Researchers studying mercury gas in the atmosphere have concluded that trees and plants absorb a vast amount of the toxic element in the same way that plants absorb carbon dioxide. Rainfall is commonly understood as a critical source of mercury pollution; however, plants absorbing mercury gas play the leading role in cycling mercury through the environment.
“Plants take up mercury from the atmosphere by accident and integrate it into their leaf tissues,” said Daniel Obrist, chair of the Department of Environmental, Earth and Atmospheric Sciences at the University of Massachusetts Lowell. “At the end of the growing season the leaves are shed, and the trees die off eventually, and as this plant material and biomass fall to the forest floor, all the mercury that has been absorbed will get transferred to the forest floor as well.”
Obrist and his colleagues synthesized more than 200 published studies with data about mercury levels in vegetation from 400 locations around the world. They concluded that about 88 percent of the mercury found in plants originates from the plants’ leaves absorbing mercury from the atmosphere. Globally, vegetation takes up more than 1,300 tons of mercury each year, accounting for 60 to 90 percent of annual deposits on land.
While some of the studies the researchers evaluated focused exclusively on mercury accumulating in leaves, Obrist said that leaves are only one part of the story. “What you normally don’t measure is branches falling off, bark being shed, lichens and mosses that fall down,” he said. “We believe that just measuring what comes down from leaves is not sufficient to quantify that flux. But clearly in our temperate forest, the full leaf drop is an important flux.”
Most of the mercury absorbed by plants originates from the emissions of power plants and other industrial sources. Mercury doesn’t harm the plants, but when it accumulates in soils, it eventually finds its way into streams, rivers, and lakes, where it concentrates in fish. The primary exposure humans and wildlife have to mercury comes from consuming fish.
Obrist said that the forests of the Northeast are a hotspot of mercury contamination, and he is working to understand why. “We were a hotspot for acid rain impacts, so maybe it’s related to that,” he said. “Air quality has improved since the 1980s and ’90s, but maybe there are other reasons why we are a sensitive area for mercury contamination.”
In a related project, Obrist is measuring how vegetation affects mercury cycling in New England forests by focusing on two sites in Maine and Massachusetts. His team is using a variety of instruments and sensors to measure the forests’ uptake of mercury in the atmosphere at various heights from above the tree canopy down to near the forest floor, allowing for daily tracking of how mercury deposition may be different in each forest and may change with the seasons. He is doing a similar study of plants in coastal salt marshes. He hopes this data will help inform regulatory frameworks in the future.