland soil pore-water variables evaluatedwere generally similar across sites, with the ex-ception of the Turtle Cove, which had a higherpore-water salinity reflecting its more mesohalinenature. Pore-water variables determined for theother five sites in this study, including salinity, pH,dissolved nitrate–nitrite, dissolved ammonia, anddissolved sulfate were similar to the results otherresearchers have found for the Lake Maurepaswetlands (e.g., Hall et al. 2008), and also fall withinthe range reported for fresh and oligohaline wet-lands in Louisiana (Sasser et al. 1991; Hester et al.2005; Meert and Hester 2009; Swarzenski et al.2008). It should be noted that soil redox poten-tial was higher for all sampling periods in thisstudy than in other studies of the Lake Maurepaswetlands (Hester et al. 2005), likely a result of


water levels in this system being unusually lowduring this study period. The soil redox poten-tials determined during this study are generallycorroborated by the relatively higher pore-waterconcentrations of ammonium and low concentra-tions of total pore-water sulfides (Delaune et al.2002). The ratio of oxidized to reduced nitrogenand sulfur compounds, in conjunction with the soilredox data, indicate that at the time these areasin the Lake Maurepas wetlands were sampled,dissimilatory nitrate reduction was the primarymicrobial metabolic pathway occurring in thesesoils (Delaune et al. 2002).

Conclusions

In summary, the total mercury concentrations insoils and plant tissues as well as the methyl mer-cury concentrations in soils determined within theLake Maurepas wetlands appear to be within therange of a normal uncontaminated oligohalinewetland. Interestingly, other recently publisheddata concerning mercury in these wetlands reportsimilar levels of total mercury in soils and gener-ally similar levels of methyl mercury in soils (Yuet al. 2008). However, this study failed to captureany sites with elevated methyl mercury concen-trations and therefore presents a more truncatedrange for soil methyl mercury concentrations thanYu et al. (2008). This may reflect the less geo-graphically extensive sampling performed in thisstudy or differences in local hydrology at the timeof sampling for these different studies. Soil char-acteristics such as the large amount of organicmatter with associated reduced sulfur functionalgroups may provide some substantial capacity torender newly deposited mercury unavailable formicrobial activity (see Skyllberg et al. 2003), thusproviding additional protection. However, exper-imental manipulation of soil mercury levels in acontrolled setting coupled with an estimate of mi-crobial bioavailability would be necessary to testthis hypothesis. No consistent seasonal variationwas detected in methyl mercury levels, or in re-lated soil characteristics, although again this waslikely due to the abnormally mild winter and lowwater levels throughout the study. Results fromthis study, as well as those from Yu et al. (2008)

and Hall et al. (2008) indicate that methyl mercurylevels in the wetlands of Lake Maurepas are eitherequivalent to or elevated in comparison to thoseof Lake Maurepas itself on average. Wetland siteswith highly elevated levels of methyl mercury insoils were completely absent from this study andrare (2 out of 35) in the study by Yu et al. (2008).Hall et al. (2008) generally found that freshwaterand brackish wetlands in the Lake PontchartrainBasin (e.g., Blind River and Bayou Lacombe)have elevated surface water methyl mercury lev-els compared with the surface waters of PassManchac and Lake Pontchartrain. This suggeststhat the surrounding wetlands are likely a sourceof methyl mercury to the adjacent lake waters,which is considered typical for wetland-surfacewater systems (St. Louis et al. 1994).

Acknowledgements This study was supported by the En-vironmental Protection Agency through the SouthwesternLake Pontchartrain Basin Research Program at Southeast-ern Louisiana University. The authors would like to thankBronwyn Duos, Laura Basirico, and Jodie Noel for theirassistance.

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Mercury concentrations in oligohaline wetland vegetation and associated soil biogeochemistry