Marshland is covered with grasses, reeds, sedges, and cattails. Theseplants all have their roots in soil covered or saturated with water and itsleaves held above water. Marshes may be freshwater or salt. Freshwater marshesdevelop along the shallow edges of lakes and slow-moving rivers, forming whenponds and lakes become filled with sediment. Salt marshes occur on coastal tidalflats.
Inland salt marshes occupy the edges of lakes. They affect the supply ofnutrients, the movement of water, and the type and deposition of sediment. Salt marshes are best developed on the Atlantic coasts of North Americaand Europe. In eastern North America the low marsh is dominated by a singlespecies, salt-marsh cordgrass. The high marsh consists of a short cordgrasscalled hay, spike grass, and glasswort. Glasswort is the dominant plant ofPacific Coast salt marshes.
Freshwater marshes provide nesting and wintering habitats for waterfowland shorebirds, muskrats, frogs, and many aquatic insects. Salt marshes arewintering grounds for snow geese and ducks, a nesting habitat for herons andrails, and a source of nutrients for estuarine waters. Marshes are important inflood control, in sustaining high-water tables, and as settling basins toreduce pollution downstream. Despite their great environmental value, marshesare continually being destroyed by drainage and filling.
Marine Life, plants and animals of the sea, from the high-tide markalong the shore to the depths of the ocean. These organisms fall into threemajor groups: the benthos, plants such as kelp and animals such as brittle starsthat live on or depend on the bottom; the nekton, swimming animals such asfishes 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 regionbounded on one side by the height of the extreme high tide and on the other bythe height of the extreme low tide. Within these boundaries organisms face asevere environment imposed by the rise and fall of tides. For up to half of a24-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 separatedinto distinct zones that reflect the length of time each zone is exposed. At thehighest position on the rocks is the black zone, marked by blue-green algae. This transition area between land and the marine environment is flooded onlyduring the high spring. Below the black zone lies the white zone, wherebarnacles are tightly glued to rocks. Living among the barnacles are rock-clinging mollusks called limpets. At low tide, barnacles keep their four movableplates closed to avoid drying; at high tide they open the plates and extend sixpairs of wandlike tentacles to sweep the water for microscopic life.
Preying onthe barnacles are hole-drilling snails called dog whelks. Below the white zone and in some places overlying the barnacles arerockweeds, 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 thebladder 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, oneof 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 forthe next high tide. Shifting and unstable, sand provides no substrate on whichlife can anchor itself.
The environment of sand-dwelling animals, however, isless severe than that of animals dwelling on rocky shores. Although the surfacetemperature on a beach varies with the tide, below the surface the temperatureremains nearly constant, as does the salinity. The upper sandy beach, like theupper rocky shore, is transitional from land to sea. It is occupied by ghostcrabs and beach fleas, animals more terrestrial than marine. True marine lifeappears at the intertidal zone. Two common inhabitants, active at high tide, arethe lugworm, which burrows through the sand and feeds on organic matter; and thecoquina clam Donax, which advances up the beach and retreats with the tides.Among the sand grains live small copepods and worms that feed on microscopicalgae, bacteria, and organic matter.On the lower beach, which remains uncovered for only a short period oftime, live clams, crabs, starfish, and sand dollars, whose calcareous skeletonslie partially buried in the sand.Category: Social Issues
