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Just as life on land stays close to the ground, bound by the need for nutrients and water tied up in the soil, so life in the oceans is concentrated near the surface of the water. Plants need three things to live: water, light and nutrients. Water is, of course, freely available in the sea, but light is found only near the surface. Water absorbs light at such a rate that below 40 metres it is too dim for plants to photosynthesize; below 600 metres it is as black as a moonless night. Plants, and the animals that eat them, must therefore live near the surface in the zone of light, called the photic zone.
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