In 1908, locals in Cape Cod, Mass., built an earthen dike across the Herring River to curtail its flow into the surrounding wetlands. Their goal was to clamp down on the number of mosquitoes. The dike destroyed the original salt marsh, replacing it with woodland, shrub, and impounded wetlands. But the community’s efforts over a century ago did far more than dry out the landscape. According to a new study, impounded wetlands can undergo an important change, shifting from carbon sinks into methane sources. The transformation turns these landscapes from ones that help mitigate climate change into ones that exacerbate it.
An impounded wetland, explains Rebecca Sanders-DeMott, a marine ecologist who led the study while working at the U.S. Geological Survey’s Woods Hole Coastal and Marine Science Center in Massachusetts, is a coastal wetland cut off from the sea. Where seawater used to naturally flow in and out with the tide, that flow has now been cut off — often by road building or the construction of dikes, dams, and other flood- and water-management infrastructure.
Although identifying impounded wetlands can be hard, since it requires knowing what the landscape was like before the infrastructure was built, scientists do know that huge areas of tidal wetlands are in a state of flux. Recent satellite mapping shows that globally almost 14,000 square kilometres of tidal wetlands disappeared between 1999 and 2009. These losses, however, were offset by the development of nearly 10,000 square kilometres of tidal wetlands over the same period through natural processes and restoration efforts. Estimates suggest that in the United States, up to one-quarter of the coastline is in some state of impoundment, although not all of this area was former wetland.
Sanders-DeMott and her colleagues’ work shows that this widespread land transformation is bad news for climate change. Comparing two sites around Herring River with two at a nearby natural salt marsh with unrestricted tidal exchange, the researchers measured gas emissions from both types of wetlands. They also used historical sediment core data to analyze how much carbon is stored in the soil and measured other environmental factors, such as the salinity of the water and the type and density of the vegetation.
“These impounded wetlands that were freshened and, in our case, invaded by invasive vegetation — which is very common in impounded wetlands — produce a lot more methane than their natural counterparts,” Sanders-DeMott says.
The scientists found that methane emissions scale exponentially with the salinity of the water. The impounded wetlands were fresher than the undisturbed tidal wetlands, with the freshest impounded site they studied producing almost 50 times more methane than the saltiest unrestricted wetland.
That increase in methane is likely caused by changes in the plant and microbial communities in the wetlands. The microbes that decompose plant matter in saltier environments produce less methane than those in fresher ecosystems.
While all wetlands store an impressive amount of carbon, the impounded study sites’ methane emissions are high enough that the wetlands have a net warming effect on the atmosphere, Sanders-DeMott says.
With people increasingly looking at restoring wetlands as a way to rebuild habitat and protect coasts against sea-level rise and storms, Sanders-DeMott says the finding shows the climate change consequences of different choices. “If we restore tidal exchange to these impounded wetlands, in addition to whatever other reasons there might be to do that, we are also likely to see a reduction in methane emissions,” she says.
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One solution for preserving the flood-protecting powers of dikes and dams that led people to impound wetlands in the first place, while avoiding the climate change-boosting consequences, could be to install storm gates, says Robert Mendelsohn, an environmental economist at Yale University in Connecticut.
While tidal gates have been used for centuries to stop seawater from entering landscapes, storm gates are a relatively new idea. “The point of storm gates is they can block storms, so you can protect yourselves from flooding, but normally they are open so the salt water gets to flow in and out with the tides. It means that the tidal wetlands can remain intact,” Mendelsohn says.
Also, because storm gates can be partially closed, Mendelsohn says they offer some other benefits over existing approaches, like giving managers the ability to control the flow of salt water into wetlands. This means, according to Mendelsohn, that they could be used to manage the ecosystem by controlling the salt balance.
Leaving wetlands alone is often considered the best way to manage them, but Mendelsohn thinks this approach fails to recognize the many forces of change — such as warming and sea-level rise — that are at play in the world. Instead, he says more active management might be a better way to preserve wetlands.