Methylmercury, an atom of mercury bound to a carbon atom and three hydrogen atoms, is one of the most toxic substances known in nature. Take a gram of mercury, the amount in one clinical thermometer, allow the bacteria in a wetland to convert it to methylmercury, and the result is more than enough methylmercury to contaminate a twenty-five acre lake. Concentrate that gram of methymercury in living tissue and it could make 2,000 fish unsafe for human consumption or decrease the reproductive success of thirty pairs of loons.

Methymercury is an unusual poison because it is both physiologically and ecologically dangerous. Because it can pass through the blood-brain barrier and affect the brain directly, very tiny amounts can affect behavior or damage the nervous system, especially in young animals. Because it can bind to proteins it accumulates in tissue and can move up the food chain from prey to predators. The concentrations of methylmercury in fish are typically a million times higher than in lake water; the concentrations in the loons that eat the fish typically ten times higher still.

Traditionally, mercury contamination was thought of as an urban problem, restricted to the areas near mercury-using industries. Then in the 1980s it was discovered that many remote lakes in Ontario and the upper Midwest had high levels of mercury. Mercury, it seemed, was a ubiquitous component in acid rain, and the acids from in the rain were facilitating its movement through watersheds and making it more available to animals.

By the mid-1990s we had learned that the Northeast had serious mercury problems as well. One study found yellow perch exceeding the Canadian limit for safe consumption on fourteen out of sixteen lakes and fish exceeding the U.S. limit in nine of sixteen lakes. Fishermen were advised not to eat more than one meal a month of fish from Minnesota, inland Maine and many lakes in the Adirondacks. Sadly, for the eagles and otters and loons on these lakes that was not an option.