20
lesson N° 3 :
coastal habitats :
Coastal habitats
Shorelines of the world are populated by a rich variety of organisms that
are well adapted to one another and to the demands of their physical
surroundings.
Because of their shallowness , coastal waters respond strongly to the
effects of waves and tides and to the daily and seasonal variations of
weather . Also , shore habitats being close to land receive a regular
infusion of river water that locally causes pronounced changes in the
salinity and water temperature of the environment. Despite these
demanding conditions , certain coastal habitats including
estuaries,
lagoons , salt marshes , mangrove forests and coral reefs , rank among
the most biologically productive ecosystems anywhere on the Earth.
Estuaries :
Oceanographers define an estuary as a semienclosed body of water
where river water mixes with ocean water. This is in contrast to lagoons
,which are likewise semienclosed coastal water bodies , but which
receive essentially no inflow of river water , so they are brackish or as
salty as ocean water.
Vegetated wetlands that surrounds estuaries , such as salt marshes , were
once considered useless land, they were used as convenient sites to
dispose of industrial and domestics wastes.Now , cleaned up more or
less and reclaimed by landfill , they have become valuable land for
urban expansion at the edge of the sea.
The biology of estuaries :
The water chemistry and current patterns in estuaries are greatly
influenced by the relative effects of river discharge and tidal mixing .
the widely fluctuating environmental conditions in estuaries make them
difficult places for plants and animals to inhabit , so species diversity is
low .However , despite these harsh living conditions , estuaries have a