Study on Marsh Resilience to Rising Seas Receives Dean’s Award for PhD Student Paper

May 16, 2016
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Contact: Tim Lucas, 919/613-8084, tdlucas@duke.edu

DURHAM, N.C. – A study showing that coastal marshes may be more resilient to sea-level rise than previously believed has received the 2016 Dean’s Award for Outstanding PhD Student Paper.

Dean Alan Townsend presented the award to doctoral students Katherine M. Ratliff and Anna E. Braswell at the Nicholas School’s Recognition Ceremony for graduates and their families on May 14.

Ratliff and Braswell published their study, “Spatial Response of Coastal Marshes to Increased Atmospheric CO2,” on Dec. 7, 2015, in the Proceedings of the National Academy of Sciences. Marco Marani, professor of ecohydrology, co-authored it.

Coastal marshes absorb and store large amounts of carbon dioxide from Earth’s atmosphere. They also filter out pollution in coastal waters; provide habitat for wildlife; protect coastlines from erosion and storm surge; and store floodwater to help reduce the threat of inundation in low-lying coastal areas.

Scientists have long worried that accelerating rates of sea-level rise linked to climate change pose a major threat to these marshes and the vital ecosystem services they provide.

Ratliff and Braswell’s research, however, finds that as levels of atmospheric carbon dioxide increase, more CO2 gets taken in by marsh plants. This spurs higher rates of photosynthesis and biomass production, allowing the plants to produce more sediment-trapping growth above ground and generate more organic soil below ground. This, in turn, increases the rates of accretion that allows marshes to keep up with rising seas and may increase the threshold for marsh drowning by up to 60 percent.

The result is that the extent of marsh loss is significantly reduced, even under high rates of sea-level rise.

The study suggests this so-called “CO2 fertilization effect” may also contribute to a stabilizing feedback in the climate system as increased biomass production and organic deposition in marshes allows them to sequester even larger amounts of carbon dioxide.

But there’s an important caveat.

While elevated atmospheric CO2 levels may offset some of the threats facing marshes from sea-level rise, another equally serious threat to marsh survival – sediment starvation – will remain.
Suspended sediments play a fundamental role in marsh survival. As more dams are built and as land use and agricultural practices in coastal regions continue to rapidly change, there has been a sharp drop in inorganic sediment delivery to many coastal marshes worldwide. This decrease significantly undercuts the marshes’ ability to build themselves up and keep pace with rising seas.

Ratliff and Braswell’s analysis finds that in sediment-poor marshes, the loss of area might range between 39 percent and 61 percent, even when the offsetting CO2 fertilization effect is accounted for, as the rate of relative sea-level rise increases beyond the initial threshold for marsh drowning.

To conduct the study, the students and Marani used a spatial model of marsh morphodynamics into which they incorporated recently published observations from field experiments on marsh vegetation response to varying levels of atmospheric carbon dioxide.

Funding came from the National Science Foundation’s Graduate Research Fellowship Program (#DGF1 1106401), and from the Nicholas School and Pratt School of Engineering.

The Nicholas School has presented the Dean’s Award for Outstanding PhD Student Paper annually since 2008 to recognize excellence in graduate student research. The award carries a $3,000 cash prize.

Brad Murray, professor of earth and ocean sciences, is Ratliff’s faculty advisor. James Heffernan, assistant professor of ecosystem ecology and ecohydrology, is Braswell’s faculty advisor.

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CITATION: “Spatial Response of Coastal Marshes to Increased Atmospheric CO2,” Katherine M. Ratliff, Anna E. Braswell, Marco Marani; Dec. 7, 2015; Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.1516286112

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