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A new study confirms that restoration of tidal flow to impounded coastal wetlands dominated by the non-native plant Phragmites australis could reduce methane emissions and help mitigate climate change.

Healthy coastal wetlands absorb and store large quantities of carbon, and saline coastal wetlands such as salt marshes and mangroves produce negligible methane emissions. Methane is a greenhouse gas many times more powerful than carbon dioxide. By limiting the amount of greenhouse gases in the atmosphere through “blue” carbon sequestration and minimal methane production, these ecosystems help to mitigate climate change. However, construction of roads and coastal defense structures, such as seawalls and dikes, can interfere with coastal hydrology and restrict tidal flow—a major factor in the global decline of coastal wetlands over the last century. When coastal wetlands are impounded, meaning that water is retained but tidal exchange is limited or excluded, it can lead to several changes that affect wetland function, including reduced salinity, altered water level, less tidal flushing, and vegetation shifts (like invasion of Phragmites). These changes are widespread nationally and globally, and they can disrupt the ecosystem’s carbon balance.  

People in field of high vegetation with masks on working with clear cylindrical chamber
Rebecca Sanders-DeMott and Adrian Mann measure carbon dioxide and methane fluxes in a clear chamber in a Phragmites wetland in Mashpee, Massachusetts.

New science provides critical confirmation that salinization (increasing the salt content) with tidal restoration could dramatically reduce methane emissions in impounded wetlands dominated by Phragmites. “We found that impounded ecosystems that are less salty than natural marshes can become net sources of climate warming due to their increased methane emissions. Restoring natural tidal exchange to our coastal wetlands improves habitat for native vegetation and wildlife, water quality, and coastal resiliency, and this study demonstrates that restoration is good for our climate, too,” said Rebecca Sanders-DeMott, lead author of the journal article. 

Dr. Rebecca Sanders-DeMott and her team designed a study to investigate how methane emissions and carbon absorption vary across a salinity gradient in impounded and tidally unrestricted Phragmites wetlands. The goal of this work was to better understand what factors control methane emissions in this invasive vegetation type that typically dominates impounded coastal wetlands. 

Research sites included one high-saline and low-saline location at both the Herring River—a dike-impounded estuary complex that is part of the Cape Cod National Seashore in Wellfleet, Massachusetts—and Sage Lot Pond marsh—a natural salt marsh complex with unrestricted tidal exchange located within the Waquoit Bay National Estuarine Research Reserve in Falmouth, Massachusetts. The research team used 12-foot-tall chambers to enclose Phragmites vegetation and assess how much methane and carbon dioxide move in and out of the ecosystem across the salinity gradient. They also used an eddy covariance tower, which consists of high-speed gas analyzers mounted high above the vegetation canopy, to monitor the continuous release and absorption of methane and carbon dioxide for a full year from the impounded wetland at the Herring River.

 

Field of tall vegetation with tall cylindrical equipment
The study used 12-foot-tall  chambers to enclose Phragmites vegetation and assess how much methane and carbon dioxide move in and out of the ecosystem across the salinity gradient.
Photograph of USGS scientist on a ladder checking equipment
Rebecca Sanders-DeMott  performing maintenance on an eddy flux tower located within the phragmites wetland at the Cape Cod National Seashore.

 

Results show that release of methane increases nearly 50-fold as salinity decreases across Phragmites-invaded wetlands. The impounded wetland at Herring River showed little variation in water-table depth or salinity during the growing season (May through October) because of disconnected and impounded hydrology. Soil temperature was therefore the strongest factor controlling both methane production and carbon absorption. Although the impounded wetland with less salinity had powerful carbon absorption and storage capabilities, it was also a strong and continuous source of methane which negatively affects the greenhouse gas balance of the ecosystem. Resalinization could significantly reduce methane emissions in impounded wetlands—helping to limit the amount of greenhouse gases in the atmosphere.

As infrastructure ages and sea levels continue to rise, management decisions for coastal areas are increasingly important. Coastal stakeholders need more information on the greenhouse gas consequences of different management actions. This study provides a better understanding of how salinity and impoundment affect methane and carbon fluxes in coastal wetlands dominated by Phragmites that will help fill this knowledge gap and advance blue carbon management as a natural climate solution—an emerging priority within U.S. and international climate policy.  

The publication team included Dr. Rebecca Sanders-DeMott (Woods Hole Coastal and Marine Science Center; WHCMSC), Dr. Meagan Eagle (WHCMSC), Dr. Kevin Kroeger (WHCMSC), Dr. Faming Wang (Marine Biological Laboratory; MBL and Xiaoliang Research Station for Tropical Coastal Ecosystems), Wally Brooks (WHCMSC), Jennifer O’Keefe Suttles (WHCMSC), Sydney Nick (WHCMSC), Adrian Mann (WHCMSC), and Dr. Jianwu Tang (MBL). The paper, “Impoundment increases methane emissions in Phragmites-invaded coastal wetlands” was published in the journal Global Change Biology on May 26, 2022. 

Impounded wetland eddy flux tower install
The research team at a related eddy covariance site in the Herring River, Wellfleet, Massachusetts.