ABSTRACT
Aquatic ecosystems are ubiquitous elements of natural or seminatural landscapes. Aquatic environments and the communities that inhabit them constitute the larger part of the biosphere as explained in the previous chapters. Water indeed covers about three quarters of the surface of our planet. The vast majority of aquatic ecosystems consist of salt water ecosystems, because almost all the water on the earth is found in the oceans and seas. Different zones of the ocean, each characterized by particular communities of organisms, can be distinguished as intertidal, benthic, and pelagic zones. Freshwater ecosystems, such as rivers, lakes, and ponds, represent only a limited portion of the water present on the planet. However these environments are home to a wide variety of organisms accounting for about 10% of all aquatic species.
