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S12 Impacts of climate change on the fate of mercury in the environment

Tuesday, 26 July, 2011

TS12-P1 — 11:00-12:00 and 17:30-18:30
MERCURY TRANSFORMATION AND SULFUR TURNOVER IN BOREAL MIRES AS A RESULT FROM A BALANCE BETWEEN SULFUR DEPOSITION AND CLIMATE EFFECTS.
Authors: ÅKERBLOM, Staffan1, NILSSON, Mats1, BJÖRN, Erik2, LAMBERTSSON, Lars2, ERIKSSON, Tobias1, BISHOP, Kevin1
(1) Swedish University of Agricultural Sciences, staffan.akerblom@slu.se; (2) Umeå University;

Transformation of inorganic mercury (Hg) to methyl-Hg (MeHg) steers the export of MeHg from catchments to freshwater ecosystems and eventually contributes to Hg concentrations in freshwater fish and wildlife that exceed environmental and health advisory limits. Hg methylation and sulfur turnover have been studied on an experimental site at Degerö stormyr in northern Sweden (64°11’N, 19°33’E; 270 masl). The experimental setup involved a full factorial design at two levels with greenhouse cover (present or absent), nitrogen deposition (3 or 30 kg N ha-1 a-1), and sulfate deposition (2 or 20 kg S ha-1 a-1) as experimental factors. The experimental sulfur addition increased the Hg methylation potential in the peat, but also significantly reduced Hg concentrations in the solid soil. The effect from the sulfur treatment was counteracted by the greenhouse treatment. In plots with combined sulfur and greenhouse treatment Hg methylation was lower than on the sulfur only treatments. Mass balance calculations of S show that about 50% of the experimentally added S to the mire was retained in the peat. Raised temperature, however, significantly reduced the effects of the S-addition on methylation even more so on S-concentrations as only 15% of the added S was found in soil treated with a combination of enhanced S-addition and raised temperature. The loss of S was attributed to gaseous evasion of S-species. As a consequence to the evasion of S from sites with increased temperature, Hg methylation and MeHg content decreased relative to sites with S-addition at ambient temperature. Therefore S concentration was more important than the effects of raised temperature on Hg methylation and the build up of the MeHg content. These effects were found after more than 14 years of experimental treatment. Our results show that chronic deposition of S influences the capacity of peatlands to retain and methylate Hg and as well as the build up of MeHg stores available for release to streams and freshwater ecosystems.

TS12-P2 — 11:00-12:00 and 17:30-18:30
EFFECT OF EXPERIMENTAL DEEPENING OF THE THERMOCLINE ON MERCURY BIOACCUMULATION IN A CANADIAN SHIELD LAKE
Authors: PERRON, Tania1, AMYOT, Marc1, CHÉTELAT, John2, GUNN, John3
(1) GRIL, Université de Montréal, tania.perron@umontreal.ca; (2) National Wildlife Research Centre; (3) Canada Research Chair in Stressed Aquatic Systems, Biology Department, Laurentian University.

Methylmercury (MeHg) is a neurotoxin that bioaccumulates in organisms and biomagnifies through food webs following its formation in anoxic waters and sediments. Thermals stratification influences the volume of anoxic hypolimnetic water and the area of anoxic sediment, although it is unclear how thermocline depth controls MeHg cycling. Environmental disturbances including forest clear cutting and climate warming can affect the thermal structure of lakes, which may in turn modify the amount of MeHg in water and aquatic food webs. In a four year experiment called TIMEX (Thermocline Deepening Experiment), one basin of a small Canadian Shield lake was isolated with a plastic curtain, and its thermocline was artificially lowered by mechanical stirring. Mercury cycling in the manipulated basin was compared to a separate reference basin in the same lake. In the manipulated basin, water MeHg concentrations at 1.5 meters above the sediment were 88 to 92 % lower than concentrations in the reference basin at the same depth. Similarly, MeHg concentrations were reduced by 19 to 45% in bulk zooplankton, by 65% in Chaoborus larvae and by 40 to 49% in brown bullhead (Ameriurus nebulosus). No effect was observed on MeHg concentrations in mayflies and creek chub (Semolitus atromaculatus). Lower water MeHg in the manipulated basin likely resulted from a reduction in the volume of anoxic hypolimnetic water and associated sediment. Thermocline deepening also affected plankton community structure and, consequently, the MeHg concentrations found in zooplankton. The two fish species responded differently to a deepening thermocline because of their distinct feeding habits. Overall, a deepening of the thermocline reduced MeHg concentrations in bottom waters and in some organisms. It is therefore likely that changes in thermocline depth resulting from climate change or watershed land use will ultimately affect mercury contamination.

TS12-P3 — 11:00-12:00 and 17:30-18:30
VARIABILITY IN HILLSLOPE RUNOFF PROCESSES AND MERCURY FLUX DURING SPRING SNOWMELT AS A FUNCTION OF INTER-ANNUAL DIFFERENCES IN SNOW ACCUMULATION
Authors: HAYNES, Kristine M.1, MITCHELL, Carl P.J.1(1)University of Toronto-Scarborough, k.haynes@utoronto.ca

Spring snowmelt is an important period of mercury (Hg) export from watersheds; contributing a large portion of the annual Hg flux. The export of dissolved solutes such as sulphate and dissolved organic carbon in meltwaters from upland soils may affect Hg methylation rates in low-lying wetlands and subsequent methylmercury (MeHg) export. Given the toxicological and teratological effects associated with mercury and methylmercury to ecosystem and human health, understanding hydrological controls on mercury mobility, especially under a changing climate, warrants further study. However, the impact of global climate change on snowpack accumulation and the volume of spring runoff, and its subsequent influence on mercury and solute mobility and transport in forested watersheds are not well understood. In order to assess the potential effects of climate change, El Niño-Southern Oscillation (ENSO)-driven climate variability was used as a proxy by comparing hydrological flows and mercury and solute mobility between the winter of 2009-2010 with severely diminished snow accumulation (with a snow water equivalent of 48 mm) and an early melt (earliest local lake ice-off date on record) to the significantly greater snow accumulation recorded for the 2010-2011 winter (with a current snow water equivalent of >100 mm).

Spring runoff samples were collected from three replicate instrumented, forested hillslope plots delineated in an upland-peatland watershed at the Marcell Experimental Forest in north-central Minnesota. Installed in each plot is a shallow subsurface runoff trench equipped with digital flow datalogging. Total mercury (THg) analysis of the spring 2010 runoff period revealed an average overall freshet load of 60 micrograms THg from each of the three replicate plots, a flux of approximately 50 ng m-2 upland forest area. This THg flux data will be compared to that from the 2011 spring melt period in order to discern the potential effects of an earlier and drier spring melt on Hg mobility and transport in forested watersheds. The spring snowmelt period for 2009-2010 also highlighted the important contribution of the melting of soil frost following snowmelt in the export of THg from the hillslope, with more than 80% of the THg mobilized as a result of soil frost melt.

TS12-P4 — 11:00-12:00 and 17:30-18:30
TEMPERATURE-DEPENDENCE OF HG METHYLATION AND DEMETHYLATION IN FRESHWATER SEDIMENTS
Authors: GILMOUR, Cynthia1, HARRIS, Reed 2, RIEDEL, Georgia S.1, BELL, James Tyler1
(1) Smithsonian Environmental Research Center, gilmourc@si.edu; (2) Reed Harris Environmental Ltd.;

Temperature is an important variable in net MeHg production; however, the influence of temperature on net MeHg production has not been tested explicitly. Field data from studies in lakes in Canada and Wisconsin in the 1980s suggested that microbial Hg methylation in sediments is more sensitive to temperature than demethylation (Furutani and Rudd, 1980; Korthals and Winfrey 1987), Further, methylmercury (MeHg) accumulation in fish across lakes of different sizes suggests that lake temperature is a strong driver of net MeHg production (Bodaly et al. 1993). In this study, we tested the temperature dependence of methylation and demethylation experimentally in small sediment microcosms, using sediments from Lake 658 in Ontario, and from Hamilton and Toronto Harbours in Lake Ontario. We also examine large datasets from a number of our own field studies for temperature-dependence relationships for both reactions, which can be expressed as equations in simulation models. We hypothesize that methylation rates are more sensitive to temperature change than are demethylation rates, since energy conservation by sulfate-reducing bacteria is low relative to most other microbial respiratory processes. Demethylation appears to be widely distributed among microbes, while methylation, as we know it, is restricted to sulfate- and iron reducing bacteria.

Sediments from all three study sites were collected in fall 2010. Five treatment temperatures were examined, from 5 – 30 degrees C. Sediments were held in microcosms with overlying water at each temperature for ~2 weeks. Water was changed frequently to maintain electron acceptor concentrations. After 18 days, surficial (0-5 cm) microcosm sediments were subsampled in a glove bag for a suite of biogeochemical measurements. Gross mercury methylation and demethylation were measured separately using enriched stable isotope spikes. A suite of microbial processes, including sulfate reduction, CO2 and CH4 production, was followed. Pore water and solid phase chemistry were examined.

The temperature optimum for CO2 production was 25C in boreal L658 sediments; but 30C in Lake Ontario sediments. Methane production was more sensitive to temperature change than CO2 production, as expected. Sulfate reduction, Hg methylation and MeHg degradation rates measurements are underway. Results are being incorporated into a mercury cycling and bioaccumulation model (D-MCM) to examine the effects of temperature and other climate-related factors on mercury cycling and bioaccumulation.

TS12-P5 — 11:00-12:00 and 17:30-18:30
CLIMATE EVENTS DRIVE EPISODIC SPIKES IN MERCURY CONCENTRATIONS IN TEMPERATE ZOOPLANKTON
Authors: TODOROVA, Svetoslava1, DRISCOLL, Charles1
(1)Syracuse University, stodorov@maxwell.syr.edu

Zooplankton play a central role in the conveyance of nutrients and contaminants through aquatic food webs. Food web transfer has been recognized as the predominant route for bioaccumulation of mercury (Hg) into higher organisms and exposure to humans and wildlife. We studied the long-term Hg bioaccumulation pattern in mixed zooplankton communities in a temperate inland lake, Onondaga Lake, NY and examined the signature of the El Nino Southern Oscillation (ENSO) events on these patterns. The goal of this event-based study was to explore the timing and consequences of the ENSO episodes and their effect on zooplankton Hg concentrations. The 20-days moving average of the zooplankton Hg concentrations demonstrated a definitive pattern during ENSO years with elevated concentrations clustered between June and mid August. During non-El Nino years the majority of the increase in total Hg concentrations was observed in the fall. Storm events, changes in photosynthetic active radiation, lake physical characteristics, and zooplankton nutrient composition are among the parameters that we found contributing to the observed pattern of zooplankton Hg concentrations. Evaluation of the trends in local temperature, wind intensity, and precipitation suggest that El Nino conditions can be used as a proxy for future changes in climate and therefore, future increases in zooplankton Hg concentration can be expected.

TS12-P6 — 11:00-12:00 and 17:30-18:30
RESPONSE OF METHYLMERCURY IN A BOREAL LAKE TO A CLIMATE CHANGE SCENARIO: A LAKE’S HYDROLOGIC DISCONNECTION FROM ITS WATERSHED
Authors: TATE, Michael1, KRABBENHOFT, David P.1, BLANCHFIELD, Paul2, PATERSON, Michael2, BEATY, Ken2
(1)US Geological Survey, mttate@usgs.gov; (2) Fisheries and Oceans Canada;

Climate change will likely have a significant impact on small boreal lakes in the Canadian Shield. For the range of predicted responses to climate change, very little is known about how methylmercury (MeHg) and its bioavailability will be affected. Under one possible scenario, changes in precipitation patterns may result in new terrestrial-aquatic relationships where runoff amounts and water quality are reduced, and lakes are disconnected from a large portion of their watersheds. This cascade of events would then likely result in proportional changes to carbon loads to aquatic ecosystems. These two factors coupled with warmer air temperatures could lead to warmer water temperatures in the epilimnion and a deeper thermocline. The watershed typically contributes a large proportion of a lake’s mercury which is largely transported on dissolved organic carbon (DOC). Dissolved organic carbon quantity and quality is integral in the abundance, distribution and production of MeHg. Increased water clarity in the epilimnion may result in photo de-methylation/reduction to greater depths and thus enhance rates of MeHg and Hg removal. At the Experimental Lakes Area (ELA) a whole-ecosystem experiment is being conducted to simulate a climate change scenario similar to the one described above. In the fall of 2010, the inflow to Lake 626 was blocked with a dyke and the water was diverted through a channel blasted through the bedrock to Lake 625 further downstream, effectively making Lake 626 a first order headwater lake with a watershed area of 69 hactares (ha). Prior to the diversion, it was a fourth order lake with a watershed area of 372 ha.

During the open water season prior to diversion, lake profile samples were taken three times at center buoy, the lake’s deepest point. Summer profiles had the highest total Hg and DOC concentrations at the surface and decreased with depth. The summer of 2010 was the second wettest and was preceded by the second lowest amount of spring runoff on record at ELA, resulted in extremely high water levels in Lake 626. The higher concentrations in the epilimnion in the summer 2010 could be the result of the shoreline being inundated and tapping a new source of mercury. Methylmercury concentration in the bottom 2 meters increased to a maximum in the fall.

Tuesday, 26 July, 2011