EFFECTS OF BELOWGROUND WARMING ON PEAT POREWATER TOTAL MERCURY AND METHYLMERCURY CONCENTRATIONS: EARLY RESULTS FROM SPRUCE
The Spruce and Peatlands Under Climatic and Environmental Change (SPRUCE) Experiment is pushing the envelope assessing the effects of whole ecosystem warming on numerous ecosystem processes. As part of the SPRUCE framework we sampled peat porewaters at 0, 30, 50 and 100 cm for totalmercury (THg)and methylmercury (MeHg) across five temperature treatments (0oC or ambient, 2.25oC, 4.5oC, 6.75oC, and 9oC) during the 2016 unfrozen period beginning in June and ending in November. Both aboveground and belowground warming is a differential compared to outside the SPRUCE chambers and belowground warming extends to >2 m. Porewater THg concentrations at 0 cm depth were significantly higher in the 6.75oC and 9oC treatments than at the 0oC, 2.25oC and the 4.5oC treatments which were similar. At 30 cm there was an opposite effect where the 9oC treatment had lower THg concentrations than at lower temperature treatments. Below 50 cm there were few temperature treatment effects on THg. Porewater MeHg concentrations were highly variable at 0 cm depth with no significant differences among temperature treatments. At 30 cm depth, the 0oC treatment was significantly lower than the 4.5oC, 6.75oC and nearly different than the 9oC treatment. At 50 cm depth all temperature treatments had significantly lower concentrations than the 9oC treatment. We had few MeHg samples above detection at 100 cm depth. Our initial data indicates that elevated soil temperatures increase peat pore water MeHg concentrations in the 30-50 cm depth zone, while also increasing THg in surface waters, but we unexpectedly found decreases in THg at the highest temperature treatment in the 30 cm zone, with few other differences below 30 cm. We will discuss possible mechanisms that lead to these results and larger implications for Hg cycling with climate change.
MERCURY AND METHYLMERCURY IN TRIBUTARY INPUTS TO KLUANE LAKE, YUKON TERRITORY, CANADA
Kluane Lake is located in the southwest of the Yukon Territory, Canada, and is part of the traditional territory of the Ln Mn Ku Dn, the Kluane Lake People. The community depends on Kluane Lake as an important source of subsistence food fishes, and the lake is well known for supporting healthy populations of Lake Trout and Lake Whitefish. Although situated in a relatively remote and pristine region, community interest in contaminant levels in food fishes supported intensive research on mercury (Hg) in the biotic and abiotic compartments in Kluane Lake, and instigated a pilot-scale investigation of Hg, methylmercury (MeHg) other metals and related ancillary measurements in the waters and sediments of its tributary inputs. We found that total Hg concentrations ([THg]) varied considerably among tributaries (<0.05 to 1.1 ng/L for filtered, and from 0.1 to 25 ng/L unfiltered), with elevated unfiltered [THg] largely reflecting a significant suspended sediment load in some glacier-fed streams. Methyl mercury concentrations ([MeHg]) were below detection (0.006 ng/L) for many streams, but two of the larger tributaries to the lake (Burwash Creek and Gladstone River) had measureable MeHg concentrations (0.010 0.040 ng/L). However, the volumetrically largest inflow to Kluane Lake, the Slims River, had dissolved [THg] and [MeHg] below detection. An examination of other water chemistry variables related to [Hg] found important relationships among [Hg], [MeHg] and the quantity and quality of dissolved organic carbon (DOC) in tributary waters. Dissolved [MeHg] was positively related to total DOC. There was no relationship between [THg] and total DOC, but [THg] was inversely related to fluorescence measures of organic matter freshness, and positively related to measures of organic matter humification, suggesting that dissolved [THg] is being derived from well-humified sources of organic matter. Currently, Kluane Lake has extremely low dissolved Hg concentrations because the largest, glacially-dominated tributary input (Slims River) has exceptionally low Hg and DOC. As the importance of glacier melt-derived inputs decrease, the relative importance of other higher Hg and DOC tributaries will increase. Although all waters in this catchment are dilute, climate-change driven shifts in runoff could ultimately drive a shift in lake water chemistry and Hg availability in the future.
EFFECT OF LOW-MOLECULAR-WEIGHT ORGANIC ACIDS ON MERCURY TRANSFORMATION IN SOIL IN WATER-LEVEL FLUCTUATION ZONE OF THREE GORGES RESERVOIR, CHINA
According to the planning and design, the water level of Three Gorges Reservoir fluctuates between 145 and 175 m, which forms a water-level fluctuation zone (WLFZ) occupying areas of more than 400 km2. Soil low-molecular-weight organic acids (LMWOAs), mainly originated from the decomposing organic matter, root exudates and microbial metabolites, are important ligands involved in complexation reactions of heavy metals and hence, their bioavailability. The WLFZ became periodically alternated wetting and drying resulting in the vegetation suffered inundation (decomposition) and growth, which may influence the Hg bioavailability in soil and bioaccumulation in aquatic food chains as the produced LMWOAs from plants. In Aril of 2016 to September, seven common LMWOAs including Citric acid (CA), Oxalic acid (OA), Tartaric acid (TA), Acetic acid (AA), Propanedioic acid (PA), Succinic acid (SA), Malic acid (MA) from root exudates, rhizosphere soil and bulk soil in WLFZ were sampled and analyzed. Then, LMWOAs (CA, TA and OA) were chosen to investigate their effect on the Hg behavior in WLFZ soil. Results were followed as, 1. TA and OA occupied for a higher percentage in root exudates, rhizosphere soil and bulk soil, but PA and CA were just different. Due to the biodegradation and clay adsorption, a lower LMWOAs concentration (M) in bulk soil was found compared to that in rhizosphere soil (mM). Statistical analysis showed a significant difference among various LMWOAs in root exudates, rhizosphere and bulk soil. 2. The presence of OA, TA, and CA promoted Hg desorption from soil to solution, and the increasing bio-available Hg speciation was found as the increasing LMWOAs concentration, which may be due to the complexation of carboxyl groups and ion exchange reactions at mineral surface. Compared to the control groups, LMWOAs increased the ratio of methylmercury to total Hg of 2.72%,1.76%, 2.02% for CA, OA and TA in anaerobic condition, respectively, which were higher than that in aerobic condition, indicating a stimulated Hg methylation was anaerobic microbes other than aerobic microbes caused by LMWOAs. Furthermore, the coexistence of various LMWOAs could enhance the activation effect to soil Hg of WLFZ.
SEASONALITY OF MEHG INPUT, OUTPUT, FORMATION AND DEGRADATION IN A HUMIC BOREAL LAKE IN RELATION TO THE CHEMICAL SPECIATION OF HG, FE AND S
Lake Ängessjön is a shallow (2.5 m maximum depth) humic boreal lake of 0.7 km2 in northern Sweden having concentrations of Hg in fish exceeding the limit set by health authorities for regular consumption of 0.5 mg kg-1 fw if standardized to1-kg pike. The total catchment area of 3.9 km2 is composed of 56% forest soils, 15% peatland soils, 10% agricultural soils and 18% lake water. Terrestrial runoff contributes about 85 % of Hg and >95% of MeHg external sources to the lake. Annual input-output budgets for 2007 and 2009 proved the lake to be a sink for Hg (16-30 % of input), MeHg (11-33% of input) and organic carbon (7-26% of input). The lake sediments provided seasonaly variable internal sources of MeHg (Hg methylation) and sinks of MeHg (demethylation). Rates of both processes were highest in the top 0-3 cm sediment and peaked during mid-summer in concurrance with maximum water temperatures and sedimentation rates of autochthonous (algal) organic matter. MeHg demethylation rates in the sediment showed an additional peak during winter. Photolysis of MeHg in the water column reduced its concentration in the bottom water drastically during summer, depleting (by diffusion) MeHg also in the top 0-3 cm sediment. Deeper sediment (3-10 and 10-20 cm) showed a clear seasonal variation in MeHg concentrations with a minimum during summer and a maximum during winter. The solubility and chemical speciation of Hg could be explained by reactions with reduced sulfur species, varying with season and depth in sediments. Because of the limited water depth, the lake does not stratify during summer and sulfide is not detected in the bottom water any time of year. In the top sediment, FeS and S0 were important components providing conditions for polysulfide formation and keeping the dissolved sulfide concentration stable between 0.5 and 2.0 µM in apparent equilibrium with the amorphous mineral mackinawite (FeSm). In deeper sediment, crystalline FeS2 (pyrite) was the dominant form (>60%) of sulfur. The concentration of organic thiol functional groups (RSH) associated with NOM was determined by use of Hg EXAFS spectroscopy. Thiols represented 7% of total S in the sediment. Thermodynamic modelling, using the latest updated constants for the formation of metacinnabar (β-HgS) and the complexation of Hg to RSH groups could reasonably well explain porewater concentrations of Hg by depth and season. We conclude that the activity of methylating anaerobic bacteria, driven by increasing temperatures and high availability of electron donors in concert with increased concentrations of dissolved Hg-sulfide and -polysulfide complexes, increased rates of MeHg methylation in boreal lake sediments during summer. Also MeHg import from terrestrial sources was significant. Yet, MeHg was net-degraded in the lake because of the very efficient photolysis of MeHg in the water column.
DISSOLVED ORGANIC MATTER AND NUTRIENT CONTROLS ON AQUEOUS MERCURY SPECIATION IN ALASKAN WETLANDS
In northern wetlands, sulfate availability is well-recognized as an important environmental variable influencing the conversion of divalent mercury (Hg(II)) to bioavailable methylmercury. However, no efforts to date provide a complete account of mercury behavior in northern wetlands by also investigating the controls on (1) total mercury abundance and (2) the dark reduction of Hg(II) to dissolved gaseous mercury (Hg(0)), a transformation proposed to limit mercury retention and bioavailability in ecosystems. A complete understanding of the factors controlling the distribution of mercury between relevant aqueous species (Hg(II), methylmercury, Hg(0)) in northern wetlands is needed to predict mercury behavior, especially considering the changes in wetland hydrologic connectivity and nutrient status expected as a result of continued warming of northern environments.
This study explores the biogeochemical controls on mercury abundance, methylation, and dark reduction in eight Alaskan wetlands that differ in nutrient status. Wetlands, selected to span a continuum from ombrotrophic bogs to minerotrophic fens, were sampled in interior and southcentral Alaska in the summers of 2015 and 2016. Concentrations of pore water constituents (dissolved organic carbon (DOC), inorganic anions, major cations, simple organic acids, sulfide) and compositional differences in dissolved organic matter (DOM) aided the interpretation of the environmental controls on mercury behavior in wetlands. Associations of Hg(II) with dissolved organic matter (DOM) were central to mercury abundance in the wetlands, as supported by site-specific correlations between pore water DOC and total mercury concentrations. Methylmercury formation in bog pore waters was minimal due to sulfate limitation, whereas methylmercury formation in fens was explained by the availability of inorganic mercury and sulfate. Good agreement was observed between the relative abundance of methylmercury in wetlands and estimated rates of Hg(II) methylation, the latter measured by stable mercury isotope injections in peat cores. Results identify sulfate-rich fens as critical locations for methylmercury formation. Across all wetlands, the relative abundance of Hg(0) was inversely related to DOC concentration, which suggests that mercury-DOM interactions limit dark Hg(0) formation in pore waters. Relationships between the absolute and relative concentrations of methylmercury and Hg(0) were not observed. Insights on the role of pore water constituents on mercury abundance and speciation in Alaskan wetlands are discussed in context of future hydrologic conditions expected in northern latitudes in response to a warming climate.
THE ROLE OF DOM IN CATCHMENT AND LAKE MERCURY CYCLING IN A BOREAL LAKE CATCHMENT
In the ecological research lake catchment Langtjern in southern Norway, monitoring programmes collect data on streamwater and lake chemistry, hydrology and climate. Here, we present 10 years of data of mercury species (Hg, MeHg) in inlets and outlet, 5 years of lake depth profiles on Hg and MeHg, in addition to dissolved organic matter (DOM) size fractionation and degradability. The data are collected to infer catchment and lake processes on mercury and DOM cycling and foodweb exposure to Hg.
We observe significant declines in streamwater concentrations of MeHg, MeHg to TOC ratio, and the MeHg to HgT ratio (a proxy for methylation potential). No temporal change is found in concentrations of Hg, TOC, and the Hg to TOC ratio. In the inlets, summer concentrations of MeHg are significantly (p<0.01) higher than in other seasons, while seasonal variation of MeHg in the outlet is minimal. Seasonal variation in HgT is stronger than for MeHg and appears to be associated with variation in TOC. In the outlet, the Hg to TOC ratio is significantly (p<0.0001) higher than in the inlets, which suggests that DOM is retained and removed in the lake while Hg is redistributed to remaining DOM compounds. Significant (5-20% of annual inputs) removal and retention of DOM is confirmed by an input-output budget of DOC for the lake. The Hg to TOC ratio in the streams is lower than in the lake, while the MeHg to HgT ratio at 1m depth is lower than at higher depth, possible reflecting losses of MeHg from photo-oxidation.
Size fractionation of DOM (tangential ultrafiltration) and studies of biodegradability of size fractions (measured as oxygen consumption) show evidence of significant changes between inlet and outlet in size fraction, biodegradability of fractionated DOM and Hg associated with the different size fractions. The smallest size fraction (<100 kDa) contains a larger proportion of the DOC in the outlet compared with the inlet, and is most labile in the outlet (expressed in oxygen consumption per g DOC).
Additionally, the Hg to DOC ratio for all size fractions is largest in the outlet. This may imply that the base of the aquatic foodweb, especially for organisms relying on bacteria as energy source, in the outlet is more exposed to Hg than in the inlets. Bacterial consumption of easily degradable, Hg-contaminated DOM is thought to be an important pathway for Hg into the aquatic foodweb.
DISSOLVED ORGANIC MATTER CONTROLS MERCURY PHOTOREACTIONS IN WATER
Methylmercury (MeHg) contamination through bioaccumulation and biomagnification is an issue in many remote ecosystems far from direct pollution sources. Quantifying why and how some ecosystems are more sensitive to contamination following atmospheric mercury deposition is key to mercury fate modeling. While we know bacterial pathways dominate the mechanism of mercury methylation, the demethylation of MeHg is less understood. Photodemethylation is thought to be one of the main processes through which MeHg can be converted into a less biologically toxic form of mercury. Previous studies highlight the importance of photodemethylation to mercury budgets, yet few have examined the magnitude and variability of photodemethylation rates as a function of associated dissolved organic matter (DOM). A temporal comparison study between summer and fall was conducted using lake water collected from 6 lakes in Kejimkujik National Park, Nova Scotia, Canada. Sample lakes were chosen based on a known range of DOM concentration. Lake waters were filtered to 0.45 m and placed in closed polytetrafluoroethylene (PTFE) bottles with >50% headspace, spiked with 3 ng/L MeHgOH, and exposed to 0, 1, 2, 3, 5, and 7 days of natural solar radiation in each experimental season. Lakes with higher DOM concentrations had significantly lower rates of photodemethylation than lakes with lower DOM concentration (p<0.001). Additionally, there were negative linear relationships between rates of MeHg photodemethylation and rates of DOM photomineralization (R2s=0.58-0.72) and DOM photobleaching (R2s=0.83-0.90). This key finding suggests that competition for photons by DOM may reduce the potential for MeHg photodemethylation in high carbon waters and that this relationship persists across seasons.
PROBING THE DOM-MEDIATED PHOTODEGRADATION OF METHYLMERCURY BY USING ORGANIC LIGANDS WITH DIFFERENT MOLECULAR STRUCTURE AS DOM MODEL
Photodegradation is the main depletion pathway of methylmercury (MeHg) in aquatic environments. The formation of MeHg-dissolved organic matter (DOM) complexes is a key step in MeHg photodegradation. However, the major functional groups in DOM mediated MeHg photodegradation have yet been clear. In this work, we systematically investigated the effects of DOM molecular structures on MeHg photodegradation by using a variety of organic ligands with different functional groups (e.g., thiosalicylic acid, thiophenol, and thioaniline). The results showed that thiol and phenyl groups may be the major functional groups in DOM dominate MeHg photodegradation, and the photodegradation rates also depends on the type (carboxyl, hydroxyl, amino group) and position (ortho-, meta-, para-) of other substituents. In addition, the addition of non-photochemically-active thiol ligands (e.g., mercaptoethanol and dithiothreitol) and high concentration Cl- could significantly inhibit the o-thiosalicylic acid-induced MeHg photodegradation, indicating that complexation of MeHg with theses ligands are necessary for MeHg photodegradation. Sparging with O2 has negligible effect on MeHg photodegradation, while sparging with N2 significantly enhances MeHg photodegradation. This finding suggests that MeHg photodegradation may be a reductive process, which was further supported by identifying the degradation products of MeHg.