From: David A
Gabel, ENN.com
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. However, 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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