REGULATING METHANE EMISSIONS FROM FRESHWATER WETLANDS

FROM:  NATIONAL SCIENCE FOUNDATION
Methane-eating microorganisms help regulate emissions from wetlands
Without this process, methane emissions from freshwater wetlands could be 30 to 50 percent higher

Though they occupy a small fraction of Earth's surface, freshwater wetlands are the largest natural source of methane emitted into the atmosphere. New research identifies an unexpected process that acts as a key gatekeeper in regulating methane emissions from these freshwater environments.

The study results are published this week in the journal Nature Communications by biologist Samantha Joye of the University of Georgia and colleagues.

The researchers report that high rates of anaerobic (no oxygen) methane oxidation in freshwater wetlands substantially reduce atmospheric emissions of methane.

New attention

The process of anaerobic methane oxidation was once considered insignificant in freshwater wetlands, but scientists now think very differently about its importance.

"Some microorganisms actually eat methane, and recent decades have seen an explosion in our understanding of the way they do this," says Matt Kane, program director in the National Science Foundation's Division of Environmental Biology, which funded the research. "These researchers demonstrate that if it were not for an unusual group of methane-eating microbes that live in freshwater wetlands, far more methane would be released into the atmosphere."

Although anaerobic methane oxidation in freshwater has been gathering scientific attention, the environmental relevance of this process was unknown until recently, Joye says.

"This paper reports a previously unrecognized sink for methane in freshwater sediments, soils and peats: microbially-mediated anaerobic oxidation of methane," she says. "The fundamental importance of this process in freshwater wetlands underscores the critical role that anaerobic oxidation of methane plays on Earth, even in freshwater habitats."

Without this process, Joye says, methane emissions from freshwater wetlands could be 30 to 50 percent greater.

Comparison of wetlands

The researchers investigated the anaerobic oxidation process in freshwater wetlands in three regions: the freshwater peat soils of the Florida Everglades; a coastal organic-rich wetland in Acadia National Park, Maine; and a tidal freshwater wetland in coastal Georgia.

All three sites were sampled over multiple seasons.

The anaerobic oxidation of methane was coupled to some extent with sulfate reduction. Rising sea levels, for example, would result in increased sulfate, which could fuel greater rates of anaerobic oxidation.

Similarly, with saltwater intrusion into coastal freshwater wetlands, increasing sulfate inhibits microbial methane formation, or methanogenesis.

So while freshwater wetlands are known to be significant methane sources, their low sulfate concentrations previously led most researchers to conclude that anaerobic oxidation of methane was not important in these regions.

Crucial process

The new findings show that if not for the anaerobic methane oxidation process, freshwater environments would account for an even greater portion of the global methane budget.

"The process of anaerobic oxidation of methane in freshwater wetlands appears to be different than what we know about this process in marine sediments," Joye says. "There could be unique biochemistry at work."

Adds Katherine Segarra, an oceanographer at the U.S. Department of the Interior's Bureau of Ocean Energy Management and co-author of the paper: "This study furthers the understanding of the global methane budget, and may have ramifications for the development of future greenhouse gas models."

Additional financial support for the research was provided by the Deutsche Forschungsgemeinschaft via the Research Center/Cluster of Excellence at the MARUM Center for Marine Environmental Sciences and department of geosciences at the University of Bremen, Germany.

-- Cheryl Dybas
-- Alan Flurry, University of Georgia
Investigators
Samantha Joye
Christof Meile
Vladimir Samarkin
Related Institutions/Organizations
University of Georgia Research Foundation Inc

NSF: RISING SEAS THREATEN WETLANDS

Credit:  Wikimedia

FROM:  NATIONAL SCIENCE FOUNDATION 
Wetlands' ability to overcome sea level rise threatened

When do wetlands reach their limit, and how are we lowering that point?
Left to themselves, coastal wetlands can resist rapid sea level rise.

But humans could be sabotaging some of wetlands' best defenses, according to results published in this week's issue of the journal Nature.

Thanks to an intricate system of feedbacks, wetlands are remarkably good at building up soils to outpace sea level rise. The questions are: When do they reach their limit, and how have we lowered that point?

Without human-caused climate change, "we wouldn't be worried about wetlands surviving the rates of sea level rise we're seeing today," says lead paper author Matthew Kirwan of the Virginia Institute of Marine Science and the National Science Foundation (NSF) Virginia Coast Reserve Long-Term Ecological Research (LTER) site.

Virginia Coast Reserve is one of 26 such NSF LTER sites around the globe in ecosystems from deserts to mountains and marshes to grasslands.

In an unchanged world, "wetlands would build vertically at faster rates," says Kirwan, "or move inland to higher elevations."

The paper's co-author is Patrick Megonigal of the Smithsonian Environmental Research Center.

A wetland is land that's saturated with water, whether permanently or seasonally. The water found in wetlands can be saltwater, freshwater or brackish water. Main wetland types include swamps, marshes, bogs and fens.

Wetlands have developed several ways to build elevation to keep from drowning.

Aboveground, tidal flooding provides one of the biggest assists. When marshes flood during high tides, sediment settles out of the water, adding new soil. As sea level rises and flooding occurs more often, marshes react by building soil faster.

Belowground, the growth and decay of plant roots add organic matter.

Even erosion can work in wetlands' favor, as sediment lost at one marsh may be deposited in another. While a particular wetland may lose ground, the total wetland area may remain unchanged.

But, if a wetland becomes so submerged that its vegetation dies off, these "positive feedback loops" are lost. Similarly, if sediment delivery to a wetland is cut off, that wetland can no longer build soil to outpace rising seas.

"This study reveals the complex, long-term interplay among processes that maintain coastal wetlands in the face of sea level rise," says Saran Twombly, program director in NSF's Division of Environmental Biology, which funds the NSF Virginia Coast Reserve LTER site.

"Humans are newcomers to this delicate balance. The future of a habitat so essential to human well-being depends on how severely we alter it."

For example, groundwater withdrawal and artificial drainage can cause the land to sink, as is happening in Chesapeake Bay.

Because of this subsidence, eight of the world's 20 largest coastal cities have relative sea level rise greater than climate change projections.

Dams and reservoirs also prevent 20 percent of the global sediment load from reaching the coast.

Marshes on the Yangtze River Delta, for example, have survived a relative sea level rise of more than 50 millimeters per year since the 7th century--until the recent building of more than 50,000 dams cut off the supply of sediment and accelerated erosion.

"Tidal marshes are amazing ecosystem engineers that can raise themselves upward if they remain healthy, especially if there is sediment in the water," says Megonigal.

"We know there are limits, however. Those limits are changing as people alter the environment."

-NSF-