New Study: Mercury Cycling in Great Bay Ecosystem

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June 2009

Biologist Celia Chen and her technician Deenie Bugge were up to their knees and elbows in thick grey marsh mud, searching for small polychaete worms buried in the Squamscott River sediment. They rinsed off by pulling a seine through the water channel at low tide, looking for mummichogs — a small estuarine fish — and green crabs.

These organisms, along with water and sediment samples, help reveal how the heavy metal mercury moves through the intricate estuarine food web and how it could impact humans near the seacoast. Exposure to high levels of this neurotoxin can impair motor coordination and sensory ability, and estuaries such as Great Bay are ideal locations for the accumulation of contaminants like mercury that settle out from surrounding rivers and industrial land use.

Deenie Bugge, a technician who works with Celia Chen at Dartmouth College, digs for small polychaete worms in the Squamscott River mud. A new study is examining how these worms and other organisms affect mercury cycling in Great Bay
Deenie Bugge, a technician who works with Celia Chen at Dartmouth College, digs for small polychaete worms in the Squamscott River mud. A new study is examining how these worms and other organisms affect mercury cycling in Great Bay.

Chen, a research associate professor of biological sciences at Dartmouth College, is collaborating with Aria Amirbahman from the University of Maine and Mary Voytek from the U.S. Geological Survey to conduct molecular analyses and DNA fingerprinting on the samples. While previous research has focused on mercury concentrations in either sediments or living organisms, the results from this N.H. Sea Grant-funded project will connect organisms with their environment to determine the method and level of mercury cycling in nearby waterways.

"We're trying to tease apart all the components of this issue," Chen said. "We want to determine how much mercury is in the water and bound to sediments, how much is available to species that live in the mud, and how much is eaten by organisms higher in the food chain," she said.

The main sources of mercury in the New England environment are coal-fired power plants, incinerators and other point sources that discharge waste directly into waterways or the atmosphere. Some mercury arrives on air currents from regions further west, including the Ohio River Valley and as far away as Asia, Chen said.

Mercury deposits on the land and eventually enters the waterways and settles into the sediments on the bottom. Tiny organisms called sulfate-reducing bacteria transform mercury into methylmercury, which can then be absorbed or consumed by other organisms that live near the sediments. Polychaete worms and other bottom-dwellers mix the top sediment layers and thus potentially enhance methylmercury cycling and eventual uptake, Amirbahman said.

Preliminary results indicate that methylmercury concentrations in polychaete worms, mummichogs and green crabs were higher for those collected in the Squamscott River channel at Great Bay in Stratham than at the Piscataqua River near Seavey Island in Kittery, Chen said. However, mercury levels were higher in both channels than in nearby salt pannes — the small pools in a salt marsh that retain water during low tide.

"The mercury levels in Great Bay in general are not alarmingly high, but they are about half as high as a very contaminated site located in New Jersey," Chen said. "By comparison, our sampling site with the lowest mercury levels is in Wells Estuary in Maine."

"The Great Bay sediments are not any more contaminated than the Penobscot River sediments, which have been subject to point-source contamination," Amirbahman added.

Tidal cycles, water circulation, surrounding land use and the presence of carbon and iron at the sites could contribute to the differences in mercury and methylmercury levels, she explained. Researchers will conduct more sampling this year to determine why the site differences are occurring. Results from this study will be used to help inform environmental policy recommendations made by local and regional agencies.

Tom Niejadlik, administrator for the N.H. Department of Environmental Services (DES) Environmental Health Division, said that mercury and methylmercury levels in Great Bay are similar to levels in the entire northeast U.S. "This isn't just a local problem," he said, "Although some places have 'hot spots' where mercury and methylmercury levels are slightly higher, in general the levels are about the same throughout the northeast."

The N.H. DES developed a strategy in 1998 to address concerns about this toxin in nearby waterways and employ pollution prevention methods to reduce mercury levels in the state's waterways, Niejadlik added. One of the most important aspects of this effort was the development of a fish consumption advisory list for freshwater and marine species in the state. This list offers guidelines for consuming certain fish species that provide the health benefits of seafood and contain relatively low levels of mercury.
For more information on Chen's research, visit: http://www.dartmouth.edu/~toxmetal/about/faculty/Celia.html. For information on Dartmouth College's Toxic Metals Research Program, visit: http://www.dartmouth.edu/~toxmetal/home/index.html.

For more information about fish consumption advisories in N.H., please visit the N.H. DES web site for the fact sheet: http://des.nh.gov/organization/commissioner/pip/factsheets/ard/documents.... The N.H. DES mercury reduction strategy can be found at: http://des.nh.gov/organization/commissioner/p2au/pps/ms/mrpptp/index.htm.

Additional information about mercury can be found at: http://www.epa.gov/mercury/.

Rebecca Zeiber, N.H. Sea Grant Science Writer