Decline in Salt Marshes in US Caused by Increased Nutrient Levels
Salt Marshes are marshy 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. They also serve an important function in stabilizing the coastlines because the plant roots anchor the otherwise highly erodible soil. Unfortunately, salt marshes have been dying away over the past 20 years without a full understanding of how and why. A new report from the Marine Biological Laboratory at Woods Hole, MA postulates that the cause of the decline is due to excess nutrients seeping into the marshes. These nutrients from sewer systems and lawn fertilizers, such as nitrogen and phosphorus, have been shown to cause salt marsh loss.
The conclusion came after a long-term, large-scale study of the salt marsh at Plum Island Estuary in northeastern Massachusetts, at the mouth of the Merrimack River. They focused on an undeveloped coastline section of the estuary.
For nine years, scientists added nitrogen and phosphorus to the tidal water to simulate typical levels of nutrient enrichment in more densely populated areas such as Cape Cod, MA or Long Island, NY.
After a few years, wide cracks began forming 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 any harmful effects to itself. In many places along densely developed areas, those limits have been exceeded.
Effects to the salt marsh from the excess nutrients occurred in different stages in different areas of the marsh. Over the first few years, the marsh grasses along the banks grew taller and greener, typical of adding fertilizer. However, the grasses produced fewer roots and rhizomes which are needed to anchor the creek bed. This caused the collapse of the banks into the creek itself.
By the sixth year of the experiment, impacts were seen at higher elevations in the marsh, away from the creek banks. More cracks began to appear, and parts of the higher marsh began sliding down into the creek. This left behind ugly patches of unvegetated mud.
"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."
The study has been published in the journal, Nature
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Salt Marsh image via Shutterstock