2h-2: Mercury cycling in response to ecosystem perturbations

At the La Grande hydroelectric complex located in northern Qubec, Canada, the evolution of mercury (Hg) was monitored for over 35 years in natural and modified environments. Hg and methylmercury (MeHg) measurements were carried out in soil, sediment and biota (plankton, insects, fish) to better understand the processes involved in the mercury cycle and ultimately, understand and predict the impact of hydroelectric reservoirs. Impoundment of reservoirs leads to the conversion (Hg to MeHg) and circulation of Hg already present in plants and flooded soil in the aquatic environment. This organic form, MeHg, is easily accumulated by living organisms, from plankton and aquatic insects to fish that can be consumed by humans. In reservoirs, concentrations in all fish species increased rapidly after impoundment, peaking after 5 to 13 years in non-piscivorous species, and after 9 to 14 years in piscivorous species. These concentrations increase at levels 2 to 8 times higher than those measured in surrounding natural lakes. Depending on the reservoir, the return to levels found in fish of natural surrounding lakes was completed after 10 to 20 years for non-piscivorous species and after 20 to 31 years in most piscivorous species. Contamination of the food chain is mainly explained by changes in the Hg form present in soil after impoundment. The progressive methylation of initial inorganic Hg content increased from 1% in natural soil and up to 30% after 13 years of flooding. The following mechanisms appear to be most important in the increasing mercury level in fish: 1) increased bacterial methylation of Hg and its passive diffusion through the water column; 2) erosion of flooded organic matter in the drawdown zone, which makes fine, Hg-rich organic particles available for aquatic filter feeders, and active transfer of Hg by aquatic insects burrowing in flooded soil rich in MeHg; 3) periphyton development on flooded soils and vegetation, which promotes the methylation of Hg and its active transfer to fish via aquatic insects and zooplankton feeding on it. The increased MeHg production generally ends 8 to 10 years after impoundment due to rapid depletion of the readily decomposable elements of flooded soil and vegetation. After this time, MeHg transfer to fish by periphyton, zooplankton and insect larvae is reduced to levels occurring in natural lakes.

Stormwater ponds are a type of green infrastructure that effectively manage erosion, flooding, and pollutant loadings related to new residential and industrial development. Stormwater ponds are effective sinks for total mercury (Hg), but very little is known about their capacity to contribute significantly to the production of methylmercury (MeHg). Moreover, the spatial variability in Hg methylation, the possible geochemical and biological controls, and the impact of active management in these systems on MeHg production are entirely unknown. This characterization is necessary to better inform green infrastructure design that reduces MeHg-associated risk; for example, understanding spatial variability is important for inferring implications related to the extent of island and mid-bay bar construction in these systems. Here, we present Hg methylation potential rate constants (K-meth) and sediment MeHg and inorganic Hg concentrations in relation to a suite of ancillary biogeochemical and plant community data from several stormwater ponds across seasons within the Greater Toronto Area in Ontario, Canada. We find significant in situ MeHg production (mean K-meth increases with pond age up to 0.06 / day) in stormwater pond sediment. Mercury methylation rate constants and MeHg concentrations in older stormwater ponds are not substantially different from most other natural and constructed wetlands, whereas MeHg production and concentrations in younger stormwater ponds are lower in comparison. The approximate 10-15 year dredging management cycle of stormwater ponds significantly reduces mercury methylation and MeHg concentrations by removing organic sediment, but only temporarily, as K-meth values and MeHg concentrations increase nearly to pre-dredging levels within a single year. The role of aquatic plants and rhizosphere processes in controlling MeHg production in stormwater ponds is ambiguous; however organic matter availability and links with nitrogen cycling are important modulators of MeHg in these systems. The capacity of stormwater ponds to produce MeHg indicates a need for better characterization of both the MeHg mass balance of these systems and potential biological risk.

Legacy mercury (Hg) contaminated sediments can be a significant source of Hg to the aquatic system and atmosphere. The release of Hg can potentially be enhanced by various sediment perturbation processes, but the biogeochemical factors controlling Hg dynamics under transient redox resuspension events remain unclear. Solubility and transformation processes of Hg were investigated at different depths (0-2, 0-5 and 0-10 cm) in Hg contaminated pulp fiber sediment microcosms subjected to different levels of oxidation. Four different chemical species of inorganic divalent Hg (HgII) and methyl mercury (MeHg), enriched in different Hg isotope tracers, were added to sediment-bottom water microcosm systems: 201Hg(NO3)2(aq), 202HgII adsorbed to natural organic matter (202HgII-NOM), 198HgII as microcrystalline metacinnabar (β-198HgS(s)) and Me204HgCl(aq). The microcosms were exposed to air for different time periods (0-24h) and thereby spanned a wide range of redox potential, as reflected by a dissolved sulfide concentration range of ≤ 0.3 - 97 µM. The potential methylation rate constant (kmeth) and net formation of ambient MeHg (MeHg/THg molar ratio) were enhanced by up to 50% and a factor of 4, respectively at intermediate oxidation of the sediment microcosms, likely because of an observed 2-fold increase in sulfate concentration stimulating the activity of sulfate reducing bacteria methylating HgII. Differences in the chemical speciation of HgII in the solid/adsorbed phase caused kmeth to vary by a factor of 11-70 in this study due to differences in HgII partitioning for the different species. Chemical speciation was a major controlling factor both for the absolute HgII methylation rate, and for the response in the rate following increased oxidation of the system. The composition of organic matter (OM) varied with sediment depth such that compared to the deeper sediments, the 0-2 cm sediment contained a 2-fold higher proportion of labile OM originating from algal and terrestrial inputs, which serve as metabolic electron donor for microorganisms. This caused kmeth to be up to a factor of 3 higher in the 0-2 cm sediment compared to the deeper ones. The kmeth was lowest and constrained by redox-driven solubility for the β-198HgS(s), intermediate and controlled by both HgII solubility and bacterial activity for the 202HgII-NOM tracer, and highest and controlled by bacterial activity for the 201Hg(NO3)2(aq) tracer. The results in this study provide important knowledge to identify “high-risk” contaminated sites regarding reactivation of Hg following transient redox resuspension events.

In the Honnedaga liming project the response of mercury (Hg) and dissolved organic carbon (DOC) are investigated in response to both watershed and in-channel additions of calcium carbonate. In northern Europe and northeastern North America, many aquatic ecosystems are experiencing increases in DOC concentrations as a result of decreases in acidic deposition. These increases in DOC have been correlated with elevated concentrations of Hg in fish. Large scale changes in biogeochemical cycling are being driven by processes such as climate change and recovery from acidic deposition. Honnedaga Lake, located in the southwestern Adirondack Park of New York State has been recovering from acidic deposition. Despite improvements in chemistry, many of the tributaries to Honnedaga Lake remain chronically or episodically acidic, which has limited successful reproduction of lake-resident heritage strain Brook Trout (Salvelinus fontinalis) in these critical spawning and nursery habitats. To improve tributary chemistry and enhance Brook Trout recruitment to the lakes population, lime was applied once in 2013 to the watershed of a chronically-acidic tributary, and every year during 2013-2015 to an episodically acidic tributary stream channel. In this study, we examine the impact of watershed and in-channel liming on streamwater Hg dynamics, including DOC concentrations. The watershed lime application resulted in increases in concentrations of DOC (18.4 mg C/L), total Hg (5.50 ng/L) and methyl Hg (1.94 ng/L) to previously unobserved levels. The MeHg concentrations were not significantly different, even though DOC and THg remained significantly elevated. Stream water of the tributary to which lime was directly applied exhibited a similar response, with short lived increases in Hg following annual applications. The results showed that mobilization of dissolved organic carbon from soils to surface waters may facilitate increases in Hg concentrations because of a strong affinity for Hg. This research suggests that concentrations of THg in streams may increase as impacted systems return to pre-acidification conditions.

While the Adirondack region of New York experiences modest atmospheric mercury deposition, it is considered a biological mercury hotspot. The Adirondacks are also recovering from severe impacts of acid deposition. Wet and litter mercury deposition, and stream and lake mercury are monitored at Huntington Forest in the central Adirondacks. We have found no changes in wet mercury deposition since measurements were initiated in 2000, but litter mercury appears to be decreasing. Arbutus Lake inlet and outlet samples at Huntington Forest show long-term decreases in concentrations and fluxes of both total and methyl mercury, despite increases in dissolved organic carbon. More broadly we examined spatial patterns and temporal trends in mercury in standard length yellow perch (Perca flavescens) across Adirondack lakes. We find elevated concentrations of mercury in fish, particularly in the western Adirondacks. Mercury in yellow perch are elevated in lakes with low pH and acid neutralizing capacity and high concentrations of monomeric aluminum, indicative of sites acidified by acid deposition. We did not observe any relationship between fish mercury and lake concentrations of dissolved organic carbon, although we did find strong correlations between total and methyl mercury concentrations and dissolved organic carbon in the water column. 17 Adirondack lakes have multiple year observations of mercury in yellow perch. Trend analysis indicates that all but one of these lake are showing statistically significant decreases in mercury concentration at a mean rate of 0.007 µg g-1 ww yr-1, with the greatest decreases occurring in lakes with the lowest pH and acid neutralizing capacity. These patterns suggest strong linkages between effects of acid and mercury deposition, and recovery of fish mercury may reflect emission control of both contaminants.

The International Joint Commission is enacting a plan (Plan 2014) to alter water levels above the Moses-Saunders power dam to simulate natural fluctuations and restore biodiversity in this river ecosystem. If the dominant macrophyte, Typha, is reduced in abundance, there is potential for large amounts of sediment, containing mercury (Hg), to become mobilized into the food web. Hg becomes hazardous if sulfate-reducing bacteria (SRB) are present. The St. Lawrence River has historically been impacted by fossil fuel emissions that deposit both sulfur and Hg. Due to extensive Typha, there is likely sufficient organic matter and phosphorous to support SRB. Here, the capacity of upper St. Lawrence River wetlands to support SRB was determined by quantifying concentrations of total mercury, sulfur, phosphorous and organic matter, as well as extracting SRB and Hg-methylating genes from sediments. Four wetland types were compared: barrier beaches, drowned river mouths, protected embayments and open embayments. Protected embayments contained significantly higher Hg concentrations than other wetland types. An estimated 71kgof Hg will be re-suspended and flushed downstream should water levels fluctuate post-enactment of Plan 2014. In addition to economic costs from decreasing water flow through the dam, costs to fish population sustainability from Hg bioaccumulation need be considered. Hg concentrations in water and methylmercury (MeHg) concentrations in wildlife, such as predatory fish and water birds, need to be monitored as Plan 2014 ensues.

Acknowledgements: Funding for Evie Brahmstedt was provided by the National Science Foundation through the Research Experience for Undergraduates program and the Great Lakes Research Consortium. This project is funded in part by the New York Sea Grant Institute.

Mentor: Dr. Michael Twiss, Department of Biology, Clarkson University

This research was funded by New Yorks Great Lakes Basin Small Grants Program (New York Sea Grant), EB was funded by NSF REU award no. 1359256 and a supplement from the Great Lakes Research Consortium (NY).

Vegetation and soil are known reservoirs for atmospheric mercury (Hg) uptake and sequestration. Wildfire is an important ecosystem perturbation, releasing stored Hg back to the atmosphere and also changing important ecosystem properties including canopy cover and soil structure and quality. A limited number of studies have quantified the release of Hg to the atmosphere from wildfire, confirming that there are losses of Hg from vegetation and soil during a fire, with the duration and intensity of heating affecting the amount of Hg loss. Beyond the release of Hg to the atmosphere at the time of the fire, it is also beneficial to understand how Hg is cycled within the ecosystem following a severe fire, specifically the ability of the soil to take up, retain, and transport Hg as the ecosystem recovers from the disturbance. This has implications for Hg transport to aquatic ecosystems as well as the fate of atmospherically deposited Hg. To assess the impact of severe fire on Hg retention and transport within forest ecosystems, we quantified the Hg and carbon (C) content of soil cores collected within the burn scars and nearby reference sites of three severe 2002 wildfires in Colorado, U.S.A. The fires encompass large precipitation and ecosystem gradients (ranging from relatively dry montane Ponderosa forests to relatively wet subalpine Fir-Spruce forests) and there is little forest regrowth more than a decade later. Soil cores were divided into 1-cm increments and separated into coarse and fine fractions. As samples were collected on the toe slope, mid-slope, and crest of the representative watershed in each site, we have a unique opportunity to look at the effects of hillslope mobilization of soil-bound Hg. Analysis of the crest sites shows relatively similar Hg content in the top 6-cm of soil (ranging from 5.27 to 16.4 g Hg/ha, on average) between burned and unburned sites, indicative of relatively low Hg accumulation. We also find the Hg:C molar ratio in the coarse and fine fractions to be significantly higher at the burned sites compared to their corresponding unburned sites, consistent with Hg uptake from atmospheric deposition in the absence of C input in the burn areas. In addition to the effects of hillslope transport, the low moisture content and relatively organic-poor soil in these Intermountain West ecosystems allow us to isolate the effect of the forest canopy on Hg uptake and retention in comparison to the burn scars with no canopy.

Erosion of watersheds following wildfire leads to the deposition of ash-laden sediment and associated mercury in receiving water bodies. This study investigated the influence of reducing conditions on total mercury retention and divalent mercury (Hg(II)) binding in ash-laden sediments. Sediments were collected from a watershed that had burned within five months. Sediments were incubated under anoxic conditions to simulate reservoir deposition. Pore water was sampled over 30 days to monitor total mercury, dissolved organic matter, and major ions as the surface sediments transitioned from oxic to sulfate-reducing conditions. The organic matter of the sediments was characterized using sulfur X-ray absorption near-edge structure spectroscopy to assess changes in sulfur oxidation state. A competitive ligand exchange technique was used to quantify changes in Hg(II) binding capacity of the sediment. Over the incubation period, sulfur oxidation state transitions were observed as a loss of highly oxidized sulfur (ΔSox = -9%), and an increase in reduced organic sulfur (ΔSred = +10%). Some of the total mercury in the sediment was initially released to pore water (0.2%), but the transition to reducing conditions produced increased association between total aqueous mercury and the ash-laden sediment. The reducing conditions resulted in a 60% decrease of filter-passing total mercury (≤ 0.2 µm) and an 80% decrease in colloidal total mercury (0.2 µm ≤ X ≤ 10 µm) in the porewater over the 30 day incubation. During the incubation, Hg(II) binding capacity of ash-laden sediment increased by 10-fold based on competitive ligand exchange measurements. The results of this study demonstrate that ash-laden sediments deposited following wildfire can sequester more Hg(II) if the sediment is subjected to reducing conditions following burial.