Canadian Arctic Contaminants Assessment Report III (2012): Mercury in Canada’s North
Mercury is a naturally occurring element that is found globally in the environment. Mercury is released into the environment through natural processes and through human activities. Mercury transport and fate in the environment are complicated, and exposure to mercury poses significant health risks to both humans and biota. In order to effectively mitigate the risks posed by mercury exposure, the processes governing mercury transport, fate and effects must be understood.
Levels of mercury in the Canadian Arctic environment have increased several-fold since the advent of the Industrial Era. This rising trend in mercury levels continues in some biota and in some areas of the Arctic despite recognition of the issue and actions by many countries to mitigate mercury releases. Progress in reducing mercury levels in the environment is hindered by some key countries that are increasing the release of mercury through industrial emissions. These mercury emissions are then distributed around the globe, including to the Arctic, through long-range atmospheric transport.
For the last two decades, Canada’s Northern Contaminants Program (NCP) which is administered by Aboriginal Affairs and Northern Development Canada has supported research and monitoring of mercury transport, fate and effects in the Canadian Arctic. This program was established in 1991 in response to concerns about human exposure to elevated levels of contaminants in fish and wildlife species that are important to the traditional diets of Arctic Aboriginal people. The NCP focuses on: 1) monitoring of contaminant levels in Arctic biota that are most relevant to human contaminant exposure and is in keeping with Canada’s monitoring obligations under international agreements; 2) research on contaminant cycling in the Arctic environment; and, 3) education and communication efforts to enable the provision of sound consumption advice. In the last several years, work supported by the NCP has been augmented by other research programs, most notably, ArcticNet and International Polar Year. These programs have supported several projects related to mercury transport and fate in Arctic ecosystems.
The purpose of the Canadian Arctic Contaminants Assessment Report III (2012): Mercury in Canada’s North is to report on the scientific progress made under these programs and to evaluate our current understanding of the environmental fate of mercury in the Canadian Arctic. This report is part of a collection of companion reports on persistent organic pollutants (POPs) and human health as well as a highlights report which synthesizes all of the findings from these companion reports. This synthesis of scientific findings is intended to inform policy makers, managers, researchers, northerners and other relevant stakeholders. The present assessment also serves to take stock of progress made during the most recent phase of the NCP and to identify knowledge gaps and the most important scientific issues concerning mercury in Canada’s northern environments. This report is the third NCP assessment of mercury in the Canadian Arctic in the last two decades and the first to focus exclusively on mercury.
Important scientific advances have been made in identifying the environmental fate of mercury and the processes that control its movement in the Canadian Arctic. The Canadian Arctic Contaminants Assessment Report III (2012): Mercury in Canada’s North provides much new information on the sources and long-range transport of mercury to the Arctic, its cycling within marine, freshwater and terrestrial environments, and its bioaccumulation in and effects on the biota that live there. New information and data are also emerging on the complex ways in which climate change is affecting mercury cycling in the Arctic.
Mercury in the atmosphere
Research and monitoring of atmospheric mercury has improved our understanding of this key transport pathway to and within the Canadian Arctic. The chemical processes resulting in enhanced deposition of mercury during the spring-time phenomenon of atmospheric mercury depletion events (AMDEs) have been better characterized. More field measurements on the speciation of atmospheric mercury during AMDEs are now available, which indicate the oxidation of gaseous elemental mercury in the atmosphere is accompanied by large increases in reactive gaseous mercury and particulate mercury concentrations. The timing of AMDEs at the High Arctic monitoring station at Alert has changed over the last two decades, with the month of maximum AMDE activity shifting from May to April. The reason for the shift in timing of depletion events is not yet understood, but may be related to changes in air temperature. Air mercury concentrations declined between 2000 and 2009 at both Alert in the High Arctic and Kuujjuarapik in the sub-Arctic. The rate of decline at Kuujjuarapik was comparable to non-Arctic monitoring sites at lower latitudes while a slower rate of decline was observed at Alert. Model simulations suggest that long-range atmospheric transport from Asia likely contributes the most mercury (from anthropogenic, natural and re-emission sources) to the High Arctic and western Arctic, followed by contributions from North America, Russia and Europe.
Considerable uncertainty remains in the quantification of atmospheric mercury contributions to Arctic ecosystems. Direct measurements of wet and dry deposition in the Canadian Arctic are extremely limited, in part due to technical challenges, and more research using a variety of methodological approaches is essential to resolve this knowledge gap, especially since any changes in emission controls would most immediately affect the atmospheric mercury flux. Better characterization of geographic variation in atmospheric mercury is also warranted, particularly over the Arctic Ocean, to determine the influence of unique marine conditions on deposition.
Biogeochemical cycling of mercury
Some of the most significant advances in Arctic mercury science have been made in the area of biogeochemical cycling. Large amounts of new data are now available for water, snow, ice and sediment, providing more information on the concentrations, fluxes and transformations of inorganic and methylmercury in these abiotic matrices. This information was virtually absent for the Canadian Arctic a decade ago.
Much research effort has focused on mercury cycling in the cryosphere, particularly snow-air exchanges, transformations in the snowpack, and mercury transport by snowmelt. Snow mercury concentrations indicate that much of the mercury deposited during AMDEs is quickly reduced and emitted to the atmosphere. The level of evasion from the snowpack is dependant on proximity to the ocean because halogen compounds from the marine environment promote the stabilization and accumulation of mercury in snowpack. Areal loads of mercury in snow were estimated for Hudson Bay and the Arctic Archipelago, which suggest that snowmelt likely constitutes a small flux of mercury to Arctic marine waters during spring. In contrast, snowmelt can be an important source of mercury to freshwater ecosystems in the High Arctic.
The first measurements of methylation rates in Arctic seawater indicate that the water column of the Arctic Ocean is an important site for the methylation of inorganic mercury to form methylmercury. Production of dimethylmercury also occurs in the water column, and the breakdown of dimethylmercury in water or in the air (after evasion) may be another source of methylmercury available for biological uptake. Arctic marine waters were also found to be a substantial source of gaseous mercury to the atmosphere during the ice-free season. Microbial organisms and photochemical processes are key drivers of mercury cycling in the Arctic.
In spite of this progress, information remains limited on the methylation, oxidation and reduction of inorganic mercury in marine, freshwater and terrestrial environments in the Canadian Arctic. Further research is needed on mercury biogeochemical processes to develop a quantitative and mechanistic understanding of abiotic and microbially-driven mercury fluxes between air and various surfaces (soil, water, snow and vegetation), and to better understand the processes regulating methylmercury entry into Arctic food webs.
Monitoring of Arctic biota
Over the last two to four decades, mercury concentrations have increased in some marine and freshwater animals, specifically in several species of Arctic-breeding seabirds, some polar bear subpopulations, and some freshwater fish from the Mackenzie River Basin. Other monitored populations, including caribou, beluga, ringed seal, and freshwater fish from the Yukon and Nunavut showed no change or a slight decline in mercury concentrations. More frequent (annual) monitoring of key Arctic biota, which was initiated by the NCP in 2005, has resulted in more powerful datasets and a better ability to detect changes in biotic mercury levels. The different mercury trends reported for Arctic biota indicate that the drivers of temporal change may be regional or habitat-specific.
Marine biota exhibit considerable geographic variation in mercury concentrations across the Canadian Arctic. The Beaufort Sea remains an area of higher mercury contamination for beluga, ringed seals and polar bears compared to other Arctic regions. Habitat use and food web structure factor into mercury levels in Beaufort Sea beluga, which vary within the population according to size and sex segregation. Elevated concentrations in polar bears from the Beaufort Sea relative to Hudson Bay are related to the higher trophic level of bears in the Beaufort Sea area and higher water concentrations of methylmercury available for entry into the food web. Latitudinal trends have also been observed for ringed seals and seabirds, with greater concentrations in the High Arctic than at sub-Arctic sites.
While strong evidence exists for variable patterns of mercury bioaccumulation both regionally and temporally within the Arctic, the drivers of the observed trends in fish and wildlife remain unclear. Further research is required to identify the underlying processes leading to temporal changes in biotic mercury concentrations at sites where increases have been observed. Multiple factors may be implicated and need to be assessed, including the roles of mercury delivery processes (e.g., atmospheric deposition, mercury methylation), shifts in food web structure, and climate change. Geographic patterns of mercury bioaccumulation in marine biota deserve further investigation because certain factors may increase the vulnerability of biota to mercury exposure in some Arctic regions.
The Arctic environment is undergoing profound change and at a rapid rate. Emerging evidence indicates that this environmental change is altering the cycling and bioaccumulation of mercury through effects on mercury fluxes and food web structure. Recent increases in algal primary production and catchment inputs may be enhancing mercury fluxes to Arctic lakes. Climate warming may be mobilizing mercury and leading to increased exposure of fish in the Mackenzie River, where a nearly two-fold increase in mercury concentration has occurred in burbot since 1985. Climate change also appears to be affecting the structure of food webs in certain regions resulting in dietary shifts and changes in mercury bioaccumulation for some marine biota such as thick-billed murres and ringed seals. However, large uncertainties remain in this emerging and complex science. A myriad of interconnected environmental changes are occurring in the Arctic, and detailed, process-focused investigations are needed to more precisely identify how these changes will alter the fate of mercury. Many aspects of the mercury cycle may be impacted including how mercury is delivered to Arctic ecosystems, its biogeochemical transformations and its trophic transfer in food webs.
Recent investigations on effects of methylmercury exposure specifically for Arctic wildlife have focused on adverse reproductive effects in seabirds, neurotoxicity in marine mammals, immunotoxicity in beluga, and the role of selenium in mercury detoxification. A laboratory egg injection study revealed that developing embryos of seabirds show species-specific sensitivity to methylmercury exposure at environmentally-relevant levels. On-going research suggests that methylmercury exposure at current environmental levels may affect brain neurochemical receptors in some Arctic top predators, although further verification is needed. Selenium is known to play an important role in sequestering and detoxifying mercury, and the formation of relatively inert mercury-selenium compounds may reduce the risk of mercury toxicity. The amount of selenium present relative to mercury in tissues of polar bears and beluga whales is variable, which suggests the protective effect of selenium against methylmercury toxicity may differ among species. Preliminary research on effects of mercury exposure on gene expression and immunotoxicity holds promise for providing new tools to better understand the toxicological effects of mercury in Arctic beluga. In general, Arctic biota have mercury concentrations that are below threshold levels of potential concern (derived from laboratory and field studies for non-Arctic species), although concentrations above threshold levels have been observed for some freshwater fish populations, Greenland shark, and some seabird species.
There is currently insufficient information to adequately assess effects of mercury exposure specifically for Arctic wildlife species, and further research should determine if and to what extent mercury exposure, combined with other stressors such as climate change, is affecting populations of Arctic fish, seabirds and marine mammals that have elevated mercury levels.
Over the last two decades, the NCP has played a pivotal role in supporting research and monitoring of mercury in the Canadian Arctic environment. Earlier phases of the NCP provided important new knowledge on the Arctic mercury cycle, which in turn, resulted in the development and refinement of program priorities related to this contaminant. This assessment documents the progress made in our understanding of the behaviour of mercury in the Arctic environment, and reaffirms the importance of continuing research and monitoring of mercury pollution in the Arctic.
While the advances in scientific research described in this report are significant, the complex nature of the mercury cycle continues to provide challenges in characterizing and quantifying the relationships between mercury sources, transport processes, and levels as well as effects of mercury exposure on biota. These gaps in our knowledge of how global anthropogenic emissions of mercury are delivered to and accumulate in Arctic environments have implications for policy development related to risk management of mercury. Of particular concern are large uncertainties in our understanding of the processes that are contributing to increasing mercury concentrations in some Arctic fish and wildlife.
Current levels of mercury affecting Arctic ecosystems are a legacy of anthropogenic emissions that began with the onset of the industrial era. With continued global economic development dependent on combustion of coal, mercury deposition to the Arctic will continue to rise. If, however, emissions can be reduced through implementation of the UNEP global mercury treaty, deposition to the Arctic can be reduced significantly, which in the future would have a positive impact on Arctic ecosystems and human health. In order to better support national and global action, and manage current mercury related risks, this assessment makes several science recommendations to improve our understanding of mercury in the Arctic. The key science recommendations of the Canadian Arctic Contaminants Assessment Report III (2012): Mercury in Canada’s North are:
- Continue research and monitoring of atmospheric mercury, with an enhanced focus on deposition measurements to facilitate quantification of atmospheric contributions of mercury to Arctic ecosystems.
- Continue temporal trend monitoring of mercury in Arctic biota, and identify the processes that are changing mercury concentrations in some species.
- Further characterize the key processes acting on mercury after atmospheric deposition and their effects on the fate of mercury in the Arctic environment.
- Better characterize the processes that link climate change with mercury transport, cycling and bioaccumulation.
- Increase efforts to determine the biological effects of methylmercury exposure on Arctic fish and wildlife.
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