Our oceans comprise almost three quarters of the surface of the earth. "The World Ocean (Oceanosphere) contains on the order of 1,340.7 million km3 of water, making up 1/800th of the total volume of the Earth (1,083.3 billion km3). Alternatively, the volume of fresh water is about 35.8 million km3. If the Oceanosphere was shown in the form of a sphere, its radius would be equal to 690 km, or 0.11 mean radii of the Earth (6,371 km)." - http://www.oceansatlas.org/unatlas/about/physicalandchemicalproperties/background/seemore1.html With an average depth Deepest point on earth is the Marianas Trench with 11.022 metres. Most oceanic waters are less than 1.000 metres or between 3.000 and 4.000 metres. of ~3.700 metres the marine habitats encompass roughly 300 times the habitable volume of the land based (terrestrial) habitats. The oceanic ecosystem is comprised of complex and interlinked systems where each organism relies on its own niche (e.g. habitat) for survival. A change in any biotic Biotic - The living parts of the environment, e.g. plants, animals and fungi, that affect ecological functions. or a-biotic Abiotic - The non-living parts of the environment, e.g. rocks, climate, and pressure, that affect ecological functions. variable, and thus a change or disappearance of a habitat, can have disastrous consequences for the organisms relying on that.
The majority of all plant and animal life on earth is found in the oceanic ecosystems. Marine ecosystems can be split up in a number of zones. While a number of classifications exists on this the following is a very practical list: Alexander, D., & Fairbridge, R. W. (1999). Encyclopedia of environmental science. Kluwer Academic encyclopedia of earth sciences series. Dordrecht: Kluwer Academic Publishers.
The main oceanic habitats are reefs, deep sea / trenches, open ocean, and intertidal / shore.
The oceanic ecosystems are populated with a tremendous amount and diversity of life. Ranging from the smallest krill to the Blue Whale, the largest mammal (up to 40m and 150tonnes in weight) known to men.
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