Marshland is covered with grasses, reeds, sedges, and cattails. These
plants all have their roots in soil covered or saturated with water and its
leaves held above water.Marshes may be freshwater or salt. Freshwater marshes
develop along the shallow edges of lakes and slow-moving rivers, forming when
ponds and lakes become filled with sediment. Salt marshes occur on coastal tidal
flats. Inland salt marshes occupy the edges of lakes. They affect the supply of
nutrients, the movement of water, and the type and deposition of sediment.
Salt marshes are best developed on the Atlantic coasts of North America
and Europe. In eastern North America the low marsh is dominated by a single
species, salt-marsh cordgrass. The high marsh consists of a short cordgrass
called hay, spike grass, and glasswort. Glasswort is the dominant plant of
Pacific Coast salt marshes.
Freshwater marshes provide nesting and wintering habitats for waterfowl
and shorebirds, muskrats, frogs, and many aquatic insects. Salt marshes are
wintering grounds for snow geese and ducks, a nesting habitat for herons and
rails, and a source of nutrients for estuarine waters. Marshes are important in
flood control, in sustaining high-water tables, and as settling basins to
reduce pollution downstream. Despite their great environmental value, marshes
are continually being destroyed by drainage and filling.
Marine Life, plants and animals of the sea, from the high-tide mark
along the shore to the depths of the ocean. These organisms fall into three
major groups: the benthos, plants such as kelp and animals such as brittle stars
that live on or depend on the bottom; the nekton, swimming animals such as
fishes and whales that move independently of water currents; and plankton,
various small to microscopic organisms that are carried along by the currents.
Shore Life, the essentially marine organisms that inhabit the region
bounded on one side by the height of the extreme high tide and on the other by
the height of the extreme low tide. Within these boundaries organisms face a
severe environment imposed by the rise and fall of tides. For up to half of a
24-hour period, the environment is marine; the rest of the time it is exposed,
with terrestrial extremes in temperature and the drying effects of wind and sun.
Life on rocky shores, best developed on northern coasts, is separated
into distinct zones that reflect the length of time each zone is exposed. At the
highest position on the rocks is the black zone, marked by blue-green algae.
This transition area between land and the marine environment is flooded only
during the high spring. Below the black zone lies the white zone, where
barnacles are tightly glued to rocks. Living among the barnacles are rock-
clinging mollusks called limpets. At low tide, barnacles keep their four movable
plates closed to avoid drying; at high tide they open the plates and extend six
pairs of wandlike tentacles to sweep the water for microscopic life. Preying on
the barnacles are hole-drilling snails called dog whelks.
Below the white zone and in some places overlying the barnacles are
rockweeds, which have no roots but attach themselves to rocks by holdfasts.
Brown algae are rockweeds that grow more than 8 ft long. The most common are the
bladder wracks, with branching thalli up to 6 in wide. In the lowest zone,
uncovered only during the spring tides, is the large brown alga Laminaria, one
of the kelps. Beneath its frondlike thalli live starfish, sea cucumbers, limpets,
mussels, and crabs.
On the sandy shores, life lies hidden beneath the surface, waiting for
the next high tide. Shifting and unstable, sand provides no substrate on which
life can anchor itself. The environment of sand-dwelling animals, however, is
less severe than that of animals dwelling on rocky shores. Although the surface
temperature on a beach varies with the tide, below the surface the temperature
remains nearly constant, as does the salinity. The upper sandy beach, like the
upper rocky shore, is transitional from land to sea. It is occupied by ghost
crabs and beach fleas, animals more terrestrial than marine. True marine life
appears at the intertidal zone. Two common inhabitants, active at high tide, are
the lugworm, which burrows through the sand and feeds on organic matter; and the
coquina clam Donax, which advances up the beach and retreats with the tides.
Among the sand grains live small copepods and worms that feed on microscopic
algae, bacteria, and organic matter.
On the lower beach, which remains uncovered for only a short period of
time, live clams, crabs, starfish, and sand dollars, whose calcareous skeletons
lie partially buried in the sand.
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