From 2000 through 2007, mercury was added to Lake 658 and its surrounding catchment at the Experimental Lakes Area (ELA) to mimic changes in mercury falling in rain. The project was designed to help us understand how mercury released into the atmosphere from coal-fired power plants and other sources affects mercury concentrations in fish. Nearly eight years after the mercury additions were stopped, scientists are still monitoring the recovery of the ecosystem. Initial results support proposed mercury emission reductions in Canada and the United States.


Mercury—especially its organic form, methylmercury—is a neurotoxin that causes cognitive problems. The primary route of exposure is by consumption of contaminated fish. Mercury is one of the most common contaminants in fish and is responsible for thousands of fish consumption advisories globally. In North America, the largest source of mercury to fish is in rain falling on lakes and their surrounding watersheds. Over half of this mercury comes from emissions to the atmosphere from coal-fired power plants. The governments of Canada and the United States have consequently proposed mercury emission standards and reductions for coal-fired power plants that could cost billions of dollars. The mercury cycle is complex, however, and a clear demonstration that these proposed regulations will be effective is required. In addition, information is needed on how long it will take for reductions of mercury air emissions to lower mercury concentrations in fish to help guide the expectations of industry, governments and the public.


At IISD–ELA, a project began in 1999 to examine the potential effectiveness of proposed mercury emission reductions to decrease mercury levels in fish. The project was called the Mercury Experiment to Assess Atmospheric Loading in Canada and the United States (METAALICUS). Unique forms of mercury called stable isotopes were added to an entire lake (Lake 658) and its surrounding watershed to mimic changes in mercury concentrations in rain. Additions were made using a crop-duster airplane and direct additions to the lake from a boat. Different stable isotopes were added to the lake itself, to the upland part of the watershed and to a wetland. By looking at how stable isotopes added to different parts of the watershed accumulated in fish, researchers could predict the response of fish in lakes in other areas where the relative size of lakes, wetlands and uplands vary. Over seven years, the total amount of additional mercury added to Lake 658 and its watershed was less than one teaspoon, increasing mercury deposition to levels experienced historically in parts of the United States and Europe. After following the effect of increasing mercury deposition, researchers are now monitoring recovery of the ecosystem.


The results of the METAALICUS project show a clear relationship between mercury added in simulated rain and methylmercury concentrations in fish. With additions of the stable isotopes of mercury, concentrations in fish quickly increased, and after additions stopped, they declined. This provides strong support for proposed legislation and indicates that reductions in mercury emissions will be effective for lowering mercury levels in fish.

a plane flies over a lake at IISD Experimental Lakes Area in Ontario