The sea floor is an area of great contrasts, from deep ocean 'deserts' of soft sediment to rich coral reefs. Exactly what lives there depends on a variety of conditions such as substrate type, current speeds and in particular availability of food. Even in deep water, there is generally some food available, mainly in the form of fragments of organic matter sinking from the overlying water, and in some areas this food supply may be sufficient to support a large population. Beneath shallow water there may also be primary production by benthic algae and photosynthetic bacteria to depths of about 50 m depending on water clarity. Kelps (e.g. Laminaria, Alaria, Saccorhiza, Macrocystis), calcareous red algae (e.g. Lithophyllum, Lithothamnion) and a variety of foliose algae, grow on shallow rocks where light is sufficient. A varied microflora of benthic diatoms and other photosynthetic protista occur on the surface of rock, sand or mud often visible to the naked eye as a thin brownish film.
Many bottom-dwelling creatures are able to live and grow to large size with relatively little expenditure of energy in hunting and collecting food because they can obtain adequate nourishment simply by gathering the particles that fall within their reach or are carried to them by the currents. Others simply digest the organic matter and associated bacteria contained within the sediment. Most of the sea bottom is covered with soft deposits which give concealment and protection to burrowing creatures. Where the substrate is hard it provides a secure surface for the attachment of sessile forms and affords protection for creatures which hide in crevices or burrow in rock. Compared with the pelagic division of the marine environment the sea bottom provides a far wider variety of habitats because the nature of the bottom differs greatly from place to place. The benthic population of the sea is correspondingly more diverse than the pelagic population.
Except in very shallow depths, the temperature, salinity, illumination and movements of the water at the bottom are less variable than in the surface layers. Below 500 m seasonal changes in these variables are negligible, and the deeper the water the more constant are the conditions.
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