Experimental Lakes Solving Mercury Puzzles

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What happens within lakes as human activities contaminate them, disrupt their surroundings, or otherwise change their environment? Laboratory experiments can suggest partial reactions, but never give the entire ecological picture. So in 1968, federal fisheries researchers got government approval to set aside, in the Pre-Cambrian shield country of northwestern Ontario, a network of lakes for environmental experiments. This was a world first, and it has yielded world benefits.

The Experimental Lakes Area (ELA) has, for example, proved how certain nutrients foster eutrophication, the over-enrichment of plankton and plant growth that consumes oxygen from lakes and shrinks their biodiversity. This led to bans on phosphorus in detergents, which helped clean up lakes in Canada and internationally. And the ELA's documentation of the alarming effects of acid rain on lake life helped create better controls on some sources of pollution.

Department of Fisheries and Oceans (DFO) researchers at the Freshwater Institute (FWI) in Winnipeg, Manitoba, led research on the 58 lakes. But John Shearer, senior biologist and operations manager for the ELA, points out that “we've attracted dozens of research partners from Canada, the United States, and overseas. These include many universities, especially the University of Manitoba, different levels of government, and hydro-power companies. Much of our research funding comes from outside DFO.”

Current ELA research on the complex question of mercury in lakes has attracted wide attention and support. Although the element occurs naturally in the earth and atmosphere, most airborne mercury, probably two-thirds, gets there from human activities, especially coal-burning power plants. When it enters lakes and watersheds, some mercury is transformed into toxic methyl mercury. Accumulating or “biomagnifying” up the food chain, methyl mercury can make fish, especially larger and older ones, dangerous to eat. In Canada, in some cases of extreme local industrial-mercury release, mercury-laden fish have caused nervous-system damage and even death in humans.

If coal-burning power plants increase mercury and other pollution, it would seem reasonable that hydro-electric power should provide a cle

aner solution to energy needs. But as governments and power companies flooded lands to create reservoirs, methyl mercury in fish often increased several times over. That phenomenon sparked a major, multi-year ELA study.

Dr. Drew Bodaly, recently retired from DFO but still involved in ELA research, describes the steps in the Experimental Lakes Area Reservoir Project, or ELARP. “In 1992, we dammed off a small pond and several acres of wetland, mainly peat bog with some black spruce,” Dr. Bodaly says. “By 1993, it had increased in depth by about a metre and a half. We watched what happened with the peat, the water, the plankton, and the fish that we added. And sure enough, methyl mercury in fish began increasing.” Toxic mercury in fish, it turned out, could reach as much as 7 times the previous levels.

The project yielded significant insights. The transformation from mercury to methyl mercury is a biological process driven by bacteria. Flooding of wetlands causes decomposition of peat, associated sphagnum mosses, and other carbon-based compounds. More organic material means more bacteria and, in turn, more methyl mercury finding its way up the food chain into fish. The worst effects come in species, such as lake trout and walleye, that feed on other fish.

In time, methyl mercury production decreased in the peat, but still kept getting into fish, through processes now being studied. ELARP also documented dramatic increases in the release of methane and carbon dioxide, greenhouse gases that contribute to climate change.

In a parallel experiment, the researchers flooded three upland areas, above the wetlands and more rocky. Here, with less carbon stored in the earth, there was less buildup of methyl mercury and greenhouse gases, and faster subsidence. In five years, mercury levels were slipping back to those before the flooding.

The research has made it clear that hydro-power, although cleaner than coal power for greenhouse gases, does damage of its own. The FWI scientists and their partners gained better understanding and developed useful guidelines for reservoir construction, including an overall lesson: if you must flood for hydro-power, use the least land you can and, if possible, do it on higher ground, away from wetlands.

Meanwhile, ELA researchers moved on to a related puzzle. It is clear that industrial growth has increased the mercury falling from the sky. Deposition is especially high in parts of Ontario, Quebec, New England, and the Maritimes that lie downwind from coal-burning power plants in central Canada and the American Midwest. But legislators, power companies, and others wanted to know more. How certain was it that mercury from the sky was getting into fish? Just how did methyl mercury production vary with mercury deposition, and what would be the ecological results of cutting emissions?

ELA researchers, along with power companies and other partners, launched a major project known as METAALICUS (Mercury Experiment to Assess Atmospheric Loading In Canada and the U.S.). To trace the movements of mercury, in a way distinguishable from the general levels already present in air, earth, and water, they needed identifiable batches. The scientists used different mercury isotopes: that is, variations that act the same as regular mercury, but have a different number of neutrons and thus a different atomic weight, detectable by a mass spectrometer.

“Starting in 2001, we applied the three isotopes separately, each year, to three different types of environment,” Drew Bodaly says. “We borrowed a pilot from the Canadian Forestry Service with a crop-duster type plane. We sprayed tiny amounts of mercury, within precise boundaries, on a small lake, the nearby wetlands, and the upland watershed. We monitored what happened, and got surprising variations.”

The mercury applied directly to the lake quickly began to appear in the food chain, raising methyl mercury levels in small fish within a year. But the mercury applied to wetlands and uplands “seemed to get stuck there,” Drew Bodaly says. “Only small amounts have gotten into the lakes and fish so far.”

After the last application of mercury, possibly in 2006, will come another crucial step. The researchers will measure the response of the lake, wetlands, and uplands as the mercury isotope additions stop.

METAALICUS as a whole will yield detailed information about what happens as atmospheric mercury increases or decreases. Both scientific understanding and regulatory programs will benefit. And the Experimental Lakes Area will notch up another achievement in understanding and protecting the world's fresh waters.