Mercury pollution at a Superfund site adjacent to a New Hampshire river is becoming more toxic because of algae in the river.

Transformation of the mercury into methylmercury occurs through a complex biogeochemical process linked to periphyton in the Androscoggin River in Berlin, New Hampshire.

Periphyton is a community of algae, bacteria, fungi and other natural material which is attached to submerged rocks, plants and other surfaces. It is a key part of aquatic ecosystems and can be a primary food source for small fish and invertebrates.

However, despite elevated levels of methylmercury in the sediment, water and periphyton, scientists from Dartmouth College and the U.S. Geological Survey found lower than anticipated levels in crayfish, mayflies and small fish downstream from the former chemical plant.

Previous studies have shown high methylmercury concentrations in large adult fish downstream from the site.

It's not yet known why bioaccumulation patterns are different between predator fish and the smaller creatures they eat, but the researchers suggest that the leaking mercury's impact may be more localized than expected.

"While our study clearly demonstrates that the chlor-alkali Superfund site is impacting this section of the Androscoggin River, future studies could investigate whether other factors such as dams, river grade, wetlands or upland drainage influence the patterns of bioaccumulation," commented senior author Celia Chen, a research professor of biological sciences and principal investigator in Dartmouth's Toxic Metals Superfund Research Program. "An even greater potential may exist for mercury bioaccumulation downstream of the Shelburne Dam, where the river broadens and slows even further."

The findings of the study have been published in the journal Environmental Toxicology & Chemistry.