Coastal and marine ecosystems

Abstract

The seas and oceans cover approximately 360 million km2 or about 70% of the Earth’s surface. The ocean basins are interconnected and currents generated by the wind keep the surface waters in continuous circulation (Figure 12.1). Deep water is brought to the surface in zones of upwelling where winds move the surface water away from the continents and further mixing occurs because of differences in water temperature and salinity. Although conditions are less variable than on land, the limits of tolerance of most marine organisms are comparatively narrow and their distribution is determined primarily by the interrelated effects of water depth, latitude and distance from shore (Barnes and Hughes, 1988). The average depth of the oceans is 3700 m, but in the deeper trenches the ocean floor is more than 10 000 m below the surface. Some organisms occur at these extreme depths but most species are found in the shallower water that surround the continents: these shallow waters represent about 8% of the total area of the oceans (Tait, 1972).