A study led by Columbia researchers published in the March issue of Wetlands, the journal of the Society of Wetland Scientists, supports observations that the salt marshes of New York City's Jamaica Bay near John F. Kennedy International Airport are eroding rapidly, and in the coming decades may disappear altogether with rising sea levels as a result of global warming.
In an analysis based on historical aerial photographs of several of the larger marsh islands, the scientists determined that 12 percent of the area's marshlands had been lost since 1959. A more comprehensive examination showed a 51 percent reduction of island marshes between 1924 and 1999, with 38 percent loss of marsh vegetation occurring since 1974. Some of the smaller islands have lost up to 78 percent of their vegetation cover since 1974.
The researchers found evidence that the rate of loss was accelerating and probably could not survive the oncoming rise of the ocean. However, they held out hope that trial restoration efforts, if begun soon, could stabilize the marshes. Because they are literally "drowning in place" from sediment starvation, remediation efforts would likely include the placement of thin layers of sediment on the marsh surface, according to Ellen Hartig, the lead author and an ecologist, formerly at Columbia, now with the Federal Emergency Management Agency.
"These efforts have had success in other wetlands," she said. One major concern of the scientists is that the loss of the wetlands in Jamaica Bay could pose a danger to surrounding residential developments, which would be further exposed to flooding from storm surges.
Hartig said while the National Park Service has been excellent in protecting the marshlands for the last 30 years, in the future the agency should take a stronger approach and adopt a hands-on role in managing the wetlands. In the past, she said, the thinking was that if the wetlands were left alone they would naturally thrive.
While measured rates of marsh grass growth appear typical of other regional wetlands, the distribution of marsh grass is rather patchy, with many bare spots and water-filled pools where normally vegetation would be found.
"From the air, many of the islands are so riddled with these pools that they resemble Swiss cheese," said Vivien Gornitz, a geologist and special research scientist at the Goddard Institute for Space Studies at Columbia.
These features indicate that the marshes are submerging, leading to plant die-off from within. Field work and mapping further substantiate that submergence is more significant than erosion of the island edges.
As part a regional report prepared for the U.S. National Assessment on Climate Variability and Change, a suite of sea-level rise projections based on historical data and climate models was compared with plausible rates of marsh growth. This analysis suggests that if enough sediment could accumulate on the marshes, the marshes would survive all but the most extreme cases of future sea level rise. The fact that the marshes are shrinking even at current rates of sea level rise (around 3 millimeters per year in New York City) suggests that lack of sediments is an important cause of the wetlands losses, said Gornitz.
In addition to Hartig and Gornitz, the study involved other researchers at the State University of New York at Stony Brook and the New York State Department of Environmental Conservation. Funding was provided by the National Science Foundation, the National Park Service, and the National Aeronautics and Space Administration.
A major cause of the vanishing wetlands in the Queens borough is the disruption of the flow of water and the spread of sediment upon which marsh grasses depend. Repeated dredging of the bay for shipping channels and to provide landfill for the expansion of Kennedy Airport (built on wetlands beginning in 1942 before laws to protect them) over time has depleted the process of sediment deposition that nourish the wetlands. The dredged shipping channels act as a kind of trap for sediment, preventing this nourishing material from washing over the marshes and maintaining their growth.
Urban development that closed off stream channels around the bay, waves from recreational boat traffic and heavy grazing by waterfowl contribute to the erosion of the marshes. In addition, development in Queens and adjacent Brooklyn and Nassau County may have diminished sediment sources upland and blocked sediments deposits from washing into the bay after storms. "Further research is urgently needed to establish the causes of marsh loss," said Gornitz.
The researchers warned that at current rates of attrition, Spartina alterniflora, the dominant marsh grass, could disappear altogether within the next few decades -- years before the full impact is felt from a projected one meter rise in the metropolitan area sea level due to global climate warming. The researchers suspect that the die-off of this marsh grass plays an important role in the marsh deterioration. An abnormally high population of mussels and apparent encroachment of sea lettuce may be symptomatic of the transformation to more marine conditions as the marshes drown, or they could be actively contributing to the grass die-off.
A rise in global temperatures due to increases in greenhouse gases would cause thermal expansion of ocean waters and melting of mountain and high-latitude glaciers. Projections call for a sea-level rise of one meter by the end of the century in metropolitan New York. "The fate of the island marshes of Jamaica Bay serves as a wake-up call regarding the hazards facing other coastal wetlands due to the intertwined impacts of human-induced stresses and sea-level rise," the report stated.
While heavy residential and industrial development and the construction of the airport has dramatically shrunk the size of the Jamaica Bay wetlands since the early decades of the 20th century it remains one of the largest coastal ecosystems in New York State. In 1972, the remaining marsh islands and Jamaica Bay Wildlife Refuge came under federal protection as part of the Gateway National Recreation Area, administered by the National Park Service. Along the Atlantic flyway for migratory birds, the salt marshes are prime nesting and feeding sites for many shorebirds, including geese and ducks.