Wetlands play an outsized role in global carbon cycling. The world’s wetlands store as much as 30% of terrestrial carbon, sequestering nearly 50 million metric tons of carbon from the atmosphere every year.
Although extensive peatlands in Alaska and Canada drive much of this storage, in recent years researchers have determined that blue carbon wetlands—those found in coastal ecosystems—store more carbon than their limited size would suggest. Should sea levels continue to rise as projected, blue carbon wetlands are expected to play a growing role in global carbon calculations.
To understand how transitions in coastal ecosystems will affect carbon budgets, Krauss et al. studied how carbon is stored, sequestered, and transported in estuaries in the southeastern United States. The researchers specifically looked at tidal freshwater forested wetlands and low-salinity marshes, two neighboring habitats in the upper estuary that may transform as ocean water encroaches farther inland in the coming decades. The study is the first to explore the carbon dynamics across these coastal environments, which occur around the world.
The researchers established transects along two rivers, the Waccamaw River in South Carolina and the Savannah River in Georgia. The transects spanned four wetland types characterized by freshwater to low-salinity conditions. (The salinity of low-salinity, or oligohaline, marshes is approximately 5.0 practical salinity units. In comparison, the salinity of marine environments is approximately 30.0 practical salinity units). At each wetland, the scientists recorded the aboveground carbon stock housed in the trees, shrubs, herbaceous plants, and downed woody debris and belowground carbon stock in soils, including plant roots and sediments. They also measured the flux of carbon between the soil, vegetation, and atmosphere and tracked how carbon moves between habitats.
The research team found that the upper estuary habitats store more carbon than several well-known carbon sponges, like boreal and temperate forests. Surprisingly, they also discovered the upper estuary carbon stocks even exceed those of seagrass and salt marsh ecosystems, two celebrated coastal carbon sinks.
However, the tidal freshwater forested wetlands and marshes differed in how they stored carbon. Whereas the tidal freshwater forests store an appreciable amount of carbon in aboveground woody material, the carbon generally migrates belowground into the roots and soil as salinity increases downstream and herbaceous vegetation becomes more prominent. The more saline lower reaches of the transects also appear to store more carbon than their upstream counterparts.
Although it is still unknown if similar wetlands behave comparably in different parts of the world, the research positions upper estuarine wetlands as carbon storage, sequestration, and flux powerhouses. These tidal wetlands could play an important role in mitigating climate change. (Global Biogeochemical Cycles, https://doi.org/10.1029/2018GB005897, 2018)
—Aaron Sidder, Freelance Writer