The isotopes allowed the scientists to distinguish between experimentally applied mercury and mercury already present in the ecosystem and to examine bioaccumulation of mercury deposited to different parts of the watershed.
The researchers found that fish methylmercury concentrations responded rapidly to changes in mercury deposition over the first three years of study.
"Up to now a direct link has been difficult to establish because of all the other factors that affect mercury levels in fish and large pools of mercury already in the environment. By adding stable mercury isotopes to an entire ecosystem for several years, our team was able to zero in on the effects of changing atmospheric mercury deposition," said lead author Reed Harris of Tetra Tech.
"The results were very dramatic. Using the stable isotope approach has revealed a great deal about the cycling of mercury in watersheds. We look forward to continuing our study to provide guidance in mitigating the legacy left by the years of high mercury deposition,' added co-author Dr. Andrew Heyes of the University of Maryland Center for Environmental Science Chesapeake Biological Laboratory.
Dr. Heyes said the study showed the "clear benefits of regulating mercury emissions, and the near-term effectiveness of emission reductions".
"This is good news. It means that a reduction in new mercury loads to many lakes should result in lower mercury in fish within a few years," added Harris.
The international research began in 2001 at the Experimental Lakes in Northern Ontario, and is featured in this week's Proceedings of the National Academy of Sciences.