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Why Are Our Salt Marshes Falling Apart?



WOODS HOLE, Mass.—Salt marshes have been disintegrating and dying over the past two decades along the U.S. Eastern seaboard and other highly developed coastlines, without anyone fully understanding why. This week in the journal Nature, MBL Ecosystems Center scientist Linda Deegan and colleagues report that nutrients—such as nitrogen and phosphorus from septic and sewer systems and lawn fertilizers—can cause salt-marsh loss.

“Salt marshes are a critical interface between the land and sea,” Deegan says. “They provide habitat for fish, birds, and shellfish; protect coastal cities from storms; and they take nutrients out of the water coming from upland areas, which protects coastal bays from over-pollution.” Losses of healthy salt marsh have accelerated in recent decades, with some losses caused by sea-level rise and development.

“This is the first study to show that nutrient enrichment can be a driver of salt-marsh loss, as well,” says David S. Johnson of the MBL, a member of the team since the project began in 2003.

This conclusion, which surprised the scientists, emerged from a long-term, large-scale study of salt marsh landscapes in an undeveloped coastline section of the Plum Island Estuary in Massachusetts. Over nine years, the scientists added nitrogen and phosphorus to the tidal water flushing through the marsh’s 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.)

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