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

Wednesday, 27 July, 2011

WS12-O1 — 8:30-8:45
Authors: HERMANNS, Yvonne1, MARTINEZ-CORTIZAS, Antonio2, KILIAN, Rolf3, ARZ, Helge4, STEIN, Ruediger5, BIESTER, Harald6
(1)Institute of Environmental Geology, Technical University of Braunschweig, y-m.hermanns@tu-bs.de; (2) Department of Pedology and Agricultural Chemistry, Faculty of Biology University of Santiago de Compostela; (3) Department of Geology, FB VI, University of Trier; (4) Leibniz Institute for Baltic Sea Research Warnemünde; (5) Alfred Wegener Institute for Polar and Marine Research; (6) IInstitute of Environmental Geology, Technical University of Braunschweig.

The role of climate in controlling mercury fluxes to aquatic systems is mostly unknown, but crucial to predict global warming related changes in mercury enrichment in aquatic systems. Although, a climate component in the mercury cycle is likely, its magnitude, especially on larger time scales, is not known. Mercury cycling in and between aquatic systems and soils is known to be coupled to organic matter dynamics and changes in climate may affect atmospheric mercury fluxes and leaching of organic matter bound mercury from soils. Moreover, recent studies suggest a relationship between increasing mercury accumu­lation in lake sediments and increased algae production during the warming past century. We investigated a 4250 years old and 450cm long sediment core from a remote lake in Patagonia (53°S) for climate driven changes in accumulation of mercury and organic matter from different sources using a multi-proxy approach and multivariate statistics (Principal Component Analyses). Differentiation of organic matter sources was based on hydrogen/carbon- and oxygen/carbon- (Rock-Eval Analyses) as well as nitrogen/carbon ratios. Mercury concentrations and accumulation rates in the organic matter dominated (~ 70 wt. %) sediments show strong variations through time varying by a factor of ~ 4, between 87 and 362 µg kg-1 (median 187 µg kg-1) and 14 to 53 µg m-2 yr-1 (median 29 µg m-2 yr-1), respectively. Mercury concentrations and mercury accumulation rates shows a striking covariance with hydrogen/carbon and N/C ratios, pointing to a relationship between mercury accumulation and within-lake productivity. Moreover, principal component analysis reveals a strong covariance of mercury with copper indicating that precipitation controlled soil leaching of both organically bound metals is a major pathway to the lake. Mercury accumulation was found to be highest during warmer and wetter and lower during cold and dry periods indicating a strong influence of climate on mercury accumulation. Besides that both processes may have an influence on mercury sedimentation we conclude, that, for mass balance reasons, changes in mercury leaching from catchment soils is the predominant factor for the observed changes in mercury accumulation in sediments of this lake. In addition, increased production of algae during warmer periods may cause enhanced scavenging of mercury bearing soil derived dissolved organic matter by sorption to or uptake by algae. This magnitude of prehistoric climatic induced variation in mercury accumulation is comparable to the recent anthropogenic forcing of the mercury cycle (factor of 3-5).

WS12-O2 — 8:45-9:00
Authors: HERMANNS, Yvonne1, BIESTER, Harald1
(1)Institute of Environmental Geology, Technical University of Braunschweig, y-m.hermanns@tu-bs.de

The coupling of mercury and organic matter has been considered as an important factor controlling the transport of mercury in and between terrestrial and aquatic systems. While some studies suggest that accumulation of mercury in lake sediments is mainly controlled by varying fluxes of organic matter bound mercury from the catchment soils, other propose changes in aquatic productivity and related scavenging by algae as a major process. Despite that both processes are to a vast extent controlled by climatic conditions long term records of mercury accumulation in lake sediments encompassing different climatic conditions have been rarely investigated. We investigated a 17,000 year old high resolved record of mercury and organic matter accumulation in sediments of a remote lake system in southern Patagonia (53°S), Chile. Sources of organic matter were examined (aquatic versus terrestrial) based on C/N ratios. To account for the contribution of different mercury sources local rocks and soils, as well as terrestrial and fresh aquatic organic material (algae) were analyzed for their mercury content. The composition of the sediment record indicates profound environmental and climatic changes throughout the past 17,000 years. On a long term accumulation of sediment organic matter increased by a factor of ~ 3 reflecting a strong increase of organic matter accumu­lation in catchment soils. Mercury accumulation rates in sediments remained nearly constant (median 29 µg m-2 yr-1), suggesting similar long term atmospheric fluxes, but changing equilibrium between mercury storage in catchment soils and Hg leaching from soils. However, short term changes in mercury accumulation vary in a wide range between 10 to 157 µg m-2 yr-1 (factor 16) even exceeding the anthropogenic forcing of modern atmospheric mercury fluxes (factor 3-5). Statistical data evaluation based on Principal Component Analyses reveals a striking covariance of mercury with other organically bound elements in soils such as copper or yttrium, but only a weak relation to total organic matter accumulation. This points towards short term changes in soil leaching of organically bound metals attributed to changes in climatic conditions e.g. precipitation. Moreover, changes in organic matter sources in the past 4500 years as indicated by C/N ratios reveal that mercury scavenging by algal derived organic matter may enforce mercury accumulation during wetter and warmer periods when mercury leaching from soils is also high.

WS12-O3 — 9:00-9:15
Authors: RYDBERG, Johan1, KLAMINDER, Jonatan 1, ROSÉN, Peter2, BINDLER, Richard1
(1) EMG, Umeå University, johan.rydberg@emg.umu.se; (2) CIRC, Umeå University;

Over time, large amounts of atmospherically deposited mercury, both from natural and anthropogenic sources, have been sequestered together with carbon in peatlands throughout the arctic and subarctic regions. Many of these peatlands are currently underlain by permafrost, which controls their stability and hydrology. With the ongoing climate change there is a concern that permafrost thawing will turn large areas of these northern peatlands from carbon/mercury-sinks into much wetter carbon/mercury-sources. By combining adjacent peat and lake sediment records we can here show that such a change in mire structure in the subarctic Stordalen mire (northern Sweden) is responsible for an increase in the export of mercury to the adjacent lake Inre Harrsjön.

The Stordalen mercury records can be divided into three different periods. I) The pre-industrial period. During this period mercury concentrations in the peat record were relatively constant (61 ±20 ng g-1). A similar stable pre-industrial mercury background level does not occur in the lake sediment record. Instead there are considerable fluctuations in the mercury levels throughout the period pre-dating 1850. During this period there is a close link between mercury and the in-transport of carbon to the lake, which indicates that organically bound mercury from the mire largely controls the lake sediment mercury levels. II) The anthropogenic period. Around 1850 the mercury concentration in both the peat and lake records started to increase due to anthropogenic mercury deposition, both on the mire and directly on the lake. The mercury concentrations continued to increase until the 1970s, when a decreasing trend starts in both records. III) The permafrost thaw period. Even though the mercury accumulation in the peat continues to decrease until present time, both the mercury levels in the lake sediment starts to increase in the beginning of the 1990s. This increase can likely be linked to an observed increase in permafrost thawing and thermocarst erosion in the Stordalen mire at this time.

Our findings indicate that, in areas with permafrost, climate and mire dynamics might have an effect on mercury loading to lakes. Because thawing of permafrost and a subsequent export of carbon is a wide spread phenomenon, substantial amounts of mercury are, if other systems behave in a similar way as the Stordalen mire, at risk of being released into sensitive arctic surface waters.

WS12-O4 — 9:15-9:30
Authors: TANG, Rex W-K1, GUNN, John M1, JOHNSTON, Thomas A2
(1)Laurentian University, ry_tang@laurentian.ca; (2) Ontario Ministry of Natural Resources.

Elevated mercury concentrations are responsible for most fish consumption advisories on lakes of northern Ontario, and atmospheric deposition is believed to be the primary source of inorganic mercury to these lakes. We predicted that recent declines in atmospheric mercury deposition would be reflected in declining fish mercury concentrations across northern Ontario. Using historic fish mercury data from the Ontario Ministry of Environment’s long-term monitoring database and current fish mercury data from a new sampling program we examined changes in mercury bioaccumulation in seven fish species (five piscivores, two benthivores) in lakes from across northern Ontario following a paired-comparisons approach. Mercury concentration vs body size slopes did not change appreciably from historic to current sampling periods. Mean predicted mercury concentrations (standardized to a 1-kg fish) increased slightly over the last 30 years in all species. However, this change was not statistically significant for any of the study species and the nature of the temporal change exhibited no systematic spatial variation across northern Ontario. Mean temporal changes in mercury concentration were positively correlated between species suggesting that the factors driving the change were acting at the ecosystem rather than population level. The potential for future changes in fish mercury concentrations in the boreal shield region will be discussed in relation to changing atmospheric mercury deposition and climate-mediated changes in methylation rates and fish growth rates.

WS12-O5 — 9:30-9:45
Authors: FJELD, Eirik1, ROGNERUD, Sigurd1
(1) NIVA, eirik.fjeld@niva.no

Mercury contamination of freshwater fish is a severe environmental problem in Norway, and the EU health advisory maximum level of 0.5 mg/kg is often exceeded in perch (Perca fluviatilis) in remote boreal lakes in the southeastern Norway. Long-range transported atmospheric depositions of mercury are the main pollution source to the watersheds in this area, and the depositions have likely decreased since the beginning of the 1990s.

Purpose: To investigate if possible overall changes in the mercury levels in perch have occurred from 1991 to 2008.

Material and Methods:
Ten boreal lakes in southeastern Norway, first investigated in 1991, were resampled in 2008. The lakes are without any known local pollution sources. The mercury concentrations in fish caught in 2008 (N=211) were compared with results from 1991 (N=226). Statistical significant differences between years were identified after adjusting for differences in length (ANCOVA, post-hoc tests).

The length adjusted mean concentration in the populations increased with 63% from 1991 to 2008, from 0.27 mg/kg to 0.44 mg/kg for a standard fish of 22 cm (arithmetic mean length) or ≈100 g. For eight of the ten populations we proved a statistically significant increase in the mercury concentrations (p < 0.05).

Mercury concentrations in the studied populations has increased substantially from 1991 to 2008. This is unexpected as the atmospheric mercury depositions most likely have decreased during the same period. Mercury in fish exists mainly as methylmercury, and factors affecting methylation and mobilization of mercury, such as a warmer and wetter climate, increased TOC (humic matter) in runnoff and lake water, and also forestry, may have contributed to the observed increase. We discuss our findings with respect to the changes during the latest decades in these overall environmental factors.

WS12-O6 — 9:45-10:00
Authors: CHUMCHALL, Matthew M.1, JARDINE, Timothy D.2, KIDD, Karen A.3, LAVOIE, Raphaël4
(1) Texas Christian University,, m.m.chumchal@tcu.edu; (2) Australian Rivers Institute; (3) University of New Brunswick; (4) Queen’s University.

Vertical trophic position is one of the most important factors determining an organism’s overall level of mercury (Hg) contamination. The development of techniques to estimate an organism’s trophic position using nitrogen isotopes (d15N) revolutionized the study of Hg biomagnification. The overall or average rate of Hg biomagnification through food webs can be determined using the slope of log(Hg) versus d15N regressions (called trophic magnification) and this parameter can be compared across systems that differ in their climate, chemical, physical or biological factors. Mercury trophic magnification was found to be variable in aquatic food webs and to date there is no consensus on the main factors driving this variation. We compiled results from over 30 studies and 100 sites spanning climate regimes across latitudes (published and non-published) that measured Hg trophic magnification in aquatic food webs using d15N. Studies ranged from freshwater to marine and from tropical to arctic ecosystems. We examined whether physical-chemical (e.g., productivity, pH, DOC and Hg in water; latitude, longitude and ecosystem size) and community structure (species composition and food chain length) variables explained among-system differences in trophic magnification. Preliminary results suggest that trophic magnification was positively related to latitude and inversely related to longitude. Mercury concentrations at the base of the food chain were negatively correlated with regression slopes. Trophic magnification was lower in systems with longer food chain length. This study is among the first to comprehensively assess factors affecting biomagnification of mercury in diverse ecosystems and will encourage similar modelling of other contaminants found to biomagnify in aquatic or terrestrial food chains.

WS12-O7 — 10:00-10:30
Authors: EVANS, Marlene1, MUIR , Derek2, KEATING , Jonathan1, WANG, Xiaowa2
(1) Water Science and Technology Directorate, Environment Canada, Saskatoon, SK, marlene.evans@ec.gc.ca; (2) Water Science and Technology Directorate, Environment Canada, Burlington, ON;

While northern Canada is largely undeveloped with a very low population density, mercury levels in fish are of concern and have been monitored for the last three decades as part of commercial fish inspections, domestic fisheries and, more recently, the Northern Contaminant Program. In the mid 2000s, researchers summarized this information to report that mercury concentrations in predatory fish commonly exceeded 0.5 µg/g, the commercial sale guideline. High mercury levels were more commonly associated with predatory fish in smaller lakes with large watersheds and with such fish reaching 12 years and older. Continued mercury monitoring in Great Slave Lake is showing that mercury levels are increasing in lake trout and burbot with a greater rate of increase observed in burbot, the more nearshore of the two species. Furthermore, mercury is increasing at a faster rate in the warmer, more productive waters of the West Basin than the deeper, cooler and less productive waters of the East Arm. Monitoring studies conducted periodically in smaller lakes have shown even larger increases in mercury concentrations in lake trout over intervals as short as 4-5 years; consumption advisories have been issued for some of these lakes. The factors driving these broad scale time trends are unclear and subject to some debate although lake size and fish biology clearly are important factors affecting the manifestation of time trends at local scales. While the Mackenzie River Basin has been experiencing a long-term warming trend, there is only a weak correlation between trends in mercury concentrations and annual or ice-free air temperatures. Wind speeds are decreasing, potentially affecting a number of lake processes including water column mixing with concomitant effects on mercury methylation rates. Another causal factor potentially affecting mercury trends is increased mercury emissions from Asia with its economic expansion. The long-term prognosis for mercury levels in fish in northern Canada is unknown. Global warming is expected to continue with many environmental consequences including enhanced productivity as the growing season is extended: if this results in an increase in fish growth rates, mercury levels may decline in fish. This may be counterbalanced by Asian emissions which are expected to continue to increase with its economic growth. Resolution of the factors affecting mercury levels and trends in fish is of outmost importance both now and for the future.

Wednesday, 27 July, 2011