Kitchen Waste Contributing to Loss of Salt Marshes

Salt marshes that are primarily made up of grass are very crucial for the coastal ecosystem as they help to protect shorelines from storms and providing habitat for a diverse range of wildlife. In addition to this they build up costal elevations by trapping sediment during floods, and produce new soil from roots and decaying organic matter.

And it appears that the losses of the salt marshes have accelerated in over the past two decades along the U.S Eastern seaboard and other highly developed coastlines. And till date no concrete reason there has been for such activity. It was believed that some losses were caused by sea-level rise and development.

But this week's journal Nature states that presence of nutrients such as nitrogen and phosphorus from septic and sewer systems and lawn fertilizers could be the prime reason for the loss of the salt marshes.

"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." 

"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.

It was after the scientists conducted a large scale study of salt marsh landscapes in an undeveloped coastline section of the Plum Island Estuary in Massachusetts they were able to trace the reason for the increasing loss of salt marshes.

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. 

A few years after initiating the experiment they noticed wide cracks in the grassy banks of tidal creeks which later dropped down and collapsed.

"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.

Prior to this the researchers were under the belief that salt marshes had a great capacity for nutrient removal and this quality did not damage 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."

According to the scientists the decay of the marshes occurs in several stages. In the first few years, the nutrients caused the marsh grass along the creek edges to get greener and grow taller. They noticed something peculiar about this grass, they noticed that the taller grass produced fewer roots and rhizomes which help to stabilize the edge of the marsh creek.

With the addition of nutrients the microbial decomposition of leaves, stems, and other biomass in the marsh peat which are responsible for destabilizing the edge of marsh creek eventually improved.

During the sixth year of the experiment the scientists started seeing impacts at higher marsh elevations, above the lower creek banks.  The number of cracks increased plus they were bigger cracks, appeared 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. They were startled on seeing 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."

"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 is part of the Plum Island Ecosystem Long-Term Ecological Research (PIE-LTER) program, supported by the National Science Foundation (NSF). 

"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.