By Edwin L. Aguirre
Toxic mercury is being deposited in forests in much higher quantities than previously thought, posing a concern for the health and well-being of people, wildlife and waterways, according to a UMass Lowell scientist investigating the source of the pollution.
“It’s the first such study that covers, at the ecosystem level, the full picture of mercury deposition at any forest site in the world,” says Obrist.
“Plants take up gaseous mercury from the atmosphere through their leaves, through their stomata (pores), and as the plants shed their leaves or die off, they basically transfer that atmospheric mercury to the ecosystems. Mercury deposits in forests ultimately run off into streams and rivers, ending up in lakes and oceans,” he explains.
The team’s data, which was gathered at a forest in north-central Massachusetts, showed the pronounced and dominant role of gaseous mercury for annual deposition, amounting to 25 micrograms of mercury per square meter of forest.
This is five times greater than mercury deposited by rain and snow, according to Obrist.
“Our observed gaseous deposition accounts for 76% of the total mercury deposition from the atmosphere. It’s also three times greater than mercury deposition through litterfall (dead leaves that fall to the ground in autumn) alone, which has previously been used as a proxy for estimating gaseous mercury deposition in forests,” he notes.
The results of the project, which is supported by a three-year, $873,000 grant from the National Science Foundation (NSF), will be published this July in an issue of the Proceedings of the National Academy of Sciences.
Other contributors to the study include Asst. Prof. Róisín Commane of Columbia University; students and postdoctoral researchers from UMass Lowell and Columbia University; and collaborators from Harvard University; the Desert Research Institute in Reno, Nevada; and the Northwest Institute of Eco-Environment and Resources and the University of the Chinese Academy of Sciences in Lanzhou. Additional research support was provided by the U.S. Department of Energy.
Understanding the Mercury Cycle
Obrist and his colleagues measured the uptake of gaseous mercury at Harvard Forest in Petersham, Massachusetts, for almost 16 months. The site is a nearly 4,000-acre temperate deciduous forest where broadleaf trees like red oak and red maple shed their leaves every year.
The team installed inlet lines connected to atomic fluorescence analyzers to measure gaseous mercury, as well as placing other automated sensors on the forest’s existing 100-foot-tall research tower at various heights, from above the tree canopy down to near the forest floor. Combining these data with turbulence measurements of the lower atmosphere, the team could directly quantify gaseous mercury exchanges.
According to Obrist, other researchers have attempted to measure forest mercury deposition by collecting plant litterfall and measuring the plant material’s mercury concentration.
“What we found out is that litterfall accounted for only a third of the mercury deposition flux we measured with our tower-based method,” he says. “Measuring only the leaves that dropped down misses a lot of the other biomass that is turning over in the forest. There are tree branches that fall off, mosses and lichens on the forest floor that take up mercury, and tree trunks that fall over or get blown down during windstorms. Litterfall provides a very incomplete picture of the entire deposition flux that is happening at the forest level.”
The mercury deposition measured by Obrist and his team is driven by the combined uptake by plants and uptake to soils and the forest floor.
“We measured how Harvard’s deposition changes over the seasons,” says Obrist. In the study, gaseous mercury deposition was highest during the active growing seasons in spring and summer, and at midday, similar to the periods when plants absorb the most carbon dioxide for photosynthesis.
“This confirmed previous findings that plants dominate as the source of mercury on land, accounting for 54% to 94% of the deposits in soils across North America,” Obrist says.
“Our study suggests that mercury loading to forests has been underestimated by a factor of about two, and that forests worldwide may be a much larger global absorber and collector of gaseous mercury than currently assumed. This larger than anticipated forest deposition may explain the high mercury levels observed in soils across rural forests, which impairs water quality and threatens the aquatic food chain.”
A Potent Neurotoxic Pollutant
is a highly toxic environmental pollutant that threatens fish, birds, mammals and humans. Vast amounts are released into the atmosphere each year by coal-burning power plants, as well as through gold mining and other industrial processes, and the pollutant is distributed by winds and currents across the globe. Long-term exposure to mercury, or consuming food containing high levels of the pollutant, can lead to reproductive, immune, neurological and cardiovascular problems.
The total global mercury deposition on land is currently estimated to be about 1,500 to 1,800 metric tons per year, but it may be more than double that amount if other forests show similar levels of deposition.
Despite COVID-19 restrictions, Obrist and his team were able to continue their study.
“Because we’re outdoors and working in a remote area in the forest, we were able to continue our year-long research and complete our record,” he says.
The researchers originally planned to travel last year to a tropical rainforest in Costa Rica to continue their work after the study in Massachusetts was concluded. But with COVID-19 raging, they couldn’t travel internationally, so they received permission from the NSF to go to another site: the nearly 600-acre Howland Forest in northern Maine.
Howland offers a distinctly different biome, or habitat, than Harvard Forest. A coniferous forest, it is full of evergreens that retain their needles year-round.
“We are currently using Howland’s research tower operated by the University of Maine and the U.S. Forest Service for the study, and so we’ll be able to compare the differences between the two forests. We can see that their litterfall dynamics and growth seasons are very different,” says Obrist.
A High-achieving Undergraduate Researcher
The mercury study provided hands-on research opportunities for undergraduate Eric Roy
, a senior majoring in meteorology and math.
“One potential application of the research is that, by constraining the role of forests and plants in the global mercury cycle, we can better set parameters to computer models that have been designed to predict where mercury might accumulate in our current and future environments and climate regimes,” says Roy, a member of Obrist’s research team.
The Lowell native entered UML in fall 2018 and joined the EEAS Department the following spring as an Immersive Scholar
, a university scholarship awarded to first-year students with high academic credentials that pays them to engage in lab work and research early on. Obrist brought on Roy to analyze the data collected from the field.
“Eric’s contributions to the study are tremendous. It’s not very common for an undergrad to play such an important role in a major, federally funded research project,” says Obrist. “His work is really impressive, and he has become more and more active in data analysis and doing complex flux calculations and data processing. He really earned himself second author position in the paper in the Proceedings of the National Academy of Sciences.”
“It’s really exciting to be a co-author,” says Roy. “It’s been a really awesome experience.”
Multiple Biomes and Ecosystems
Obrist, an expert in atmospheric cycling and biogeochemistry of mercury, has been studying the deposition impact of this environmental hazard across multiple biomes and ecosystems for more than a decade.
Beginning in 2014, Obrist led a two-year, comprehensive international study to find the source of mercury pollution in the Arctic tundra
. The team’s findings, which were published in the journal Nature, showed that the absorption of mercury from the atmosphere by tundra vegetation is shown to drive high loads of the element in tundra soils. Mercury runoff from tundra soil then supplies 65 to 85 tons of the toxic heavy metal to Arctic lakes and rivers and the Arctic Ocean each year.
In summer 2020, the NSF awarded Obrist a three-year, $385,000 grant to determine the origin and fate of mercury in a Massachusetts saltwater marsh ecosystem in Plum Island Sound and the mercury’s potential export to the ocean. The project is being conducted in collaboration with the Marine Biological Laboratory in Woods Hole, Massachusetts.
“The Plum Island salt marsh estuary is an area previously shown to be a mercury contamination hotspot, the source of which is largely unknown,” Obrist says. “We want to see if the contributions of plants also play a key role in this contamination. Estuaries are not just bodies of water or wetlands; they are very productive ecosystems.”