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.)
A few years after the experiment began, 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," Deegan says.
Until this study, it seemed that salt marshes had 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," Deegan says. "And in many places along the Eastern seaboard -- such as Jamaica Bay in New York, where marshes have been falling apart for years -- we have exceeded those limits."
The disintegration of the nutrient-enriched marsh in this study happened in several stages, the scientists report. In the first few years, the nutrients caused the marsh grass (primarily cordgrass Spartina spp.) along the creek edges to get greener and grow taller, "just like when you add fertilizer to your garden," Deegan says. This taller grass also, however, produced fewer roots and rhizomes, which normally help stabilize the edge of the marsh creek. The added nutrients also boosted microbial decomposition of leaves, stems, and other biomass in the marsh peat, which further destabilized the creek banks. Eventually, the poorly rooted grass grew too tall and fell over, where the twice-daily tides tugged and pulled it. The weakened creek bank then cracked and fell into the creek.
By year six of the experiment, the scientists started seeing impacts at higher marsh elevations, above the lower creek banks. Three times more cracks, and bigger cracks, emerged at the top of the banks parallel to the creeks, than in a control marsh where no nutrients were added. Eventually, parts of the higher marsh also broke off and slid down toward the creek (which the scientists call the 'toupee effect,' because it leaves behind patches of bare, unvegetated mud). All told, at least 2.5 times more chunks of marsh fell into the creeks in the nutrient-enriched marsh than in the control system.
"We honestly did not anticipate the changes we measured," says Deegan. "Based on prior small-scale experiments, we predicted nutrient enrichment would cause the marsh grass to grow better and remain stable. But when we allowed different parts of the ecosystem to interact with the nitrogen enrichment over time, the small process changes we saw in the first few years resulted in the creek banks later falling apart. This could not have been extrapolated from the smaller-scale, shorter term studies."
Nutrient enrichment of coastal areas is known to cause harmful algae blooms, which create low-oxygen conditions that kill off marine life. "Now we understand that nutrient enrichment also causes a very important loss of salt marsh habitat for fish and shellfish," Deegan says. "This is one more reason why we need better treatment of household waste in our towns and cities." Individuals can help by not using fertilizers on their lawns and gardens. "If you have a green lawn because you are fertilizing it, you are contributing to loss of salt marshes and ultimately of fish," Deegan says.
This study could not have been accomplished without the cooperation and fore-sightedness of officials from the towns of Ipswich, Mass., and Rowley, Mass., and the Essex County Greenbelt Association, the scientists say.
"They recognized the importance of the work," Johnson says. "They understood that our work would not affect the much larger Plum Island Estuary, since the area manipulated was small relative to the large area of the sound and the marsh is able to process a lot of the nutrients before they get anywhere near the sound. They realized that whatever we discovered would help their towns, and society in general, make better decisions about treating the excessive nutrient enrichment of our coast."
This 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.
In the next phase of research, the scientists will study the recovery of the nutrient-enriched marsh. "After we stop adding the nitrogen, how long does it take the system to rebound to its natural state?" Deegan asks. This information will be important in reclaiming the health of salt marshes that are currently suffering from nutrient enrichment.
In addition to Deegan, Johnson, and Bruce J. Peterson of the MBL, co-authors of this study in Nature include: R. Scott Warren of Connecticut College; John W. Fleeger of Louisiana State University; Sergio Fagherazzi of Boston University; and Wilfred M. Wollheim of The University of New Hampshire.
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