Bacteria take in tiny particles of mercury, combining it with methane to produce methyl mercury. This is the most toxic form of mercury, and it can damage brain cells. When the feathers stop growing, they stop taking in mercury. By analyzing loon feathers, scientists can determine how much mercury a loon was exposed to during the time it molted into its feathers. The mercury stays in the feathers as long as the feathers exist.
All natural systems have tiny amounts of mercury.
One lake with moderate levels is Little Rock Lake, in northern Wisconsin. In Little Rock Lake, mercury levels are about 0.0001 ppb. This means 0.0001 grams of mercury for every billion grams of water. Loons 1250 ppb Insects not tested EPA scientists studying Little Rock Lake have measured the amount of mercury in various living things in the lake. At each level of the food pyramid, the animals have more mercury buildup than the level beneath.