Researchers have revealed the declining health of coastal marshes several years before visible signs of decline, providing an early warning and opportunity to protect an ecosystem that serves as the first line of defense against coastal flooding.
Scientists from Colorado State University, the University of Georgia and the University of Texas at Austin developed a model to detect early signs of marsh decline using satellite observations. The model identified vulnerable marshes along Georgia's coast by locating declining root production – a harbinger of marsh failure.
The research, published in June in Proceedings of the National Academy of Sciences , is the first to predict broad trends in marsh deterioration using remote sensing.
"Our findings show widespread belowground decline over the past decade and suggest this is an early warning sign of marsh deterioration and loss," said lead author Kyle Runion, a research scientist at the University of Georgia who was at UT Austin's Marine Science Institute during the study. "By pinpointing where belowground loss is happening, we can get a head start on conservation and restoration projects to more effectively prevent marsh loss."
Coastal marshes filter water, store carbon, act as habitat for wildlife and provide food and fishing livelihoods for people. Marshes also absorb storm surges and sea-level rise, preventing worse flooding inland.
"These marsh areas might be in someone's backyard," said senior author Jessica O'Connell, a CSU professor and Runion's adviser. "It's important to have this early warning and a chance to do something before you lose these special landscapes that people have an economic or emotional connection to."
O'Connell added, "Conserving the natural landscape is a lot cheaper than losing it and then trying to come up with an environmentally engineered way to do the things that the land used to do."
The study focused on the marsh grass Spartina alterniflora, which is found along much of the U.S. coast. The researchers examined the entire Georgia coast – an area for which they had extensive field data – and found belowground biomass has declined across 72% of Georgia's coastal marsh since 2014, with nearly 30% suffering substantial loss.
They are now working to make the model universally applicable to all marsh plants and coastlines.
Declines in belowground biomass indicate marsh vulnerability to loss
The research team developed the Belowground Ecosystem Resiliency Model (BERM) using more than a decade of field data gathered through the Georgia Coastal Ecosystems Long Term Ecological Research Program at the University of Georgia. The program, led by co-author Merryl Alber and funded by the National Science Foundation, collected data on marsh grass leaves and stems aboveground, along with roots and rhizomes belowground.
This rigorous fieldwork detailed the complicated – and sometimes contradictory – relationship between above- and belowground marsh grass growth. The study found that marsh grass might appear to be healthy or even thriving aboveground, while the roots could be receding.
Coastal marshes are adapted to salt water and fluctuating water levels. Periodic flooding or slow sea-level rise can boost plant growth by flushing out excess salt buildup and bathing plants in nutrient-rich sediment, which contributes to the soil in which they grow.
"Coastal wetlands such as salt marshes historically have responded dynamically to keep pace with sea-level rise in part by developing extensive root networks, which expand the soils with organic-rich material," Runion said. "This accumulation of 'blue carbon' drives elevation gain and enables the marsh surface to maintain elevation relative to rising sea levels."
However, if the water stays too high, marsh vegetation can drown as roots are deprived of oxygen. Root system decline is the first sign that a marsh is failing.
"The declines in vegetation were closely related to flooding pressure, which will only worsen with the rapid acceleration of sea-level rise that the southeast U.S. is experiencing," said Alber, a UGA professor and director of UGA's Marine Institute. "That is why this work is so important."
BERM predicts both above- and belowground biomass based on plant characteristics that are observable from space and environmental factors, such as elevation, sea level and climate. The researchers applied it to all Georgia locations of Spartina alterniflora, mapped by co-author Christine Hladik, a geography professor at Georgia Southern University.
The study was also co-authored by Deepak Mishra at the University of Georgia and Mark Lever at the University of Texas at Austin Marine Science Institute. Research was funded by the National Aeronautics and Space Administration, the National Oceanic and Atmospheric Administration and the National Science Foundation.
