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In 1908, the people of Cape Cod, Massachusetts built an earthen levee on the Herring River to reduce its flow into surrounding wetlands. Their goal was to control the number of mosquitoes. The levee destroyed the original salt marsh, replacing it with retained woods, shrubs and wetlands. But community efforts more than a century ago did more than parch the landscape. According to a new studthere, selected wetlands may undergo a significant shift from carbon sinks to methane sources. Transformation transforms these landscapes from those that help mitigate climate change to those that exacerbate it.
A confined wetland, says Rebecca Sanders-DeMott, a marine ecologist who led the study while working at the Woods Hole Coastal and Marine Science Center in Massachusetts, is a coastal wetland — once with water from sea flowing in and out naturally with the tide – in which the tidal exchange has been cut off, often by building roads or constructing levees, dams and other flood and water management infrastructure.
Although it can be difficult to identify retained wetlands, as it requires knowing what the landscape looked like before the infrastructure was built, scientists know that huge areas of tidal wetlands are in flux. Recent satellite mapping shows that globally, nearly 14,000 square kilometers of tidal wetlands disappeared between 1999 and 2009. These losses were, however, compensated by the development of nearly 10,000 square kilometers of tidal wetlands during of the same period through natural processes and restoration efforts. Estimates suggest that in the United States as much as a quarter of the coastline is in a hold-up state, although not all of this area was former wetland.
The work of Sanders-DeMott and his colleagues shows that this widespread land transformation is bad news for climate change. By comparing two sites around the Herring River with two in a nearby natural salt marsh with unrestricted tidal exchange, the researchers measured the gas emissions from both types of wetlands. They also used historical sediment core data to analyze the amount of carbon stored in the soil and measured other environmental factors, such as water salinity and vegetation type and density.
“These retained wetlands that have been refreshed and, in our case, overgrown with overgrown vegetation — which is very common in retained wetlands — produce significantly more methane than their natural counterparts,” says Sanders-DeMott.
Scientists have found that methane emissions scale exponentially with water salinity. The retained wetlands were cooler than the undisturbed tidal wetlands, with the coolest retained site they studied producing nearly 50 times more methane than the saltiest unrestricted wetland.
This increase in methane is likely caused by changes in wetland plant and microbial communities. Microbes that break down plant matter in saltier environments produce less methane than those in cooler ecosystems.
While all wetlands store an impressive amount of carbon, methane emissions from selected study sites are high enough that wetlands have a net warming effect on the atmosphere, Sanders-DeMott says.
As people increasingly consider restoring wetlands as a way to rebuild habitat and protect coastlines from rising sea levels and storms, Sanders-DeMott says the discovery shows the consequences on climate change of different choices. “If we restore the tidal exchange in these retained wetlands, in addition to all the other reasons that could justify that, we will also probably see a reduction in methane emissions,” she says.
One solution to preserve the flood protection powers of levees and dams that led people to retain wetlands in the first place, while avoiding the consequences of climate change, could be to install storm gates, says Robert Mendelsohn, environmental economist at Yale University. in Connecticut.
While tidal gates have been used for centuries to prevent seawater from entering landscapes, storm gates are a relatively new idea. “The beauty of storm gates is that they can block out storms, so you can protect yourself from flooding, but normally they are open so that salt water can come in and out with the tides. This means that areas tidal wetlands can remain intact,” says Mendelsohn.
Additionally, because storm gates can be partially closed, Mendelsohn says they offer other advantages over existing approaches, such as 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 seen as 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.