Geoscience Reference
In-Depth Information
they are large enough to survive on the sea bottom. Many of the more than a million waterfowl that depend
upon the bay exploit sea grasses—redhead grass is likely named for redhead ducks that prefer it as fodder.
Collapses of this grass due to poor water quality caused a marked decline in the numbers of redhead duck
utilizing the bay, from eighty thousand a half century ago, to fewer than ten thousand today. But overall, un-
derwater grasses throughout the Chesapeake are making a comeback as measures to improve water quality
take effect, an upswing which bodes well for the health of the bay.
NARRAGANSETT BAY AND Long Island Sound are remnants of large river basins at the margin of the glacier's
most southerly advance, where today the glacial debris forms cobble beaches along their shores. These protec-
ted bays harbor relatively simple intertidal communities compared with the rocky shores north of Cape Cod,
which acts as a barrier to the richer boreal fauna found there.
Cobble beaches, composed of small stones, have a high intertidal zone dominated by the cordgrass Spartina
alterniflora, which acts as a buffer against wave shock and erosion. Cobbles nestled among the root mat of the
cordgrass are often encrusted with barnacles and mussels, and typical salt-loving plants, halophytes such as
goosefoot and glasswort, establish behind the cordgrass stands, which trap the seeds of these annual plants and
prevent them from being washed away. In so doing, cordgrass acts as a bioengineer, over time transforming
cobble beach habitats into high marsh plant communities.
Barnacles and fleshy algal crusts cover the cobblestones of the middle intertidal zone. But the most con-
spicuous organism is the common European periwinkle, which occurs in astronomical numbers, six hundred to
one thousand per square meter, in Narragansett Bay and Long Island Sound. This herbivorous snail is an invas-
ive species that appeared in Nova Scotia in the mid-19th century and moved south, reaching Chesapeake Bay
by the 1950s. It is a habitat generalist now found in all northeast shoreline habitats. In salt marshes it has re-
placed the native mud snail, and on estuarine cobble beaches it severely restricts the growth of green algae.
Such an algal canopy would lead to sediment accumulation and the establishment of tube-building organisms,
such as worms and crustaceans, typical of soft-sediment communities. The periwinkle, however, is restricted to
the intertidal zone—perhaps by predatory whelks or crabs—allowing an algal canopy of Irish moss, dead
man's fingers, and sea lettuce to develop in the subtidal zone.
Humans are undoubtedly the most effective bioengineers, although our impacts on ecosystems are often in-
advertent. The Hudson River estuary and New York Harbor are among the most polluted estuarine environ-
ments in the United States, repositories of industrial wastes, such as heavy metals and toxic organic com-
pounds, and untreated sewage. Commercial fisheries in these waters shut down in 1976 because of pcb con-
tamination. Pollution has been dramatically reduced and a number of native fish species have rebounded in re-
cent decades, including American shad, striped bass, short-nose sturgeon, and the Hudson's behemoth, the At-
lantic sturgeon, which can reach 4 meters (13 feet) and 360 kilograms (800 pounds).
The area between Boston and Washington, D.C., is one of the most densely populated urban areas in the
world, home to tens of millions of people whose cities, towns, and suburbs meld into a single urban entity.
Even so, the shore and offshore environment near this megalopolis remain surprisingly vital. As we have seen,
it is host to a large number of marine birds, both breeders and nonbreeders. A number of these local breeders,
such as gull-billed tern, roseate tern, and Forster's tern, nest at a very limited number of sites, as do an import-
ant proportion of the western Atlantic population of royal terns. The Chesapeake feeds impressive numbers of
bald eagles and is home to the largest population of osprey—some two thousand nesting pairs—in the world.
The invertebrates and plants of both the Chesapeake and Delaware also support large numbers of waterfowl;
tundra swans, Canada geese, greater snow geese, and a variety of ducks, including pintails, canvasbacks, ei-
ders, and ruddy ducks, winter here.
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