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Researchers from the Wisconsin Department of Natural Resources (DNR), the University of Wisconsin (UW) Water Chemistry Program and Lake Superior State University of Sault Sainte Marie, Mich., believe that microscopic organisms living at the bottom of Lake Superior are causing the dangerous levels of mercury in fish.
The researchers are studying how mercury enters the food chain and accumulates in certain fish. Some researchers believe it is the form of mercury (methyl mercury) that etners the lake from wetlands and tributaries rather than by processes in the lake itself.
"Phytoplankton take up the methyl mercury and are eaten by zooplankton, zooplankton are eaten by small fish, and small fish are eaten by large fish," explained Jim Hurley, a DNR water chemist an assistant firector of research for the UW Water Resources Institute. "Each time the mercury moves up a rung of the chain, there is at least a threefold increase in methyl mercury. Some large fish have enough mercury in them to warrant fish advisories."
This mercury conversion, he added, occurs at different rates in different waterbodies. That is why waterbodies with high levels of total mercury, but low rates of conversion to methylmercury, may not carry fish advisories while others do. Hurley said, "We think that bacteria in the watershed, particularly in wetlands, are producing high levels of methyl mercury, even in zones remote from any direct pollution source."
(Source: Environment News Service)