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A salt marsh, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open salt water or brackish water that is regularly flooded by the tides. It is dominated by dense stands of salt-tolerant plants such as herbs, grasses, or low shrubs. 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 emerged from a long-term 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.
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.
In the first few years, the nutrients caused the marsh grass along the creek edges to get greener and grow taller. This taller grass, 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 down.
The coast is a highly attractive natural feature to humans through its beauty, resources, and accessibility. As of 2002, over half of the world’s population was estimated to being living within 60 km of the coastal shoreline, making our coastlines highly vulnerable to human impacts from daily activities that put pressure on these surrounding natural environments.
“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.”
For further information see Salt Marsh Collapse.
Marsh image via Wikipedia.
