Mercury Concentrations In Fish Respond Quickly To Increased Mercury Pollution

"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," said lead author Reed Harris of Tetra Tech. "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."

"The results were very dramatic," said co-author Dr. Andrew Heyes of the University of Maryland Center for Environmental Science Chesapeake Biological Laboratory. "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."

The international team's research began in 2001 at the Experimental Lakes in Northern Ontario.  To directly test the response of fish contamination to changing mercury deposition, researchers conducted a whole-ecosystem experiment, increasing the mercury load to a lake and its watershed by the addition of enriched stable mercury isotopes. The isotopes allowed the team 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.

For three years, the mercury load to the lake ecosystem was increased by roughly three times--bringing the total annual mercury load up to a level comparable to that on the east coast of the United States. This large-scale, whole ecosystem approach was important because the complex behavior of ecosystems can be difficult to predict from smaller-scale experiments.

To distinguish the mercury they added to the lake from the existing mercury in the study ecosystem, the researchers used a sophisticated analytical method that had never been used in this way at such a large scale. Mercury in the natural environment is made up of seven stable isotopes that do not vary much in proportion to one another. To dose the lake, the scientists used mercury that is heavily enriched in one of those isotopes, enabling them to trace the mercury they added through the complex environmental mercury cycle.

Fish methylmercury concentrations responded rapidly to changes in mercury deposition over the first three years of study.

Additionally, the study team found that mercury added directly to the lake surface was rapidly accumulated into fish, while essentially none of the mercury deposited to the lake's watershed was found in fish after three years. This suggests that lakes should exhibit a two-phase response to load reductions. Initially, mercury in fish should decline rapidly (within years) as deposition to the lake itself is reduced. A slower, more prolonged decline (perhaps decades long) should follow in response to decreases in mercury deposition in the watershed.

"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 Cynthia Gilmour of the Smithsonian Environmental Research Center and University of Maryland MEES participating faculty. Harris went on to say "The study shows the clear benefits of regulating mercury emissions, and the near-term effectiveness of emission reductions."

Mercury levels in the environment have increased several-fold on a global scale since pre-industrial times due to emissions from coal-fired power plants, metal smelting, and other sources. Mercury is persistent in the environment, and toxic to humans and wildlife. There are currently thousands of advisories against eating fish from lakes in both Canada and the United States.

The research is to be published in the Proceedings of the National Academy of Sciences.

The research was funded by grants from both U.S. and Canadian sources. Dr. Heyes, along with his colleagues Cynthia Gilmour and Robert Mason of the University of Connecticut have been funded by the United States Environmental Protection Agency, the National Science Foundation and the Electric Power Research Institute.

The article, "Whole-ecosystem study shows rapid fish-mercury response to changes in mercury deposition," is the first synthesis of results from the Mercury Experiment to Assess Atmospheric Loading in Canada and the United States (METAALICUS) experiment and is co-authored by 24 authors from Canada and the United States.