The West Coast's tidal swamps are supercharged carbon sinks

A collaborative cross-border study digging into forested tidal swamps in the Pacific Northwest has determined these ecosystems are carbon storage superheroes.

Found upstream from coastal estuaries and shorelines, but still subject to the flux of ocean tides, the woody wetlands feature a tangle of shrubs, grasses and trees, like willows and Sitka spruce, that can trap about nine million tonnes of organic carbon per hectare — the equivalent to the amount of carbon burned by two million gas-powered cars every year.

Researchers from Canada, the U.S. and Mexico pooled data and analyzed nearly 1,300 sediment samples from the length of the Pacific Northwest coast to examine coastal ecosystems, like mangroves, marshes, mudflats and swamps, that deposit organic matter, like leaves and sediments, by locking it away in low-oxygen conditions and storing it as “blue carbon.”

“Northwest tidal swamps provide a really high carbon bang for your buck, so they’re really important areas for conservation,” said Margot Hessing-Lewis, a researcher with B.C.’s Hakai Institute, who participated in the University of Oregon study.

However, the pockets of woody wetlands aren’t widespread across the coast, and many carbon hotspots have been destroyed by logging activity or development, she said, adding the next step would be to start mapping where they still exist or could be restored along the B.C. coast.

“It’s a unique but very understudied ecosystem that’s quite threatened. They’re like the temperate version of mangroves,” she said.

In addition to finding that forested tidal swamps store an amount of carbon, they also seem to emit less methane, the study found, which may make them more valuable from a climate perspective and when examining areas suited for the carbon offset market.

A surprising aspect of the study was the finding that tidal mudflats without vegetation were found to store carbon as well or better than eelgrass meadows, Hessing-Lewis said.

While both of the near-shore habitats aren’t as effective at storing soil carbon as the tidal swamps, they still contribute significantly because they cover larger geographical areas, she said,

All coastal ecosystems, most of which are disappearing rapidly, are vital for multiple reasons beyond blue carbon values, she added.

“Carbon storage is just one habitat asset,” Hessing-Lewis said.

“Eelgrass meadows act as nurseries for fish. They also filter pollution and do other things beyond climate mitigation.”

The study led by Chris Janousek is part of the work being done under the umbrella of the Pacific Northwest Blue Carbon Working Group, said co-author Scott Bridgham, Emeritus professor at the University of Oregon. The working group involves scientists, conservation leaders, habitat restoration groups, carbon offset market experts and land managers and government staff that are looking to examine how coastal ecosystems store or release carbon, he said.

Studies are also being done to help assess which coastal ecosystems might emit high levels of methane, a gas with 80 times the planet-warming power of carbon dioxide over the first 20 years it enters the atmosphere, Brigham said.

The group is also examining how areas like swampy tidal wetlands could buffer the impacts of flooding and storm surges with sea-level rise to protect important conservation areas or communities, Bridgham added.

The data contributions from international scientists spread across the Pacific coast strengthened the study’s outcomes and offered insights that can help inform decisions to protect nature or coastlines at a regional or national level, said James Holmquist, at the Smithsonian Environmental Research Center.

Pooling and sharing scientific information from sources, like the Smithsonian Carbon Atlas, the blue carbon working group and among regional and international scientists, leads to richer outcomes and avoids data being siloed in individual “filing cabinets,” Homquist said.

“Coastal wetlands store tremendous amounts of carbon, but they are under-researched and over developed and vulnerable throughout the world,” he said.

“We really need large data sets to answer just basic questions about their ecosystem properties.

“By building trust and building relationships, [scientists] can do something that is more impactful than any one of us can do on our own.”