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As we were talking about salt marshes on a recent podcast, the following is information provided by the Massachusetts Bays Program:
The Essex Salt Marsh is part of the 17,000 acre Great Marsh that extends from Cape Ann into New Hampshire. Salt marshes are found in coastal areas. These unique ecosystems are formed within protective estuaries and support numerous plants and animals. Salt marshes are among the most productive lands on earth, outcompeting even the best-managed farms. Two-thirds of all marine fish and shellfish depend on salt marshes during some portion of their lives.
Salt marshes are divided into two general vegetation zones. The Low Marsh is flooded twice daily by the incoming tide and is dominated by Spartina alternifolia (low salt marsh grass). The High Marsh is flooded sporadically and is dominated by Spartina patens (high salt marsh grass). Salt marshes contain tidal creeks, pools, and islands of high ground, and serve as highly efficient pollution filters.
Nationwide, vast areas of salt marsh have been destroyed by filling, dredging, and developing upland areas. The Great Marsh has escaped much of this destruction, but it is impacted by pollution runoff and mosquito control ditches built in the 1930s, and by road and rail crossings, which restrict tidal flows to upstream marshes.