Evaluation of wetland methyl mercury export as a function of experimental manipulations

Methylmercury Biogeochemistry in Freshwater Ecosystems: A Review Focusing on DOM and Photodemethylation

Mercury contamination is a growing concern for freshwater food webs in ecosystems without point sources of mercury. Methylmercury (MeHg) is of particular concern, as this is the form of mercury that crosses the blood-brain barrier and is neurotoxic to organisms. Wetlands and benthic sediments have high organic content and low oxygen availability. Anaerobic bacteria drive the metabolic function in these ecosystems and subsequently can methylate mercury. The bioavailability of MeHg is controlled by physicochemical characteristics such as pH, binding affinities, and dissolved organic matter (DOM). Similarly, photodemethylation is influenced by similar characteristics and thereby the two processes should be studied in tandem. The degradation of MeHg through photochemistry is an effective destruction mechanism in freshwater lakes. This review will highlight the uncertainties and known effects of DOM on subsequent photoreactions that lead to the occurrence of mercury photodemethylation and reduction in mercury bioavailability in freshwater ecosystems.

Total and methyl mercury concentrations in sediment and water of a constructed wetland in the Athabasca Oil Sands Region

In the Athabasca Oil Sands Region in northeastern Alberta, Canada, oil sands operators are testing the feasibility of peatland construction on the post-mining landscape. In 2009, Syncrude Canada Ltd. began construction of the 52 ha Sandhill Fen pilot watershed, including a 15 ha, hydrologically managed fen peatland built on sand-capped soft oil sands tailings. An integral component of fen reclamation is post-construction monitoring of water quality, including salinity, fluvial carbon, and priority pollutant elements. In this study, the effects of fen reclamation and elevated sulfate levels on mercury (Hg) fate and transport in the constructed system were assessed. Total mercury (THg) and methylmercury (MeHg) concentrations in the fen sediment were lower than in two nearby natural fens, which may be due to the higher mineral content of the Sandhill Fen peat mix and/or a loss of Hg through evasion during the peat harvesting, stockpiling and placement processes. Porewater MeHg concentrations in the Sandhill Fen typically did not exceed 1.0 ng L(-1). The low MeHg concentrations may be a result of elevated porewater sulfate concentrations (mean 346 mg L(-1)) and an increase in sulphide concentrations with depth in the peat, which are known to suppress MeHg production. Total Hg and MeHg concentrations increased during a controlled mid-summer flooding event where the water table rose above the ground surface in most of the fen. The Hg dynamics during this event showed that hydrologic fluctuations in this system exacerbate the release of THg and MeHg downstream. In addition, the elevated SO4(2-) concentrations in the peat porewaters may become a problem with respect to downstream MeHg production once the fen is hydrologically connected to a larger wetland network that is currently being constructed.

Experimental dosing of wetlands with coagulants removes mercury from surface water and decreases mercury bioaccumulation in fish

Mercury pollution is widespread globally, and strategies for managing mercury contamination in aquatic environments are necessary. We tested whether coagulation with metal-based salts could remove mercury from wetland surface waters and decrease mercury bioaccumulation in fish. In a complete randomized block design, we constructed nine experimental wetlands in California's Sacramento-San Joaquin Delta, stocked them with mosquitofish (Gambusia affinis), and then continuously applied agricultural drainage water that was either untreated (control), or treated with polyaluminum chloride or ferric sulfate coagulants. Total mercury and methylmercury concentrations in surface waters were decreased by 62% and 63% in polyaluminum chloride treated wetlands and 50% and 76% in ferric sulfate treated wetlands compared to control wetlands. Specifically, following coagulation, mercury was transferred from the filtered fraction of water into the particulate fraction of water which then settled within the wetland. Mosquitofish mercury concentrations were decreased by 35% in ferric sulfate treated wetlands compared to control wetlands. There was no reduction in mosquitofish mercury concentrations within the polyaluminum chloride treated wetlands, which may have been caused by production of bioavailable methylmercury within those wetlands. Coagulation may be an effective management strategy for reducing mercury contamination within wetlands, but further studies should explore potential effects on wetland ecosystems.

Spatial and temporal variation of total mercury and methylmercury in lacustrine wetland in Korea

The spatial and temporal variations of total mercury (THg) and methylmercury (MeHg) concentrations (n = 120, respectively) in water and sediments of the Yangsuri lacustrine wetland in Korea were measured. The average concentrations of THg and MeHg in surface water were 2.04 ± 1.97 and 0.05 ± 0.05 ng L(-1), respectively. The concentrations of THg and MeHg in sediments ranged from 1.28 to 85.83 and ≤0.01 to 0.35 ng g(-1), respectively, and varied depending on the location. In the vegetated zone located near residential areas, the highest concentrations of THg and MeHg in both surface water and sediments were obtained near the residential areas, especially during the fall season. This result might be due to increased methylation rate of Hg by water turnover in the fall season. While THg and MeHg concentrations in water were not correlated, they were significantly correlated in sediment (r = 0.75; P < 0.01). Log-transformed Hg concentrations in sediments were highly correlated with log-transformed organic matter (OM) and acid volatile sulfide (AVS) concentrations (P < 0.01). We also collected five species of fish near the output point of the wetland, and MeHg concentrations in fish tissue varied from 21.30 to 154.66 μg kg(-1) w/w, which was significantly dependent on fish species (P < 0.05). This is the first reported study which measured the levels of mercury, especially MeHg in the wetlands, and freshwater fish species in Korea.

Mercury cycling in agricultural and managed wetlands: a synthesis of methylmercury production, hydrologic export, and bioaccumulation from an integrated field study

With seasonal wetting and drying, and high biological productivity, agricultural wetlands (rice paddies) may enhance the conversion of inorganic mercury (Hg(II)) to methylmercury (MeHg), the more toxic, organic form that biomagnifies through food webs. Yet, the net balance of MeHg sources and sinks in seasonal wetland environments is poorly understood because it requires an annual, integrated assessment across biota, sediment, and water components. We examined a suite of wetlands managed for rice crops or wildlife during 2007-2008 in California's Central Valley, in an area affected by Hg contamination from historic mining practices. Hydrologic management of agricultural wetlands for rice, wild rice, or fallowed - drying for field preparation and harvest, and flooding for crop growth and post-harvest rice straw decay - led to pronounced seasonality in sediment and aqueous MeHg concentrations that were up to 95-fold higher than those measured concurrently in adjacent, non-agricultural permanently-flooded and seasonally-flooded wetlands. Flooding promoted microbial MeHg production in surface sediment of all wetlands, but extended water residence time appeared to preferentially enhance MeHg degradation and storage. When incoming MeHg loads were elevated, individual fields often served as a MeHg sink, rather than a source. Slow, horizontal flow of shallow water in the agricultural wetlands led to increased importance of vertical hydrologic fluxes, including evapoconcentration of surface water MeHg and transpiration-driven advection into the root zone, promoting temporary soil storage of MeHg. Although this hydrology limited MeHg export from wetlands, it also increased MeHg exposure to resident fish via greater in situ aqueous MeHg concentrations. Our results suggest that the combined traits of agricultural wetlands - slow-moving shallow water, manipulated flooding and drying, abundant labile plant matter, and management for wildlife - may enhance microbial methylation of Hg(II) and MeHg exposure to local biota, as well as export to downstream habitats during uncontrolled winter-flow events.

Evidence for sites of methylmercury formation in a flowing water system: impact of anthropogenic barriers and water management

The Truckee River, California-Nevada, USA is impacted by mercury (Hg) contamination associated with legacy gold mining. In this work, we investigated the potential for hot-spots of methylmercury (MeHg) formation in the river. Mercury concentrations in multiple media were also used to assess the impacts of anthropogenic barriers, restoration, and water management in this flowing water ecosystem. Water samples were collected on a seasonal time step over 3 years, and analyzed for total Hg (THg) and MeHg concentrations, along with a variety of other water quality parameters. In addition, we measured THg and MeHg in sediments, THg in macroinvertebrates, and THg and δ(15)N and δ(13)C concentrations in fish. Differences in stable isotopes and Hg concentrations in fish were applied to understand the mobility of fish in the river. Mercury concentrations of specific macroinvertebrate species were used to identify sites of MeHg production. In general, loads of Hg and nutrients in the river reach above the Reno-Sparks metropolitan area were similar to that reported for pristine systems, while within and below the city, water quality impacts were observed. Fish isotope data showed that in the city reach food resources were different than those upriver and downriver. Based on Hg and isotope data, mobility of the fish in the river is impacted by anthropogenic obstructions and water manipulation. Below the city, particle bound Hg, derived from the legacy mining, continues to be input to the Truckee River. This Hg is deposited in riparian habitats and areas of river restoration, where it is methylated and becomes available to biota. During spring, when flows were highest, MeHg produced and stored in the sediments is mobilized and transported downriver. Fish and macroinvertebrate concentrations increased downriver indicating passive uptake from water. The information presented here could be useful for those doing river restoration and water manipulation in mercury contaminated environments.

Landscape factors and hydrology influence mercury concentrations in wading birds breeding in the Florida Everglades, USA

The hydrology of wetland ecosystems is a key driver of both mercury (Hg) methylation and waterbird foraging ecology, and hence may play a fundamental role in waterbird exposure and risk to Hg contamination. However, few studies have investigated hydrological factors that influence waterbird Hg exposure. We examined how several landscape-level hydrological variables influenced Hg concentrations in great egret and white ibis adults and chicks in the Florida Everglades. The great egret is a visual "exploiter" species that tolerates lower prey densities and is less sensitive to hydrological conditions than is the white ibis, which is a tactile "searcher" species that pursues higher prey densities in shallow water. Mercury concentrations in adult great egrets were most influenced by the spatial region that they occupied in the Everglades (higher in the southern region); whereas the number of days a site was dry during the previous dry season was the most important factor influencing Hg concentrations in adult ibis (Hg concentrations increased with the number of days dry). In contrast, Hg concentrations in egret chicks were most influenced by calendar date (increasing with date), whereas Hg concentrations in ibis chicks were most influenced by chick age, region, and water recession rate (Hg concentrations decreased with age, were higher in the southern regions, and increased with positive water recession rates). Our results indicate that both recent (preceding two weeks) hydrological conditions, and those of the prior year, influence Hg concentrations in wading birds. Further, these results suggest that Hg exposure in wading birds is driven by complex relationships between wading bird behavior and life stage, landscape hydrologic patterns, and biogeochemical processes.

Factors regulating the bioavailability of methylmercury to breeding rusty blackbirds in northeastern wetlands

Rusty blackbirds are undergoing rapid population decline and have elevated Hg concentrations while breeding in the Acadian ecoregion of North America. Factors regulating the bioavailability of methyl-Hg (MeHg) within this population's habitat were determined using water, invertebrates, and blood from adult rusty blackbirds collected for Hg-speciation, along with additional water column parameters: MeHg and THg, dissolved organic carbon, pH, dissolved oxygen, conductivity, redox potential, and temperature. Both DO(2) and pH were negatively related to biota MeHg, while water MeHg concentrations were positively related. Both invertebrate MeHg concentration and %MeHg increased with trophic level. Invertebrate MeHg concentrations were among the greatest reported when compared with those reported elsewhere for wetlands and waterbodies-often several times greater for similar taxa-while percent MeHg of THg were similar. An environment with high bioavailability of MeHg in combination with a high trophic position best explains elevated Hg concentrations for this species regional population.

Temporal and spatial distributions of sediment mercury at salt pond wetland restoration sites, San Francisco Bay, CA, USA

Decommissioned agricultural salt ponds within south San Francisco Bay, California, are in the process of being converted to habitat for the benefit of wildlife as well as water management needs and recreation. Little is known of baseline levels of contaminants in these ponds, particularly mercury (Hg), which has a well established legacy in the Bay. In this study we described spatial and short-term temporal variations in sediment Hg species concentrations within and among the Alviso and Eden Landing salt ponds in the southern region of San Francisco Bay. We determined total Hg (Hg(t)) and methylmercury (MeHg) in the top 5 cm of sediment of most ponds in order to establish baseline conditions prior to restoration, sediment Hg(t) concentrations in a subset of these ponds after commencement of restoration, and variation in MeHg concentrations relative to sediment Hg(t), pH, and total Fe concentrations and water depth and salinity in the subset of Alviso ponds. Inter-pond differences were greatest within the Alviso pond complex, where sediment Hg(t) concentrations averaged (arithmetic mean) 0.74 microg/g pre and 1.03 microg/g post-restoration activity compared to 0.11 microg/g pre and post at Eden Landing ponds. Sediment Hg(t) levels at Alviso were fairly stable temporally and spatially, whereas MeHg levels were variable relative to restoration activities across time and space. Mean (arithmetic) sediment MeHg concentrations increased (2.58 to 3.03 ng/g) in Alviso and decreased (2.20 to 1.03 ng/g) in Eden Landing restoration ponds during the study. Differences in MeHg levels were related to water depth and pH, but these relationships were not consistent between years or among ponds and were viewed with caution. Factors affecting MeHg levels in these ponds (and in general) are highly complex and require in-depth study to understand.

Wetland influence on mercury fate and transport in a temperate forested watershed

The transport and fate of mercury (Hg) was studied in two forest wetlands; a riparian peatland and an abandoned beaver meadow. The proportion of total mercury (THg) that was methyl mercury (% MeHg) increased from 2% to 6% from the upland inlets to the outlet of the wetlands. During the growing season, MeHg concentrations were approximately three times higher (0.27ng/L) than values during the non-growing season (0.10ng/L). Transport of Hg species was facilitated by DOC production as indicated by significant positive relations with THg and MeHg. Elevated concentrations of MeHg and % MeHg (as high as 70%) were found in pore waters of the riparian and beaver meadow wetlands. Groundwater interaction with the stream was limited at the riparian peatland due to the low hydraulic conductivity of the peat. The annual fluxes of THg and MeHg at the outlet of the watershed were 2.3 and 0.092microg/m2-year respectively.

Mercury and drought along the lower Carson River, Nevada: II. Snowy egret and black-crowned night-heron reproduction on Lahontan Reservoir, 1997--2006

Mercury concentrations in the floodplain of the Carson River Basin in northwestern Nevada are some of the highest ever reported in a natural system. Thus, a portion of the basin including Lahontan Reservoir was placed on the U.S. Environmental Protection Agency's Natural Priorities List for research and cleanup. Preliminary studies indicated that reproduction in piscivorous birds may be at risk. Therefore, a 10-year study (1997--2006) was conducted to evaluate reproduction of snowy egrets (Egretta thula) and black-crowned night-herons (Nycticorax nycticorax) nesting on Gull Island in Lahontan Reservoir. Special attention was given to the annual flow of the Carson River, the resultant fluctuation of this irrigation reservoir, and the annual exposure of snowy egrets and night-herons to methylmercury (MeHg). The dynamic character of the river due to flooding and drought (drought effect) influenced snowy egret and night-heron reproduction more so than did MeHg contamination of eggs. During an extended drought (2000--2004) in the middle of the study, snowy egret nests containing eggs with concentrations of MeHg (measured as total mercury [THg] approximately 100% MeHg) > or =0.80 microg THg/g, ww, all failed, but in 1997 and 2006 (wet years with general flooding), substantial numbers of young were produced (but fewer than at nests where eggs contained <0.80 microg/g). Thus, a variable reproductive threshold of tolerance to MeHg may be associated with habitat quality (food type and abundance). Clearly, drought was the most important factor affecting snowy egret annual productivity. In contrast to snowy egrets, night-herons generally had fewer nests meeting the 0.80 microg THg/g criterion, and those above the criterion were less sensitive to mercury than were snowy egrets. Furthermore, night-herons appeared more tolerant of drought conditions than snowy egrets because they nested earlier, selected more protected nesting sites, and had a more generalist diet that provided additional food options including terrestrial organisms, which also reduced exposure to MeHg. A putative biological effect threshold of 2.0 microg THg/g in whole blood for young of both species was evaluated, which was frequently exceeded, but with no evidence, while still in the colony, of an association with direct mortality. An evaluation of physiological associations with blood residues and post-fledging survival will be presented in future reports in this series.