Geoscience Reference
In-Depth Information
Just as they were created by rising sea level, barrier islands must continue to adapt to the encroachment of
the sea. Barrier islands are ever on the move toward the land and in the process undergo continuous change
while, paradoxically, remaining the same. The position of the island migrates, but the ecological units of the is-
land—dune, maritime forest, salt marsh, and tidal flat—are retained in their regular sequence, just displaced
landward. The beach acts as a living thing, adapting to the environmental stresses imposed on it by rising sea
level while at the same time retaining its essential character.
The movement and transformation of the island is often initiated by a major storm that causes waves to
wash over the island or to push through low areas between the dunes, carrying with them large loads of sand,
which are deposited when the storm surge loses its energy. Sand is deposited either on top of the salt marsh or,
if the surge and tides are great enough, in the sound or lagoon behind the island itself. The deposits are called
overwash fans to describe the shape of the sand deposit. Overwash is a regular event in the Outer Banks of
North Carolina, which consists of low-lying islands that lack high, well-stabilized dunes. It is a less frequent, if
not uncommon, event in areas such as Cape Cod, where dunes rise to dramatic heights.
When the overwash process delivers the sand into the bay behind the island, the landward movement of the
island is clearly extended, although parts of the fan may remain underwater. Often, however, the overwash fan
is deposited on top of a living salt marsh. This kick-starts an ecological response in which the barrier island es-
sentially rolls over on itself, moving landward but maintaining the succession of habitats from its seaward to
its landward edge. American beach grass colonizes the new sand deposits, usually beginning on the landward
side of the overwash fan. Then prevailing onshore winds deliver new sand from the seaward side of the over-
wash deposits, which stimulates growth of the grasses and stabilization of the new dune. Eventually, a new,
high foredune is created with extensive dune fields and other habitats, such as maritime forest and salt marsh,
behind it—and the whole island is displaced toward the land. Dunes, and therefore islands, can also migrate
when the wind moves sand and dunes toward the back of the island.
The landward displacement of the barrier island, whether it occurs through a dramatic incursion of the sea or
by the gradual but steady force of the wind, continually recreates the same succession of habitats. From the
beach (or swash zone) landward, in simplified terms, these habitats are dunes, shrub thicket and maritime
forest, salt marshes, and tidal flats, with a sheltered lagoon or bay separating the island and the mainland. In
each case a community of plants and animals has adapted to the demands and rewards of the habitat.
Beginning in the swash zone, where the turbulent surf crashes ashore, we are likely to encounter a number
of well-adapted and visible creatures that live in the subtidal area bordering the beach and on or under the
beach surface itself. The most impressive shells on the beach belong to the Atlantic surf clam, the largest clam
living along the Atlantic coast, where it can be found from the Gulf of St. Lawrence to Cape Hatteras. The tri-
angular shell can grow to a maximum of 23 centimeters (9 inches) during the mollusc's twenty-fiveto thirty-
year lifetime. Surf clams are prolific, maturing sexually as early as three months of age, and older individuals
can broadcast as many as 13 million eggs at a time. They burrow into the sand at the lowest intertidal level, but
most make their home below the tide line to depths of 60 meters (200 feet). Surf clams are filter feeders
equipped with two siphons, one that draws water across a large set of gills, which extracts food and oxygen,
and the other used to expel water and waste. These siphons are relatively short, however, so the clams can bur-
row only just below the surface. As a result, they are vulnerable to predators such as gulls, which pluck the
clams exposed by the pounding surf and then break them open by dropping them on hard surfaces such as
rocks, or even nearby parking lots. The clams have two large adductor muscles that hold the shells together,
and the sweetness of these muscles makes them the target of a clamming industry. Moon snails also seek out
surf clams and open them expertly, first softening the calcium carbonate shell with an acidic secretion and then
drilling a hole through the shell with their rasplike radula, which acts like a conveyor belt equipped with
hardened teeth.
