2h-1: Mercury cycling in response to ecosystem perturbations

Global contamination by anthropogenic mercury is an issue of great concern as highlighted by the Minamata Convention on Mercury. The organic form of mercury, methylmercury (MeHg), is a ubiquitous, bioaccumulative and toxic contaminant that biomagnifies in marine food webs, exhibiting the highest concentrations in apex predators. In the face of climate change, ocean warming and acidification may potentially intensify methylmercury bioaccumulation in food webs with implications for food safety and public health. Several studies suggest that climate change is already changing pollutant fate and transport in the ocean, and affecting pollutant exposure and accumulation in marine organisms that can subsequently result in adverse effects on ecosystem and human health. However, the interplay of the combined effect of climate change and methylmercury exposure and bioaccumulation has been rarely quantified. Using an ecosystem modelling approach (EwE model with the Ecotracer module), we simulated potential future scenarios for mercury bioaccumulation in a regional food web (i.e. Chinook salmon-resident killer whale food web from the Northeastern Pacific, including British Columbia, Canada and Washington State, USA) under climate change forcing, including Representative Concentrations Pathway (RCP), i.e. RCP 2.6 = strong climate change mitigation (low CO2 emissions), and RCP 8.5 = business as usual (high CO2 emissions). The outcomes were compared against a baseline scenario (no climate change forcing). The model simulations showed that MeHg bioaccumulation in the food web is exacerbated by ocean warming and acidification. In comparison to the baseline scenario, the percentage increase in mercury concentration accumulated in Chinook salmon is 1% and 10% under RCP 2.6 and RCP 8.5 scenarios, while in it is 1% and 8% in killer whales, respectively. The bioaccumulation simulated by the model echoes the importance of mercury uptake rates, which are faster in salmon relative to that in fish-eating killer whales; with the latter having a slower elimination rates. This level of bioaccumulation would have negative effects on all seafood consumers, but considering that seafood consumption by coastal First Nation communities from British Columbia is 15 times (64% of consumed seafood is salmon) that of the average Canadian consumers, Indigenous people will be exposed to much more mercury in comparison to non-Indigenous people in the long-term. The socio-economic implications of mercury-contaminated salmon fisheries intensified by climate change are discussed. This study improves our understanding of the interactions of climate and pollution impacts of multiple human stressors, highlighting key areas for concerted research and potential mitigation policies.

Rapid warming in the Arctic has resulted in unprecedented occurrence of ice-free waters and opportunities for new commercial fisheries. Global mercury (Hg) emissions from anthropogenic sources have greatly enriched levels of the toxic and bioaccumulative monomethylmercury (MeHg) in global marine food webs. Higher levels of MeHg have been reported in many Arctic food webs than at mid-latitudes and enrichment of tissue burdens across a suite of biota have been estimated at greater than 10-fold the pre-industrial levels. Most of this increase has been attributed to anthropogenic pollution but these prior analyses have not evaluated the contribution of changing ocean biogeochemistry. Climate related warming in the Arctic is nearly double that of mid-latitude ecosystems, leading to melting sea ice and permafrost, warmer seawater temperatures, higher freshwater discharges and marine productivity. Previous GEOS-Chem modeling has suggested reduced sea-ice cover will lead to increased oceanic evasion of elemental Hg (Hg0) and lower total Hg concentrations but the implications for MeHg production have not been explored. The overall result of these conflicting effects on MeHg levels in commercially important fish species is unknown. Here we construct a biogeochemical model (MITgcm) for the Arctic that combines cycling of MeHg and uptake into biota. We use new measurements from the central Arctic basin in 2015 to experimentally evaluate the role of changing temperature, productivity, and light for production and degradation of MeHg in Arctic Ocean seawater, sea ice and food web. Modeled fish MeHg levels are most sensitive to changes in temperature and diet shift; we find that a 1°C rise in seawater temperatures can lead to a 30% increase in MeHg content of some important commercial fish species despite a significant reduction in global Hg emissions.

Areas of the St Louis River watershed in northern Minnesota have been chronically impacted by sulfate loading from iron mining for over a century. Multiple studies were conducted on several natural systems over a wide range of sulfate concentrations to assess the effects of elevated sulfate loading on Hg dynamics, methylmercury (MeHg) bioaccumulation and Hg methylation. Chronically sulfate-impacted systems were characterized by high sulfate (100s to 1000s ppm) in the water, high dissolved magnesium, low solid-phase Fe:S ratios, and elevated dissolved sulfide in sediment porewater. However, the high sulfate systems were not systematically elevated in MeHg in the water, sediment, or biota (dragonfly larvae). In a principal component analysis of surface water measurements in the St Louis River, sulfate concentrations were associated with magnesium concentrations while dissolved MeHg concentrations grouped with Fe, DOC, and THg concentration. This finding is consistent with MeHg, THg, DOC, and Fe being delivered primarily via watershed inputs while Mg and sulfate are discharged from mining activities. MeHg in stream-dwelling dragonfly larvae were correlated with dissolved MeHg concentrations over a wide range of sulfate concentrations during two relatively high flow years where MeHg inputs from local watersheds appeared to dominate over any in-stream MeHg transformation processes. Dissolved and solid-phase MeHg concentrations in lake and wetland sediment in the watershed were statistically similar across sulfate-impacted and un-impacted systems. Chronically sulfate-impacted systems had significantly lower potential methylation rates compared to sulfate-limited systems due to accumulation of dissolved sulfide which limited long term MeHg accumulation, perhaps as a result of less bioavailable Hg-S complexes or enhanced partitioning of MeHg into the aqueous phase. These studies were conducted during relatively high flow conditions in which non-point source, precipitation-driven mobilization of MeHg from the landscape appear to have overwhelmed any potential impacts of chronic sulfate loading on Hg methylation and MeHg bioaccumulation in near-stream areas. In wetland and lake sediments, iron and water level fluctuations were identified as key controls on the extent to which sulfate and sulfide impact sediment MeHg production and accumulation. Further research is needed to determine how MeHg in sulfate-limited freshwater systems might respond to initial increases in sulfate loading and how iron and sulfur loads interact to define porewater sulfide and Hg bioavailability; however, chronically impacted systems do not appear to continually accumulate or produce MeHg at rates different from un-impacted systems in the St Louis River watershed.

Methylmercury (MeHg) is a well-known neurotoxic that bioaccumulates and biomagnifies through the food web and the factors that control its production and accumulation remain an important and complex question. Eutrophication is widely believed to stimulate mercury methylation and MeHg accumulation, while algae blooms are thought to reduce MeHg accumulation due to a dilution effect. Here we present data from a unique environment located in the tropics but at an altitude of more than 3800 m above the sea level. The emblematic Lake Titicaca is also naturally enriched with sulfate (frequently above 200 mg L^-1) and partly polluted with domestic and industrial wastewater mainly at Cohana Bay (DOC up to 6.1 mg L^-1). The gradient of eutrophication among different locations of the lake served as a natural experiment scenario. As expected we found an increased concentration of MeHg in more eutrophic compartments (> 60% of MeHg) than in more oligotrophic compartments (< 10% of MeHg) of the same lake. Such difference is in agreement with the hydrogen sulfide concentrations found close to the sediments and mainly produce by sulfate reducing bacteria (nearly complete inhibition of H2S production with molybdate), which increases with eutrophication and may reach more than 2 mg L^-1 in water overlying sediments. Therefore, the most likely explanation would be that eutrophication increases MeHg production by stimulating sulfate-reducing bacteria and possibly by attenuating UV radiation-mediated demethylation effect, which is critical considering the high altitude of the system. Surprisingly, something very similar was observed during a massive algae bloom in April 2015. Hydrogen sulfide production increased significantly (up to 155 µg L^-1 in surface waters) as well as MeHg concentrations through the lake by a factor of up to 40%. The algae bloom was located in the first 10 cm of the water surface, blocking light penetration and probably MeHg photodemethylation, while reducing oxygen saturation in the water column from more than 100% to 40% during the day. Such oxygen concentrations were likely lower during the night and may have allowed microbial sulfate reduction through the water column and the consequent increase of methylmercury production. The main algae of the bloom (Carteria sp.) is known to produce appropriate electron donors for sulfate-reduction. Consequently the particularities of this environment allowed the stimulation of MeHg production and inhibition of photodemethylation to overcome any dilution effect associated to the algae bloom. The degree in which our results can be extrapolated to other sulfate-rich environments is uncertain, but to our knowledge this is observed in natural environments and may have important implications regarding the management of sulfate-rich and mercury polluted environments.

The central European lake district extends within the Bohemian forest and Bavarian forest Mountains. It includes 8 glacial lakes whose catchments were strongly acidified in the 70 and 80s of the 20th century. The lakes in altitudes from 935 to 1087 m a.s.l. have been oligotrophic and prevailing tree species within the lake catchments was Norway spruce (Picea abies).

The research was mainly focused on Plešné lake (PL) catchment located at 1087 m .a.s.l. covering area of 0.67 km2. In 2004, forest at PL catchment was infested by the bark beetle (Ips typographus) and 88%–99% of trees had died by 2011.

To assess changes in Hg distribution within the soil profile due to forest dieback we compared the soil data from year 1999 with the situation in 2015. While the mean Hg concentrations in the O horizons decreased from 424 to 311 µg/kg, in A horizons the situation was reversed and we detected increase from 353 to 501 µg/kg. The means of Hg concentration in mineral soil remained relatively similar at 145 and 121 µg/kg. The increase in Hg concentrations within A horizons was concurrent with increase in organic C from 24.5% in 1999 to 39.9% in 2015. But the Hg/C ratio in the A horizon remained rather comparable (1.27 and 1.47). In O horizons Hg/C ratio decreased from 0.9 to 0.5 comparing 1999 and 2015 due to changes in litterfall composition and overall deposition due to canopy absence since 2005.

Tributaries, lake water and precipitation solutes were assessed to estimate the fluxes of Hg within lake catchment. In hydrological year 2016, mean annual Hg concentration in bulk precipitation reached 3.0 ng/L and bulk annual Hg deposition amounted at 4.6 µg/m2. PL water contained on average 4.4 ng/L of Hg and 8.2 mg/L of DOC. Mean annual Hg concentration in four lake tributaries ranged from 2.0 to 16.5 ng/L. The differences in Hg concentrations among individual streams were driven by DOC concentrations ranging from 2.1 to 21.2 mg/L.

The financial support was provided by the Czech Science Foundation project No. GA16-14762S.

Wildfires alter forested ecosystems, which include large stores of mercury (Hg) and organic carbon, two compounds that are closely linked in vegetation, terrestrial soils and streamwater. Studies have shown that wildfires release elevated levels of mercury to the atmosphere which can be locally redeposited and subsequent to fires, charred organic material (vegetation and litter) remains on the soil surface. Both of these new fire-altered sources of Hg (and carbon) have the potential to be mobilized into lakes and steams, particularly during high-flow precipitation events. However, no studies have conducted a detailed evaluation of mercury and carbon concentrations and dynamics in streams immediately following a wildfire.

This study investigates the coupled transport of mercury and carbon at Twomile Run, a headwater stream located in the forested mountains of Shenandoah National Park, in the year following a wildfire (April 16-May 2, 2016) which burned the entire watershed. Since June 2016, we have been collecting weekly baseflow samples and bi-hourly high-flow samples during storm events. Samples are analyzed for dissolved and particulate mercury (HgD and HgP, respectively), dissolved organic carbon (DOC), UV absorbance at 254 nm (UV254, surrogate for DOC character), total suspended solids (TSS), and volatile solids (VS). The chemical concentrations and dynamics will be compared to those in control streams, located in nearby catchments that were not burned in the fire. Initial evaluation of summer and fall stream samples indicate that, under baseflow and high-flow conditions, HgD concentrations and the HgD:UV254 ratio are similar to those observed in the unburned streams (DOC data not yet evaluated). Under baseflow conditions, HgP concentrations and the HgP:TSS ratio are also similar to those measured in the unburned streams. However, under high-flow conditions, HgP is approximately an order of magnitude higher per unit of TSS and VS and the HgP:TSS ratio is relatively unstable as compared to those observed in the unburned streams. These initial findings illustrate the variable response of this forested headwater system to wildfire disturbance. We will continue to collect and evaluate stream samples in the winter and spring to quantify mercury and carbon dynamics over the full range of discharge conditions, gaining a more comprehensive understanding of the impact of wildfire on downstream Hg and carbon transport. These findings will provide insight into how Hg cycling might change in the future, as wildfires are expected to increase, specifically in the mid-latitudes, with climate change.

Following its emission through anthropogenic activities, mercury is transported globally through the atmosphere, deposited, and impacts local ecosystems as a result of methylation and bioaccumulation in food webs. We are studying potential atmospheric mercury contamination of the Great Lakes region and examining the extent to which local, regional and global regulation of emissions are required to bring deposition to desired levels. This study is part of a larger integrative study examining scenarios of mercury contamination of Great Lakes Region fish. Previous studies have shown that deposition has increased roughly three-fold above pre-industrial levels in this region. The global 3-D chemical transport model GEOS-Chem has been applied to estimate future atmospheric deposition rates of mercury in the Great Lakes region for selected future scenarios of emissions, climate, and land use/land cover. Three scenarios (aspirational, policy-in-action and failure-to-govern) reflecting varying levels of policy actions taken at the regional and global scales to curb anthropogenic mercury emissions are considered. Future changes in climate, land use/land cover and biomass burning emissions are estimated following the IPCC A1B scenario. Changes in biomass burning emissions reflect future alterations in fuel type and availability (land use/land cover), fire meteorology (climate change), ignition agents and anthropogenic suppression of fires in the region. We find that, assuming no changes in climate, annual mean net deposition flux of mercury to the Great Lakes Region may increase by approximately 32% over 2005 levels by 2050, without global or regional policies addressing mercury, air pollution, or climate. In contrast, we project that the combination of global and North American action on mercury could lead to a 21% reduction in deposition from 2005 levels by 2050. U.S. action alone results in a projected 18% reduction over 2005 levels by 2050. We also find that, assuming no changes in anthropogenic emissions, climate change and biomass burning emissions would, respectively, cause annual mean net deposition flux of mercury to the Great Lakes Region to increase by approximately 5% and decrease by approximately 2% over 2000 levels by 2050. Changes in land use/land cover are expected to cause no net change in mercury deposition to the region in 2050.

Mercury (Hg) is a widespread global pollutant and neurotoxin that bioaccumulates and biomagnifies in aquatic food webs. Humans are most frequently exposed to Hg, particularly the more toxic form methylmercury (MeHg), through consumption of marine fish. Therefore, it is important to understand the environmental factors that influence bioaccumulation of mercury and how changes in climate will alter the fate of Hg in marine food webs. Environmental conditions that are predicted to change with climate include increases in temperature, precipitation, nutrient and carbon loading, and freshwater influx. Here, we used the estuarine amphipod, Leptocheirus plumulosus, to conduct single factor laboratory experiments to investigate the effects of organic carbon, temperature and salinity on Hg and MeHg bioaccumulation in amphipods feeding in estuarine sediments. In addition, we examined how the interaction of temperature and carbon impacted bioaccumulation in amphipods using two-factor experiments. Current results indicate that salinity had a slight impact on bioaccumulation of Hg, bioaccumulation of Hg increased as organic carbon decreased, and Hg bioaccumulation increased as temperature increased. However, we found no significant temperature carbon interaction. A biogeochemical model of Hg in sediments has been developed to predict the net effect of climate change on mercury bioaccumulation in marine organisms.