Geoscience Reference
In-Depth Information
ance with the diurnal tides common to the Atlantic coast, but at least once a month, when the highest tides, the
spring tides, occur around the new and full moons. The tides bring with them nutrients in the form of inorganic
soils scoured from the sea bottom.
The major sources of the salt marsh's great primary productivity are two salt-tolerant cordgrasses, Spartina
alterniflora and Spartina patens. The former grows on the low marsh, which is flooded daily, and the latter,
also known as salt marsh hay because it was once widely harvested for cattle fodder, grows on the high marsh,
which is touched by the tide less frequently. Much of this productivity is exported by the tide, or in winter by
the ice, as dead plant matter (or detritus) to the adjacent marine zone, where it fuels the offshore food web.
Salt marshes play another critical role in the ecology of the marine zone, acting as nurseries for fishes and
invertebrates, many of which, including herring, smelt, and flounder, are of commercial importance. Juvenile
fishes frequent marshes, in part because they rely on detritus or the microbes associated with decaying plant
matter as food and because of a relative absence of predators. Large salt marshes occur along the shores of
Chesapeake and Delaware bays, as well as in the lee of the barrier islands in the Mid-Atlantic region. Salt
marshes are also common in the southern Gulf of St. Lawrence, as part of the barrier island and lagoonal sys-
tem that has developed there, and in the inner Bay of Fundy, where the world-famous tides flood large areas.
The ebbing tides also expose vast tidal flats, or mudflats as they are more commonly called, which in Fundy
can be as wide as 5 kilometers (3 miles). Mudflats occur in any areas along the shoreline with sufficient sedi-
ment and tidal range and are therefore more common in the sediment-rich areas south of Cape Cod. Organisms
like clams and worms, which live buried in the sediment, as well as predators that feed upon them, such as
shorebirds at high tide and fishes at low tide, ultimately depend on the productivity of the adjacent salt marsh,
or in some cases sea grasses, such as eelgrass, that grow in the sediments themselves. A second and significant
source of primary productivity is the microscopic diatoms that coat the mudflats with a living membrane and
whose silica shells act as tiny solar greenhouses. The sediments can vary in size from fine muds to coarse
sands, depending on the source of the sediments and the force of the water movements that mobilized and
transported them. In the barrier island system, for example, on the seaward side exposed to wind-driven waves,
the beaches are sandy, whereas on their less energetic landward side, mudflats and salt marshes develop.
