3. Mercury in the Environment

The Canadian Mercury Science Assessment (the Assessment) published in 2016  is the first comprehensive scientific evaluation and synthesis of information on mercury in the Canadian environment (Environment and Climate Change Canada, 2016). In Canada, environmental monitoring programs measure levels of mercury in air, water, plants, and animals. This monitoring is undertaken by Environment and Climate Change Canada primarily through the Addressing Air Pollution Horizontal Initiative and Chemicals Management Plan monitoring programs as well as by Crown-Indigenous Relations and Northern Affairs Canada, including through the Northern Contaminants Program. The monitoring data summarized below are (unless otherwise noted) drawn from the assessment.

3.1 - Air

Mercury levels in the air decreased by 18% on average between 1995 and 2011. Total concentrations of mercury in gaseous form were (with some exceptions) generally 1.2 – 1.9 nanogram^{Footnote 1} per cubic metre (ng/m³) throughout Canada. Mercury deposited as a result of precipitation averaged 100 – 800 ng/m² per month (Cole et al., 2013). In addition, many areas have experienced greater than average reductions in atmospheric mercury concentrations during the same period.

There are a lot of factors that affect the amount of mercury in the air. For example, it will take much longer for mercury levels in air to decline in and around areas where mercury has accumulated over many years, such as areas around base metal smelting facilities or other industrial sites. Another factor includes the movement of mercury from sources outside Canada. This is reflected, for example, in recent findings of increasing mercury levels in the air in two places in Canada: Little Fox Lake in the Yukon Territory and Whistler Mountain, British Columbia. Additionally, localized areas of high mercury deposition have been noted along the west coast and in coastal areas in the western Arctic and sub-Arctic. Trends in mercury deposition are changing over time and are now different than reported in the Canadian Mercury Science Assessment. These observations have been attributed to mercury emissions from sources in Asia and variations in weather conditions as a result of climate change.

3.2 - Water

Monitoring of surface water levels of mercury, and in some cases levels of methylmercury, is done for major fresh water bodies across Canada, including in the Pacific region, tributaries in the Athabasca River oil sands region, the Great Lakes and connecting channels, the Saint-Lawrence Seaway, and Hudson’s Bay, as well as Atlantic Canada. Air pollutants such as nitrogen oxides and sulphur oxides from industrial sources are known to acidify lakes and streams. Nutrient-deficient bodies of waters that are either naturally acidic or acid-impacted tend to have higher methylmercury concentrations than nutrient-rich, less acidic bodies of water. Acidic lakes typically contain fish and fish-eating wildlife with relatively higher mercury levels compared to non-acidic lakes. In studies reviewed for the Canadian Mercury Science Assessment, no water samples exceeded the Canadian Water Quality Guidelines for the Protection of Aquatic Life for Inorganic and Methylmercury values of 26 ng/L for mercury and 4 ng/L for methylmercury.

3.3 - Animals

The amount of methylmercury in the environment determines the mercury levels in animals. Methylmercury (the most toxic form of mercury) accumulates in animals’ bodies. Animals and humans tend to absorb and retain over time most of the methylmercury in the food they consume (bioaccumulation). At each step in the food chain, predators accumulate mercury from their prey. As a result, over their lifetime, they have much higher mercury levels in their bodies than their prey (biomagnification). Consequently, large predatory fish, fish-eating mammals, and fish-eating birds are at greatest risk of impaired health and reproduction from exposure to methylmercury.

Monitoring of methylmercury levels in animals has revealed a wide variation between species and regions. Overall, methylmercury levels have remained stable in nearly half (48%), decreased in 21%, and increased in 31% of the populations monitored. Most of the fish or wildlife species reviewed in the Assessment had methylmercury levels below the level thought to cause death or physical harm. However, despite this, the Assessment also highlighted that the levels of methylmercury in some populations of fish-eating fish and wildlife species in Canada may be high enough to cause behavioural changes and affect reproductive success.

Of the populations showing an increase in methylmercury levels, the large majority (83%)^{Footnote 2} are in the Arctic. The environmental conditions in the Arctic are rapidly changing due to climate change and large amounts of foreign mercury emissions are deposited there. These two factors are thought to have played a role in the increases in methylmercury levels observed in many Arctic populations. The greatest increases in methylmercury levels have been seen in seabirds. According to data from short-term monitoring, the only animal population in the Arctic showing a decreasing trend in methylmercury levels is fish. Despite this decrease, levels in some types of fish may still be a concern because the traditional diets of Indigenous communities in this region rely heavily on fish and sea mammals.

The Great Lakes region has shown the largest and most frequent declines in methylmercury levels in individual populations (40% of fish and seabird populations reported). However, recent data illustrate that some populations that have previously shown declines or stable trends in methylmercury concentrations are now showing increasing trends (Environment and Climate Change Canada and the United States Environmental Protection Agency, 2017). Climate change, changes in food web structure due to invasive species, and fluctuating water levels are thought to be contributing factors to the observed increasing trends (Blukaz-Richards et al., 2017). Additional data is being collected through Canadian and American monitoring programs to better understand why these increases are occurring.

3.4 - Conclusion

Overall, environmental monitoring data outlined above show decreases in levels of mercury in air, water and biota across Canada. However, increasing trends in mercury levels have been noted in the air and some animals in some locations, particularly in the Arctic and some areas of western Canada. The Arctic’s unique chemistry, weather conditions, climate change and being downwind of foreign sources of mercury emissions may be contributing factors to the observed increases. The increases in methylmercury levels in some animal populations in the Arctic are concerning because of the importance of fish and sea mammals in the diets of northern and Indigenous communities. Environmental monitoring should continue to better understand spatial and temporal trends of mercury in the environment. This ongoing monitoring is particularly important since changes in emissions and releases and ecosystem shifts are altering the trends of mercury levels in the environment.