Despite its dangers, the shore is often densely populated by a variety of organisms excellently adapted to the difficult conditions. So great are the numbers on some shores that every available surface is colonized, and there is severe competition for living space. A numerous population indicates an abundant supply of food, and this is derived from several sources. On rocky coasts there is often a thick cover of seaweed. The rapid growth of these plants is favoured by the excellent lighting conditions and by a good supply of nutrients. The nutrients are continually released and replenished by wave disturbance of sediments, weathering of the coastline and input from fresh water flowing off the land, and are well distributed by water movements. Some animals browse directly on these seaweeds. Many more obtain food from the masses of organic debris formed by the break-up and stranding of pieces of seaweed. Even where there are no large plants, the surface of the beach may be covered by a film of microscopic algae.
In addition to primary production on the shore, large quantities of food are brought in by the sea. Inshore water often contains a rich plankton on which innumerable shore creatures feed, replenished at each rise of the tide. Also, pieces of plant material torn from the sea-bed below low tide level become deposited on the shore. The land, too, makes a contribution, various organic substances of terrestrial origin being stranded on the beach.
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