Infrastructure protectionHumans threaten wetlands' ability to overcome sea level rise

Left to themselves, coastal wetlands can resist rapid sea level rise. Humans, however, could be sabotaging some of wetlands’ best defenses, according to results published in this week’s issue of the journal Nature.

Thanks to an intricate system of feedbacks, wetlands are remarkably good at building up soils to outpace sea level rise. The questions are: When do they reach their limit, and how have we lowered that point?

Without human-caused climate change, “we wouldn’t be worried about wetlands surviving the rates of sea level rise we’re seeing today,” says lead paper author Matthew Kirwan of the Virginia Institute of Marine Science and the National Science Foundation (NSF) Virginia Coast Reserve Long-Term Ecological Research (LTER) site.

An NSF release reports that Virginia Coast Reserve is one of twenty-six such NSF LTER sites around the globe in ecosystems from deserts to mountains and marshes to grasslands.

In an unchanged world, “wetlands would build vertically at faster rates,” says Kirwan, “or move inland to higher elevations.”

The paper’s co-author is Patrick Megonigal of the Smithsonian Environmental Research Center.

A wetland is land that is saturated with water, whether permanently or seasonally. The water found in wetlands can be saltwater, freshwater or brackish water. Main wetland types include swamps, marshes, bogs and fens.

Wetlands have developed several ways to build elevation to keep from drowning. Aboveground, tidal flooding provides one of the biggest assists. When marshes flood during high tides, sediment settles out of the water, adding new soil. As sea level rises and flooding occurs more often, marshes react by building soil faster.

Belowground, the growth and decay of plant roots add organic matter.

Even erosion can work in wetlands’ favor, as sediment lost at one marsh may be deposited in another. While a particular wetland may lose ground, the total wetland area may remain unchanged.

If a wetland becomes so submerged that its vegetation dies off, however, these “positive feedback loops” are lost. Similarly, if sediment delivery to a wetland is cut off, that wetland can no longer build soil to outpace rising seas.

“This study reveals the complex, long-term interplay among processes that maintain coastal wetlands in the face of sea level rise,” says Saran Twombly, program director in NSF’s Division of Environmental Biology, which funds the NSF Virginia Coast Reserve LTER site.

“Humans are newcomers to this delicate balance. The future of a habitat so essential to human well-being depends on how severely we alter it.”

For example, groundwater withdrawal and artificial drainage can cause the land to sink, as is happening in Chesapeake Bay.

Because of this subsidence, eight of the world’s 20 largest coastal cities have relative sea level rise greater than climate change projections.

Dams and reservoirs also prevent 20 percent of the global sediment load from reaching the coast.

Marshes on the Yangtze River Delta, for example, have survived a relative sea level rise of more than 50 millimeters per year since the 7th century — until the recent building of more than 50,000 dams cut off the supply of sediment and accelerated erosion.

“Tidal marshes are amazing ecosystem engineers that can raise themselves upward if they remain healthy, especially if there is sediment in the water,” says Megonigal.

“We know there are limits, however. Those limits are changing as people alter the environment.”

— Read more in Matthew L. Kirwan and J. Patrick Megonigal, “Tidal wetland stability in the face of human impacts and sea-level rise,” Nature 504 (5 December 2013): 53–60 (doi:10.1038/nature12856)