Study Shows Forests May Take in Far More Toxic Mercury Than Thought

Atmospheric mercury in gaseous form appears to be adding heavily to the load.

Earth Institute
July 12, 2021

Researchers studying a Massachusetts forest say that the toxic element mercury may be deposited in forests across the globe in much greater quantities than previously understood. The study suggests that the mercury load carried by forests has been underestimated by half. This mercury accumulates in soils, and ultimately runs off into streams and rivers, ending up in lakes and oceans. The study appears this week in the Proceedings of the National Academy of Sciences.

Mercury threatens fish, birds, mammals and humans. The pollutant is released into the atmosphere each year by coal-burning power plants, as well as gold mining and other industrial processes, and distributed by winds and water currents across the globe. Long-term exposure or consuming food containing high levels of it can lead to reproductive, immune, neurological and cardiovascular problems in people.

The total global amount of mercury deposited to land currently is estimated at about 1,500 to 1,800 metric tons per year. It may actually be more than double that if further research at other forests confirms the results of the new study.

For 16 months, the team measured exactly how mercury in the atmosphere gets deposited at Harvard Forest in Petersham, Mass., a nearly 4,000-acre site that includes deciduous trees such as red oak and red maple that shed their leaves every year. Instruments placed at various heights on the forest’s 100-foot-tall research tower looked at how mercury gets deposited from the tree canopy to the forest floor.

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Instruments on a 100-foot-tall tower in Howland Forest, Maine, take measurements in an ongoing study of mercury deposition. Newly published research was performed at Massachusetts’ Harvard Forest. (Courtesy Daniel Obrist, UMass Lowell)

It turned out that three quarters of the deposition came from mercury in its gaseous form—five times greater than that deposited by rain and snow, and three times greater than that deposited through litterfall, in which mercury is transferred by leaves falling to the ground. Most previous studies did not address gaseous mercury directly; litterfall had previously been used by other researchers as a proxy for it. However, in the new study, the researchers measured gaseous deposition directly.

Lead author Daniel Obrist of the University of Massachusetts, Lowell, said the study is the first to fully examine how mercury in the atmosphere is deposited at any rural forest.  “Trees take up gaseous mercury from the atmosphere through their leaves, and as plants shed their leaves or die off, they basically transfer that atmospheric mercury to the ecosystems,” he said.

“By measuring trace gases from the top of the canopy to the forest floor, we can understand how those gases move within the forest and where the various gases are taken up and emitted,” said study coauthor Roísín Commane, an atmospheric researcher at Columbia University’s Lamont-Doherty Earth Observatory.

The researchers are continuing their work at a second forest, in northern Maine. The state’s Howland Forest, a nearly 600-acre research site full of evergreens that retain their leaves year round, offers a distinctly different habitat. Assessing both forests will allow researchers to examine differences in mercury accumulation between different forest types, Obrist said.

Other contributors to the study include students from UMass Lowell and Columbia; and collaborators from Harvard University; the Desert Research Institute in Reno, Nevada; the Northwest Institute of Eco-Environment and Resources; and the University of the Chinese Academy of Sciences in Lanzhou. The study was supported by the U.S. National Science Foundation; additional research support came from the U.S. Department of Energy.

Adapted from a press release by the University of Massachusetts, Lowell.