An aerial image with false colors shows marsh elevations in the Venice Lagoon. Credit: Marco Marani, Duke University
Posted: 18 Dec 2015 05:43 AM PST
Rising seas threaten coastal marshes worldwide. But a new study finds marshes are more resilient than previously believed. Elevated levels of atmospheric CO2 boost plant biomass production, allowing marshes to trap more sediment and generate more organic soil. This may elevate the threshold rate of relative sea-level rise at which marsh drowning is initiated by up to 60 percent and partially offset the effects of reduced sediment delivery and accelerating sea-level rise. The research, published this month in the Proceedings of the National Academy of Sciences, shows that the significant boost in marsh plant productivity associated with elevated levels of atmospheric carbon dioxide will allow marshes to trap more sediment and create more organic soil. This, in turn, will result in increased rates of accretion that will allow marshes to keep up with rising sea levels and may increase the thresholds for marsh drowning by up to 60 percent.
Coastal marshes absorb and store large amounts of carbon dioxide from Earth’s atmosphere; they help filter out pollution in coastal waters; provide habitat for wildlife; help protect coastlines from erosion and storm surge; and can store huge amounts of floodwater, reducing the threat of flooding in low-lying coastal areas. “Essentially, we found it’s a self-rising mechanism marshes use to build themselves up,” said Marco Marani, professor of ecohydrology at Duke’s Nicholas School of the Environment and Pratt School of Engineering. “As levels of atmospheric carbon dioxide increase, more CO2 gets taken in by marsh plants. This spurs higher rates of photosynthesis and biomass production, so the plants produce more sediment-trapping growth above ground and generate more organic soil below ground.”
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 sequester 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,” said Katherine M. Ratliff, a PhD student at Duke’s Nicholas School, who was lead author of the study. “Suspended sediments play a fundamental role in marsh survival,” she said. “As more dams are built and as land use and agricultural practices in coastal regions continue to rapidly change, we’re seeing 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.”…
…The new study 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 their study, the researchers 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. “While the effect of direct carbon dioxide fertilization has so far been neglected in marsh modeling, our research shows it is central in determining possible marsh survival under the foreseeable range of climatic changes,” Marani said….
Katherine M. Ratliff, Anna E. Braswell, Marco Marani. Spatial response of coastal marshes to increased atmospheric CO2. Proceedings of the National Academy of Sciences, 2015; 201516286 DOI: 10.1073/pnas.1516286112
Thorne, KM, BD Dugger, KJ Buffington, CM Freeman, CN Janousek, KW Powelson, GR Gutenspergen, JY Takekawa. 2015. U.S. Geological Survey Open-File Report 2015-1204, 54 p. plus appendixes, http://dx.doi.org/10.3133/ofr20151204.
In the Pacific Northwest, coastal wetlands support a wealth of ecosystem services including habitat provision for wildlife and fisheries and flood protection. The tidal marshes, mudflats, and shallow bays of coastal estuaries link marine, freshwater, and terrestrial habitats, and provide economic and recreational benefits to local communities. Climate change effects such as sea-level rise are altering these habitats, but we know little about how these areas will change over the next 50–100 years. Our study examined the effects of sea-level rise on nine tidal marshes in Washington and Oregon between 2012 and 2015, with the goal of providing scientific data to support future coastal planning and conservation. We compiled physical and biological data, including coastal topography, tidal inundation, vegetation structure, as well as recent and historical sediment accretion rates, to assess and model how sea-level rise may alter these ecosystems in the future. Multiple factors, including initial elevation, marsh productivity, sediment availability, and rates of sea-level rise, affected marsh persistence. Under a low sea-level rise scenario, all marshes remained vegetated with little change in the present configuration of communities of marsh plants or gradually increased proportions of middle-, high-, or transition-elevation zones of marsh vegetation. However, at most sites, mid sea-level rise projections led to loss of habitat of middle and high marshes and a gain of low marshes. Under a high sea-level rise scenario, marshes at most sites eventually converted to intertidal mudflats. Two sites (Grays Harbor and Willapa) seemed to have the most resilience to a high rate of rise in sea-level, persisting as low marsh until at least 2110. Our main model finding is that most tidal marsh study sites are resilient to sea-level rise over the next 50–70 years, but that sea-level rise will eventually outpace marsh accretion and drown most habitats of high and middle marshes by 2110.
Posted: 21 Dec 2015 04:34 PM PST
Coastal wetlands are in retreat in many locations around the globe — raising deep concerns about damage to the wildlife that the marshes nourish and the loss of their ability to protect against violent storms. The biggest cause of their erosion is waves driven by moderate storms, not occasional major events such as Hurricane Sandy, researchers now have shown.