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Cape Cod Hikes: The Great Marshes of Barnstable
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As the longshore current extended Sandy Neck to the east, tidal currents swept fine-grained sediments, silts and clays, into the quiet waters behind the barrier spit. (See my article on Sandy Neck). Eventually shallow mudflats developed on which a hardy salt-tolerant plant called Spartina alterniflora grew. Commonly called Cordgrass, this broad-leaved grass trapped more sediment and built the mudflats higher and wider. Cordgrass, a perennial, dies to the roots in the fall and grows back again in the spring, so over the years the cordgrass built a layer of marsh peat several feet thick. When the cordgrass had built the marsh near high tide level, it was replaced by another grass, Spartina patens, or salt meadow grass. With the arrival of the patens the marsh became a permanent fixture, and other plants and animals began to move in. The Great Marshes of Barnstable grew in this way, sharing the 3000 year history of Sandy Neck Beach. As the barrier grew east so did the marshes, until now more than 4000 acres of marsh fill Barnstable harbor.
Salt marshes are the most productive of earth's ecosystems. Biotic communities such as marshes, tropical rain forests, woodlands and savannahs, are compared by the efficiency with which energy from the sun is passed through the food chain from plant to animal. The efficiency of the salt marsh is unsurpassed. On one acre of salt marsh the grasses produce up to nine tons of organic matter each year. Our own New England forests produce only five and a half tons, grasslands only two and a half tons, and a wheatfield only one and a half tons. The excess organic material that is not used as food or shelter by the inhabitants of the saltmarsh (they use only fifty-five percent) is of vital importance to the surrounding coastline. Two-thirds of the fish and shell-fish harvested along the Atlantic coast depend in some way on the salt marshes for survival. They need the marshes and their intricate creek systems as a source of food, protection, and as a place to spawn.
From the rear dunes on Sandy Neck you can look out over the entire expanse of the marshes. At first it seems to be nothing more than a field of grass, but within its borders are thousands of different varieties of plant, animal and insect life. The Spartinas share the marsh with spike grass, black grass, sea lavender and the Salicornias-- salt-wort, glasswort and chickentoe. Grasshoppers, crickets, stickbugs, and caterpillars feed on the grasses. They in turn are eaten by beetles, flies, dragon flies, wasps and spiders. Sharptailed sparrows and seaside ducks, black ducks and clapper rails swoop down on more than 400 different species of insects. Crabs feed on the low-tide mud. Fiddler crabs-- Uca pugilator, Uca minax and Uca pugnax-- stuff chunks of algae and decomposing grasses down their digestive tracts. Their foraging helps aerate the soil and recycle dead matter in the marsh.
Ribbed mussels also help the marsh recycle waste. They filter one gallon of seawater through their gills every hour, extracting small food particles and returning larger particles to the marsh surface. These "pseudofeces" are so rich in phosphates that they renew the total phosporous in the marsh waters every fourteen days.
Salt marshes are delicate and complex ecosystems teeming with life, and yet there are those who see them as only prime development land. The entire Back Bay of Boston, Fenway Park, and the Public Gardens were all once marsh land. Barnstable county alone lost 3000 acres of marsh in one twenty year period. When marshes are filled in the effect on coastal ecosystems is subtle but wide spread. One has only to look at the declining yields of clams and mussels over the years to see just one area of harm.
Directions: Best seen from the rear dunes on Sandy Neck.
For more information:
Dave McShaffrey Summary of How Energy Moves Through Ecosystems
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