Salt Marsh Decline Due to Nutrient Levels

According to a new report from the Marine Biological Laboratory at Woods Hole, MA shows that the cause of the decline of salt marshes is caused from excess nutrients soaking into the marshes. Nutrients such as nitrogen and phosphorus from sewer systems and lawn fertilizers have been linked to salt marsh loss.

Salt Marshes are sodden areas found near estuaries and low-energy coastlines. The water can vary from completely fresh to completely salt water, and is greatly affected by the tides. Salt marshes support diverse wildlife up and down the east coast of the United States. Marshes also serve an important function in stabilizing coastlines because the plant roots anchor the otherwise highly erodible soil. Without a full understanding of the causes and reasons, salt marshes have been dying for the past twenty years.

Scientists found the answers to the decline of salt marshes after a large-scale study had been conducted at Plum Island Estuary at the mouth of Merrimack River in Massachusetts. In this study, scientists added nitrogen and phosphorus to tidal water to simulate the typical areas of nutrient enrichment in more densely populated areas.

After just a few years, wide cracks formed in the grassy banks of the tidal creeks. The banks eventually collapsed into the creek, resulting in a downgrading of the ecosystems. The long-term effect was a conversion of vegetated marsh into a mudflat, a much less productive ecosystem with fewer benefits to humans and wildlife. The study reveals that there is a limit to the amount of nutrients that a salt marsh can absorb and remove without experiencing any harmful effects.

"We honestly did not anticipate the changes we measured," says Linda Deegan, senior scientist at Woods Hole. "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."