Mercury levels on the rise in remote Canadian lakes.

Phillips, VJA, VL St. Louis, CA Cooke, RD Vinebrooke and WO Hobbs. 2011. Increased mercury loadings to Western Canadian alpine lakes over the past 150 years. Environmental Science and Technology http://dx.doi.org/10.1021/es1031135.

Recent increases in mercury pollution into remote Canadian lakes is linked to a combination of erosion from the watershed and inputs from the atmosphere, shows a study that examined historic mercury levels in lake sediments.

The findings help explain why some of the most remote ecosystems have high levels of mercury pollution. The study's results are published in the journal Environmental Science and Technology.

Mercury is a ubiquitous contaminant that can be found even in the most pristine environments. Scientists have struggled to understand how mercury reaches remote regions far away from its industrial sources, such as coal-burning power plants, and whether high mountain regions are more vulnerable to mercury pollution than more urban regions.

Scientists in Alberta collected sediment cores from the bottom of nine remote, high elevation lakes in national parks in the Canadian Rocky Mountains. The sediments were aged and measured for mercury that is preserved in buried sediments. The scientists compared historical mercury levels in these lakes with changes in climate, nutrients and erosion from surrounding lands.

They found that mercury inputs to the relatively pristine mountain lakes almost doubled on average since pre-industrial times. For most lakes, the highest mercury levels were very recent – within the last decade in five of them and within the last 50 years in three of the other lakes. Mercury levels in one of the lakes declined over time.

The scientists discovered that the surrounding land is a big source of mercury pollution in several of the lakes. Many factors can play into how mercury enters lakes. In the atmosphere, mercury latches onto small particles and precipitation that eventually fall directly into lakes.

As this study highlights, mercury also hitches a ride on sediments and organic debris that wash down into lakes from the surrounding land. The researchers found that the amount of mercury attached to organic matter varied 10-fold. The findings help explain how – and perhaps why – the pollution levels can drastically differ from lake to lake.

Evidence from the study challenges the long-held notion that climate change and the amount of algae in the lake influence the amount of mercury stored in the lake bed. However, more research will be needed to tease apart the climate analysis since it was limited to just three of the nine lakes. Also, algae levels may be more important in controlling mercury in lakes with different degrees and types of algae production, such as in arctic or urban areas.  

Although the lakes studied did not have fish, a major source of human exposure to mercury – and its more harmful form methylmercury – is through eating fish. Some of the highest mercury levels are found in fish from freshwater lakes. High mercury exposure can result in adverse health effects on the heart, brain, nervous system and immune system. Children and the developing fetus are especially vulnerable to low levels because they are growing and changing.  

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