Pollution from agricultural production degrades coastal salt marshes more quickly than previously thought, according to a study published in the journal Nature. The decline of these ecosystems, the authors add, not only harms the plants and animals that inhabit them but undermines the storage capacity of one of the world's primary carbon sinks.
"Perhaps the most obvious [benefit that salt marshes provide] is they're nursery grounds for many fish, shellfish and birds, especially migrating birds that use them as feeding stops," said John Fleeger, an emeritus professor of biology at Louisiana State University and one of the researchers involved in the study.
Coastal marshlands, he added, also provide protection for cities. As storms pass over marshes, they lose energy; the wetlands absorb storm surges that can batter coastal settlements.
"That's a very important function, especially if you think of New Orleans and Katrina," Fleeger said. "Many people feel the strength of Katrina was heightened by the loss of salt marshes in the last 50 years or so."
Salt marshes also play a significant role, he said, in mitigating the onset of climate change and helping reduce the vulnerability of coastal cities to the impacts of rising sea levels.
Just as these ecosystems buffer storm surges, Fleeger said, they also absorb the imperceptible rise in sea levels brought about by a warmer atmosphere.
Marshes also sequester carbon. Degrading marshes means less carbon is being pulled from the atmosphere but also that the carbon stored in those soils is emitted back into the atmosphere.
A study published last year in the journal Frontiers in Ecology and the Environment estimated the carbon storage capacity of mangroves, salt marshes and sea grass beds -- known collectively as "blue carbon" -- to be "comparable to those of terrestrial forest types."
Lastly, Fleeger said, marshes filter nitrogen from agricultural runoff before it reaches the oceans and seas.
Destroying the coast's defenses against storms
More nitrogen-enriched fertilizer is being used in agricultural production around the world in order to meet growing demand for food. But some of that fertilizer ends up in rivers and is carried to open oceans and seas.
Too much nitrogen in the water column can trigger algal blooms that deplete oxygen concentrations, creating "dead zones" where marine life cannot survive.
Each year, for example, thousands of square miles of the Mississippi Delta become a dead zone due to the soil runoff from the nation's agricultural belt up and down the Midwest.
The microbes that are abundant in the soil of salt marshes as well as the grasses that inhabit them, Fleeger said, take nitrogen from the water and convert it to nitrate gas.
In other words, they remove nitrogen from the organic cycle and pump it into the atmosphere, where it doesn't lead to dead zones.
Too much nitrogen, though, and salt marshes collapse and turn to mudflats that have far fewer ecosystem services, particularly in the fight against climate change.
"If you think of any farm or field or garden, you add nitrogen and the plants tend to grow better. So when you add nitrate to the water, the plants take it up through their leaves," said Fleeger. "So plants grow taller. Normally, we want that to happen. In a cornfield, we want that to happen."
Salt marsh grasses, though, tend to have complex root systems rather than abundant leaf structures. So nitrogen-enriched fertilizer diminishes nitrogen uptake in the roots and leads to a loss of soil integrity that reaches a point where the marshland simply collapses.
'Hope of recovery'
"This is a soil that is just chock-full of root matter. If you were to take a plug, a little core, of salt marsh sediment, it's literally full of these roots," Fleeger said.
Previous studies of the impacts of nutrient-rich runoff on salt marshes were conducted on much smaller scales than what Fleeger and his colleagues undertook.
Researchers previously would sprinkle powdered fertilizer into small areas of marshland. "But the problem with that," Fleeger said, "is it's a small area and it's not a good mimic of nature."
Instead Fleeger's team pumped nutrient-enriched water stored in a large tank into a northeastern Massachusetts creek as the tide was rising. They were able to see how dissolved nitrogen affected the ecosystem as the water washed over a much larger area.
"Instead of studying something that is a meter square," he said, "we studied something that is 30,000 meters square." What the researchers found was that nutrient enrichment was affecting areas of the marsh that prior studies had failed to identify, primarily the intertidal zone on the edge.
The team conducted its experiments over nine years and found that the degradation of marsh soils occurred not only in areas previously unidentified but on a much quicker time scale. "The fact that some of these impacts were showing up in five years was not something we expected," Fleeger said.
Future study needs to be done, he added, on how salt marshes might rebound after depletion and how marshes differ from one place to another. However, he said, degradation in Long Island Sound and Jamaica Bay in New York mimicked his team's findings in Massachusetts.
Anywhere from a third to half of the world's salt marshes have been lost. But, Fleeger said, "there is hope of recovery."