October 18, 2012
Reason U.S. Salt Marshes Are Disappearing Found
Brett Smith for redOrbit.com — Your Universe OnlineSalt marshes along the eastern coast of the United States have been disappearing over the past two decades and a group of American researchers led by Linda Deegan of the Marine Biological Laboratory (MBL) in Woods Hole, Mass. has shown that an influx of phosphorus- and nitrogen-based nutrients is partly to blame.
These nutrients come from untreated sewage systems and fertilizer runoff that leach into the marshy areas and disrupt the natural cycles that have kept these ecosystems stable for hundreds of years, according to the team´s report that was published recently in the journal Nature.
"Salt marshes are a critical interface between the land and sea," Deegan said. "They provide habitat for fish, birds and shellfish, protect coastal cities from storms and take nutrients out of the water from upland areas, which protect coastal bays from over-pollution."
"This is the first study to show that nutrient enrichment can be a driver of salt marsh loss, as well," added co-author David Johnson of MBL, a member of the team since the project began in 2003.
The scientists said finding the negative effects of these nutrients was quite unexpected and was the result of a long-term study around Plum Island estuary in Massachusetts. For over nine years, the scientists added nitrogen and phosphorus to the tidal marshes around the island that were consistent with what marshes around Cape Cod and Long Island might be experiencing.
After a few years, cracks in the land began to appear along the tidal creeks that feed into water into the marsh. Eventually, these cracks expanded, slumped, and collapsed into muddy creeks.
Researchers found that the nutrients caused the marshes to become muddier by supplementing the grass that lived there to grow bigger than the sizes that the soil and waterways could support. The bigger grasses had comparatively smaller roots since the amount of nutrients in the soil did not necessitate their expansion. The results were a less stable soil base and taller grass that got too big, fell over, and got pulled out by the tidal forces that swept through the marsh twice a day.
"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 to habitat for fish and wildlife," Deegan said.
In addition to causing the disappearance of tidal salt marshes, high amounts of nitrogen and phosphorus have been known to cause massive algal blooms that can choke the life out of tidal zones.
"Now we understand that nutrient enrichment also causes a very important loss of salt marsh habitat for fish and shellfish," Deegan warned. "This is one more reason we need better treatment of household waste in our towns and cities."
She added that individuals can help prevent the ill effects of phosphorus and nitrogen leaching into the soil by responsibly maintaining 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 said.
The researchers said their future studies will focus on how these nutrient-enriched marshes might recover.