Geoscience Reference
In-Depth Information
the Chesapeake, with only about one-tenth the volume of water compared to most of the other major bays in
the world, is more than a hundred times as productive.
The classic bay circulation is characterized by lighter freshwater moving across the top of the bay toward
the sea, while the heavier salt water moves under this lighter layer toward the head of the bay. Winds fre-
quently turn these layers over, and flushes of freshwater can also upset this classic circulatory pattern, turning
it topsy-turvy. This constant mixing of salt waters and freshwaters means that the bay's dominant species must
be able to tolerate wild swings in salinity. The salinity of the water affects all aspects of the bay's ecology, in-
fluencing spawning and nursery areas for crabs and fish, for example, and determining what plants grow
where. Year-round residents, such as blue crabs, oysters, striped bass, and blue herons, must be adaptable to
these ever-changing conditions.
Despite such vicissitudes, the turnover of waters in the bay does have a seasonal rhythm. In spring, the
rivers bring a fresh supply of nutrients and oxygen, triggering a profusion of phytoplankton growth. The
phytoplankon, in turn, attract fish and other consumers but eventually deplete the oxygen in these waters. In
the fall, when river levels are low, tidal waters push farther into the bay and bring with them fresh supplies of
oxygen. At the same time, the surface waters are cooling and therefore begin to sink, while the oxygenated salt
waters now rise. In this way, the oxygen, and with it phytoplankton and zooplankton, are redistributed
throughout the water column.
The shallowness of the bay—10 percent of it is less than a meter (3 feet) deep and 20 percent is less than 2
meters (6 feet) deep—allows sunlight to penetrate much of the water column, fueling phytoplankton growth
and supporting underwater meadows of sea grasses. More than a dozen varieties of sea grasses grow from the
head to the mouth of the bay, depending on their varying tolerances to salinity, and provide food for waterfowl
as well as nursery areas for shrimp, crabs, fish, oysters, and sea horses. The extensive tidal marshes that fringe
the eastern shore of the bay—the so-called “Everglades of the North”—are the largest salt marshes in the Mid-
Atlantic region. Here, the dominant cordgrasses—the Spartinas—rule, providing food or shelter for an array of
marine creatures—from fish to oysters, clams to worms—and foraging areas for mammals such as mink,
muskrat, nutria, and river otter. In these marshes and the adjacent shallow waters and tidal flats, eagles, osprey,
and great blue herons also feed.
It takes six months for water from the Susquehanna, with its dissolved nutrients, to pass from the river to the
ocean. During this period it is recycled several times, from top to bottom, a process abetted by the bay's shal-
lowness. Nutrients taken up by the phytoplankton eventually sink to the bottom and are recycled by the worms
and clams, thus moving it through the food web. Underwater grasses absorb the phosphorus and nitrogen de-
livered to them by the river in spring and store it in their tissues until fall, when they begin to die back, thus re-
leasing the nutrients to the system. They also bank nutrients in their roots, which are cropped by migrant swans
and geese. Ultimately, this recycling of nutrients contributes to the bay's stability, a state of equilibrium that
ecologists call homeostasis.
