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Excess Nutrients Collapsing East Coast Salt Marshes, Scientists Show
By David Sims
EOS Science Writer

October 17, 2012

DURHAM, N.H. -- In a paper published in the current issue of the journal Nature, scientists explain why salt marshes have been disintegrating over the past two decades along the U.S. Eastern seaboard and other highly developed coastlines. Unexpectedly, they discovered that nutrients such as nitrogen and phosphorus from septic and sewer systems and lawn fertilizers can cause salt marsh loss.

The researchers, including aquatic ecosystem ecologist Wilfred Wollheim of the University of New Hampshire, based their findings on a long-term, large-scale study of salt marsh landscapes in an undeveloped coastline section of the Plum Island Estuary in Massachusetts. A nitrogen flux model Wollheim developed was used to demonstrate potential areas of global vulnerability.

"With nutrient enrichment increasing globally due to human activities, these results suggest salt marsh vulnerability to nitrogen pollution could be a widespread concern," says Wollheim, an assistant professor in the UNH department of natural resources and the environment and the Institute for the Study of Earth, Oceans, and Space (EOS).

"Salt marshes are a critical interface between the land and sea and provide habitat for fish, birds, and shellfish, protect coastal cities from storms, and take nutrients out of the water coming from upland areas, which protects coastal bays from over-pollution," says lead author Linda Deegan of the Marine Biological Laboratory in Woods Hole, Mass. "Losses of healthy salt marsh have accelerated in recent decades, with some losses caused by sea-level rise and development. The Plum Island experiment is the first to show that nutrient enrichment can also be a driver of salt-marsh loss."

Deegan notes that until this study it appeared salt marshes had an unlimited capacity for nutrient removal, with no harmful effects on the marshes themselves. "Now we really understand that there are limits to what salt marshes can do," she says.

Moreover, in many places along the Eastern seaboard - such as Jamaica Bay in New York, where marshes have been falling apart for years - those limits have been exceeded.

For their nine-year experiment, the scientists added nitrogen and phosphorus to the tidal water flushing through the study area's marsh creeks at levels typical of nutrient enrichment in densely developed areas, such as Cape Cod, Mass., and Long Island, N.Y. Usually, nutrients originating from septic systems, sewerage, and soil fertilizers on land flow with rainwater down to the coastal ocean.

In the first few years, the nutrients caused the marsh grass along the creek edges to get greener and grow taller, but the taller grass also produced fewer roots and rhizomes, which normally help stabilize the edge of the marsh creek. Over time, wide cracks began forming in the grassy banks of the tidal creeks, which eventually slumped down and collapsed into the muddy creek.

The long-term effect is conversion of a vegetated marsh into a mudflat, which is a much less productive ecosystem and does not provide the same benefits to humans or habitat for fish and wildlife.

Wollheim broadened the results of the study by applying a global nitrogen flux model. The model quantifies changes in nitrogen inputs to the coastal zone due to human activity in different parts of the world, and accounts for the role of freshwater ecosystems to help lessen the impacts between pollution sources and the coastal zone.

The model suggests that while aquatic ecosystems (streams, rivers, and lakes), can reduce fluxes to some degree, they are insufficient to prevent increased inputs to coastal zones.

Maps of changes in nitrogen fluxes were overlaid with the distribution of salt marshes globally by Wollheim's colleagues Stanley Glidden and Rob Stewart of the EOS Water Systems Analysis Group. The resulting composite map shows global regions where salt marshes may be at risk due to increasing nitrogen loading.

"This analysis makes clear that the implications of the field findings extend well beyond the local study area, and suggest that additional study of other global regions is needed," Wollheim says.

The study is part of the Plum Island Ecosystem Long-Term Ecological Research (PIE-LTER) program, supported by the National Science Foundation (NSF). The PIE-LTER conducts basic science and provides information to coastal managers to help them make more informed decisions.

"This is a landmark study addressing the drivers of change in productive salt marsh ecosystems, and a stellar example of the value of supporting LTER sites," says David Garrison, program director in NSF's Division of Ocean Sciences, which supports the LTER program along with NSF's Division of Environmental Biology.

The paper, "Coastal eutrophication as a driver of salt marsh loss," is available to download here:

Co-authors include David S. Johnson and Bruce J. Peterson of the MBL, R. Scott Warren of Connecticut College, John W. Fleeger of Louisiana State University, and Sergio Fagherazzi of Boston University. This work is supported by grants from NSF (DEB0816963, DEB0213767, OCE0923689, OCE 0423565, OCE0924287), NOAA and The Mellon Foundation.