The Confounding Simplicity of Coastal Carbon

What do a Florida mangrove swamp, a Connecticut cattail-lined salt marsh, and an Oregon tidal freshwater forest all have in common? Sure, that funky low-tide smell is shared by each, but they’re all different in essential ways—some are cold, others warm, and in each setting different plants and creatures live among the salty mud.

New research, however, reveals that in one important parameter these wetlands are remarkably similar: a cubic meter of mud from each setting holds almost exactly the same amount of carbon. This finding, which came as a surprise to the team led by Lisamarie Windham-Myers of the United States Geological Survey, shows that sometimes, nature is simpler than it seems.

Though Windham-Myers spends plenty of time venturing into knee-deep coastal muck to collect data firsthand, this finding emerged from an analytical foray into a dataset of over 1,500 sediment cores. The dataset, compiled by biogeochemist and ecologist James Holmquist of the Smithsonian Environmental Research Center, is the most comprehensive to date, and its high data density helped the researchers demonstrate just how narrow the distribution in carbon stock really is.

Pieces of the theoretical basis for this finding have been around for some time. It was already known that soil with high carbon content has a relatively low bulk density and vice versa, theoretically creating a consistent carbon level. In inland ecosystems, however, differences in environment drive large differences in carbon stock per unit volume. The prevailing assumption was that this variability occurs in coastal wetlands too.

As for why carbon behaves differently on the coasts, Windham-Myers is blunt. “We don’t really know,” she said. She proposes, however, that the rules governing carbon concentration along the coasts are not the same as in interior ecosystems.

The ebb and flow of tides may also be a great equalizer for coastal wetlands. Tides have a fairly consistent influence along different coasts; their regulation of geomorphic processes could help account for the regularity of the coastal carbon stock. “If we do see variability in soil stocks, it probably will be an effect of variability in tidal influence,” Windham-Myers said.

Though tidal wetlands only make up two percent of land area worldwide, they hold 50 percent of the organic carbon buried in ocean sediment, making them a pivotal element in the global carbon budget. Sea level dictates the volume of soil within which carbon can be stored. These findings suggest that changing sea level, more than changing temperature and biological productivity, will control the amount of carbon sequestered in coastal wetlands.

Exactly how coastal wetlands will respond to changing sea level remains uncertain. Rising sea levels may create new tidal wetlands that take carbon dioxide out of the atmosphere by trapping organic matter in the soil. At the same time, however, the seas will drown existing wetlands and release their carbon through erosion. The balance between these two processes affects carbon dioxide concentrations in the atmosphere, making this feedback of particular interest to climate change modelers.

This research will likely have a more immediate impact in the carbon offset market, where individuals, companies and governments can fund the restoration and preservation of wetlands in return for a carbon offset credit. In essence, they are paying to keep wetland carbon in the ground to offset carbon emissions they have made elsewhere. With an improved understanding of how much carbon is retained in coastal wetlands, their value on the carbon market will likely increase, and with it the likelihood that they will be protected or restored.


1. Interview with Lisamarie Windham-Myers, January 29, 2018