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Production and Retention of Methylmercury in Inundated Boreal Forest Soils Kristofer R Rolfhus, James P. Hurley, Richard A Bodaly, and Gregory Perrine Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505398z • Publication Date (Web): 10 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015
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Production and Retention of Methylmercury in Inundated Boreal Forest Soils
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Kristofer R. Rolfhus1*, James P. Hurley2, Richard A. (Drew) Bodaly3, and Gregory
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Perrine1
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Author Addresses:
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* corresponding author; email:
[email protected]; phone: 608-785-8289; fax: 608-
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785-8281
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Wisconsin-La Crosse, La Crosse, WI 54601
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2
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Department of Civil and Environmental Engineering, Environmental Chemistry and Technology Program, University of Wisconsin-Madison, Madison, WI 53706
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Department of Chemistry and Biochemistry, River Studies Center, University of
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Penobscot River Mercury Study, 115 Oystercatcher Place, Salt Spring Island, BC, Canada V8K 2W5
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TOC/Abstract Art:
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Abstract
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The Flooded Uplands Dynamics Experiment (FLUDEX) was an ecosystem-scale study
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examining the production of methylmercury (MeHg) and greenhouse gases from
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reservoirs constructed on an upland boreal forest landscape in order to quantify their
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dependence upon carbon stores. We detail the within-reservoir production and storage of
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MeHg before, during, and nine years after the experiment. The reservoirs were net
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MeHg producers during the first two years of flooding, and net demethylating systems
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afterwards. During years 1-3, a rapid pulse of MeHg and total Hg was observed in
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floodwater, followed by substantial increases in MeHg in seston and sediment. Re-
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sampling of the dry reservoirs nine years after the experiment ended indicated that
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organic soil MeHg was still 8 to 52-fold higher than pre-flood conditions, and averaged
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86% of the levels recorded at the end of the third flooding year. Both total Hg and MeHg
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retention in soil were a strong function of organic carbon content. The timescale of soil
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MeHg retention may help explain the decadal time lag frequently observed for the
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decrease of piscivorous fish Hg concentrations in new reservoirs. Predicted extreme
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precipitation events associated with climate change may serve to make landscapes more
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susceptible to this process.
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Keywords: mercury, methylmercury, soil, flooding, inundation, reservoir
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Introduction Mercury (Hg) contamination of aquatic and terrestrial systems has led to elevated
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concentrations in ecosystems and food webs and enhanced exposure of the neurotoxin
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methylmercury (MeHg) to humans and wildlife1,2. Mercury is methylated by aquatic
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bacteria at oxic/anoxic interfaces, which may be exacerbated in reservoir systems that
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experience water level change. For example, changes in the Hg content of age-0 yellow
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perch (Perca flavescens) in lakes within Voyageurs National Park, MN, were linked to
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changes in water level from the previous year3, while a survey of 18 Finnish boreal
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reservoirs linked fish Hg levels to reservoir age and water level fluctuation4. While
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MeHg may be formed rapidly on timescales of weeks to months, its effects may be
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lasting; elevated predatory fish Hg values have been observed for 10-30 years in
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Canadian reservoirs5.
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Methylation is enhanced by elevated levels of inorganic mercury, sulfate ion, and
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organic matter, with a growing evidence of association with the metabolism of a variety
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of aquatic microbes, including iron reducing, methanogenic, acetogenic, and cellulolytic
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species 6-10. Mercury exhibits a high degree of association with soil organic matter,
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forming complexes that are strongly dependent upon thiol content11,12. Indeed, sulfur
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redox cycling may play an important role in MeHg formation in periodically flooded
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ecosystems13 where sulfate is reduced microbially and oxidized by drying events and
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sulfide oxidizing bacteria as observed in a Canadian reservoir14 and the Florida
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Everglades15. The rate of supply of organic matter to inundated ecosystems is also an
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important factor, whether the source is from the flooded soil, vegetation, marginal
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erosion, or is autochthonous in origin16.
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Soils and surficial vegetation are important sinks for mercury deposition, with an
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estimated global anthropogenic enrichment factor of 5.8 for the rapidly-exchangeable
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terrestrial Hg pool17. Recent work suggests that there may be an increasing latitudinal
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trend in soil Hg and MeHg storage, possibly due to slower carbon cycling, vegetation
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type, or condensation/evaporation cycles18,19.
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Inundation represents a potentially large vector for Hg re-mobilization into food
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webs. For periodically inundated systems, production of MeHg may be greatest in soils
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that contain the highest levels of inorganic Hg, labile organic matter, and moderate levels
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of porewater sulfate during flooding14,20,21. It is unclear whether MeHg production is a
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function of the preceding duration of dry conditions. One possibility is that due to
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continuous atmospheric Hg and sulfate deposition, longer dry periods would result in
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increased MeHg associated with the labile organic matter pool. Predictions from the
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latest Intergovernmental Panel on Climate Change Technical Summary22 and the 3rd
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National Climate Assessment23 state that storm effects and coastal surges are likely to
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become stronger through the 21st century, with precipitation becoming flashier and more
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extreme. Resultant seasonal flooding and high-precipitation events may therefore
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enhance MeHg formation and exposure to food webs in terrestrial ecosystems. The
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potential exists for similar effects in flooded coastal zones, but these remain largely
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unconstrained.24,25
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The first ecosystem-level study to investigate inundation effects on MeHg
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formation examined an experimentally-flooded boreal forest wetland at the Experimental
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Lakes Area (ELA; northwestern Ontario, Canada), and termed the “ELA Reservoir
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Project (ELARP)”26,27. This study, thought to represent a “worst-case scenario” of
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flooding a wetland ecosystem, found that MeHg was rapidly produced from peat, though
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most of what was produced was not hydrologically or atmospherically exported27. A
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similar recent study was conducted in the Florida Everglades21, where a constructed
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wetland was a net source for MeHg during the first two years of flooding, and a sink
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afterwards.
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The follow-up study to ELARP, the Flooded Uplands Dynamics Experiment
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(“FLUDEX”), measured MeHg production and greenhouse gas emission relative to a
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gradient of stored C in three experimentally inundated upland boreal forest
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catchments28,29. Prior FLUDEX papers have focused on construction of a reservoir mass
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balance29 and effects of reservoir creation on Hg in zooplankton30. Here, we present
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additional results from the FLUDEX project that test the hypothesis that flood-derived
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soil MeHg is retained over annual time scales. Herein, we detail 1) the response of
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MeHg and total mercury in boreal forest soils to cyclic inundation, 2) evidence of flooded
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soils as the principal source of MeHg to the reservoirs, and 3) the relationship between
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Hg and C in reservoir components before, during, and after the project.
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Experimental
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Study Location.
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Three sub-hectare reservoirs were created at the Experimental Lakes Area,
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Ontario, Canada (Figure SI-1; Table SI-1). The high-C reservoir contained substantially
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more total carbon (soil plus vegetation) than the medium and low reservoirs, and
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possessed far more soil carbon relative to the medium and low reservoirs, which were of
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similar quantity. The high-C reservoir was a mixed wet-dry forest composed of paper
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birch (Betula papyrifera), jack pine (Pinus banksiana), black spruce (Picea maricana),
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Labrador tea (Ledum groenlandicum), and mosses (Sphagnum spp. and Polytrichum
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spp.)29. The medium-C reservoir was a dry forest ecosystem of mainly birch, jack pine,
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and shrubs (Vaccinium spp. and Alnus spp.), while the ridge-top low-C reservoir was
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approximately 70% dry birch/jack pine forest and 30% exposed granitic bedrock covered
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by lichen. The soils were characterized as poor, acidic brunisols of Precambrian granitic
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origin31, whose depth ranged from bare bedrock in the low-C reservoir to approximately
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1 m in the high-C reservoir. The reservoirs were filled and drained annually from 1999
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(“Year 1”) through 2003 (“Year 5”). Feed water from a nearby oligotrophic lake (Roddy
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Lake) was continuously pumped into the reservoirs during the summer period, and
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flowed out of a notched weir at the opposite end of the inlet. Typical hydraulic residence
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times were on the order of one week (Table SI-1), and water depth was on average
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approximately 1 meter. Reservoirs were typically filled in late May/early June, and
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drained in late September/early October. The average duration of inundation was
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approximately 100 days.
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Sample Collection
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The initial “pre-flood” Hg content of each site was determined by collecting intact
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soil cores every 20 m along two crossing transects (total 29 cores; September 1998), as
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well as 9 cores for detailed vertical profiling that were 0.050). However, samples from the high-C reservoir
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associated with patches of peat were up to 20-fold higher in MeHg than soils just 10 m
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away. For mineral soils, total Hg was significantly higher in the high-C relative to the
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low-C reservoir, and MeHg was significantly higher in the high-C compared to the
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medium-C reservoir (p
