Mercury - The Hidden Danger of Arctic Warming?
Posted on 27 July 2012 by Steve Brown
If you've been following the global warming debate over the past few years, you'd be forgiven for thinking that the problems it poses are restricted to the impacts of higher temperatures, sea level rise, extreme weather, and ocean acidification. Unfortunately, we're only just beginning to understand the potential of other more subtle risks and impacts.
The Arctic is the region that has experienced the greatest warming in recent decades and is a unique environment. On the surface, it may appear to be a relatively pristine place that has escaped the touch of man; however, a combination of atmospheric and oceanic circulation has made it a sink for numerous contaminants emitted from industrial activity around the World, particularly Asia. Prevailing ocean and atmospheric circulation cause many of these contaminants to be transported to the Arctic where they can be absorbed by plants and animals, or locked into snow, ice, and permafrost.
Figure 1: Arctic contamination pathways (AMAP, 2002)
A risk to human health?
Indigenous populations in the Arctic, such as the Inuit, are at potential risk from these contaminants due to their reliance on hunting various species for food that may have been exposed to elevated concentrations of contaminants due to their progressive magnification through the food web. For example, algae in lakes may absorb a contaminant during photosynthesis. The algae are a food for small critters, which are then eaten by progressively bigger animals that in turn concentrate the contaminant in their tissue. At the top of the food-chain, human hunters that eat those animals in their regular diet may accumulate those contaminants in their own body tissue, with potential health side effects. Such scenarios exist for the main hunted food sources in the Arctic, including fish, whales, seals and birds.
One such contaminant is mercury, which has received recent attention by the Arctic Monitoring and Assessment Programme (AMAP) that published a 2011 Assessment of Mercury in the Arctic (Warning: 36MB PDF download). Mercury poses a particular danger to unborn children during pregnancy, as it can prevent healthy foetal development. AMAP has highlighted studies suggesting that indigenous Arctic populations are being exposed to levels of mercury that exceed World Health Organisation safety levels due to a diet of wild country foods.
The influence of climate change
Environmental changes in the Arctic due to warming are now generating great unknowns as to how food webs in the Arctic will be affected in the coming decades, and whether this will increase the health risks from mercury contamination. SKS'ers may be familiar with the carbon cycle, but the Arctic mercury cycle has just as much complexity and then some (AMAP, 2011). The mercury cycle has links and affinities with the organic carbon cycle, particularly with the formation of methylmercury, which is especially toxic. A store of mercury from human industrial sources has built up in the ice sheets, glaciers and snow fields over the past 200 years. Mercury is mobile in air, water, soil, flora and fauna. As scientists have examined this issue in recent years, the result is an ever increasing picture of complexity.
The Arctic has strong seasonality due to its high latitude, as well as large areas of melting and re-freezing sea ice affecting circulation patterns and biological turnover. Continuous sunlight in summer also allows continuous photosynthesis. All of which play their part in the biological uptake of mercury and its propagation through the food web. The ongoing reduction in summer sea ice extent is expected to have significant effects on mercury cycling and its availability to pass into the food chain.
A recent literature review by Stern et al (2012) elaborates on the impacts and uncertainties of how mercury pollution will be affected by climate change in the Arctic region. Factors considered include: changes in sea ice and snow cover, melting permafrost, and changes in animal behaviour and feeding habits. All of these reactions to Arctic warming will affect the transport of mercury and other contaminants through the environment and food web.
A further mechanism that has been a concern in recent years are Atmospheric Mercury Depletion Events (AMDE), that comprise a rapid oxidation and deposition of mercury from the atmosphere during the onset of Arctic spring; a photochemical reaction that has similarities with the mechanism responsible for the creation of the Ozone Hole. An estimated 243 tonnes of mercury is deposited in the Arctic each year, most of which is due to AMDE, though further photochemical reactions subsequently reduce a large proportion of the deposited mercury which becomes volatile, returning around 80% of it back to the atmosphere.
Warmer temperatures are expected to decrease AMDE deposition, though expanded areas of open sea due to reduced sea ice cover may result in up to 60 tonnes per year of AMDE mercury being absorbed by the ocean. Even this estimate is made uncertain by the possibility of enhanced transfer of mercury from the ocean to the atmosphere due to there being a greater area of open sea in the Summer.
Another result of changes in sea-ice distribution that has been observed is an increase in mercury levels in seals linked to their changing feeding habits as they adapt to the disruption of regular periods of sea-ice cover.
Mercury on the move
Snow that has accumulated mercury of atmospheric origin will transfer that mercury into soil and lakes during the spring melt. Increased river discharge from snow and ice melt is expected to increase mercury concentrations in the Arctic Ocean and lakes, especially in conjunction with increased soil and sediment mobilisation, which has been observed in the McKenzie River in Canada. The Arctic Ocean currently has a reservoir of 8000 tonnes of inorganic mercury available for biological uptake. The melting of Greenland glaciers is expected to increase the flux of centuries of stored mercury into the environment.
Mobilization of nutrients and stored mercury in soil as permafrost melts will promote microbial and algal activity in water catchment areas. Increased water discharge will increase mercury particulates in rivers due to erosion. In the McKenzie river, a 30% increase in discharge has been found to nearly double mercury concentration.
Conversion of mercury to form highly toxic methylmercury occurs in bacteria and algae, which then introduce it into the food web. Fish and mammals that occupy higher levels in the food chain typically accumulate methylmercury through their diet.
The amount of methylmercury in organisms at higher levels in the food chain can increase significantly with only small increases in mercury at lower levels in the food-chain. This biomagnification can result in methylmercury being 90% of the total mercury in tissues of high level predators.
Climate change is expected to affect mercury in unpredictable ways. Loss of sea ice and snow cover affects carbon cycle and mercury pathways. Warming and changing ocean circulation affects atmospheric circulation and influences the delivery of mercury from different regions. Ocean acidification from rising CO2 can also promote the formation of toxic methylmercury. Increased temperatures will increase biological activity, which in turn results in the creation of more methylmercury. Warmer spring and summer will also increase the time available for methylmercury formation to occur.
Thawing permafrost is estimated to release 200 micrograms of mercury per square-metre each year, which is significantly higher than the total from atmospheric deposition. Thawing permafrost is also expected to promote the formation of methylmercury due to increased microbial activity in the soil, while thawing glaciers are expected to temporarily increase mercury through release of stored mercury from historic human sources. In Hudson Bay alone, the input of mercury from coastal erosion is 0.25 tonnes per year.
Changes in the Arctic environment due to climate change may affect the behaviour and diet of animals, which in turn may affect how concentrations of methylmercury are magnified and accumulated by biological processes in food webs. A known path of this bioaccumulation is for mercury in lakes to be absorbed by algae, that is eaten by water fleas, which themselves are a food source for fish. The rate of methylmercury creation due to bacterial activity is expected to increase with increasing average temperature in the region, though this may decrease in some lakes affected by anaerobic conditions that would limit biological activity.
There are numerous further puzzles, for example, increased biological activity could dilute the amount of mercury in the environment. However, reduced ice cover over lakes and ocean will increase photosynthesis, which may biomagnify it. Evidence from the Bering Sea indicates that loss of sea ice may convert methylmercury to less toxic forms of mercury by photo-chemical reactions, thus reducing its impact on marine life.
An Arctic riddle
Research into Arctic mercury still has many open questions that are being hotly debated. Will ice free summer seas increase the transfer of atmospheric mercury across the air/ocean interface? Are chemical reactions involving mercury going to change with warming temperatures? Will environmental changes affect the biomagnification of mercury in food webs? As snow and ice melts, will there be a release of stored mercury to the environment? Until these questions are definitively answered it will be difficult to determine the net balance of mercury contamination in the Arctic food chain and the health risk to the indigenous people and wildlife of the Arctic.