Although the mixing effected by the oceanic circulation ensures that the major parameters vary but little throughout enormous volumes of water, there are nevertheless some strong contrasts between different parts of the sea. The cold, dark, slowly moving bottom layer of the deep ocean is obviously a very different
environment from the well-illuminated, wave-tossed waters of the sea surface, or the strong currents and fluctuations of temperature and salinity that often occur near the coast. We therefore need a classification of subdivisions of the marine environment which takes account of different conditions of life in different parts of the oceans.
There are broadly two ways in which organisms live in the sea; they float or swim in the water, or they dwell on or within the sea bottom. We can correspondingly make two major divisions of the environment, the Pelagic and the Benthic, the Pelagic Division comprising the whole body of water forming the seas and oceans, and the Benthic Division the entire sea bottom (Figure 1.8).
In shallow water there is usually more movement and greater variations of composition and temperature than occur where the water is deep. We can therefore subdivide the Pelagic Division into (a), the Neritic Province, the shallow water over the continental shelf, and (b), the Oceanic Province, the deep water beyond the continental edge.
In deep water, conditions change with level and it is useful to distinguish four zones as follows:
(a) The Epipelagic Zone from the surface to 200 m depth, in which there are sharp gradients of illumination, and often temperature, between the surface and the deeper levels; and also diurnal and seasonal changes of light intensity and temperature. In many areas the temperature gradient is irregular, involving discontinuities or thermoclines (see Section 4.2.2). Water movements may be relatively rapid.
(b) The Mesopelagic Zone from 200 to 1000 m depth, where very little light penetrates, and the temperature gradient is more even and gradual without much seasonal variation. An oxygen-minimum layer (see Section 4.3.2) and the maximum concentrations of nitrate and phosphate (see Section 4.3.3) often occur within this zone.
(c) The Bathypelagic Zone between 1000 m and 4000 m, where darkness is virtually complete except for bioluminescence, temperature is low and constant, and water pressure high.
(d) The Abyssopelagic Zone below 4000 m; dark, cold, with the greatest pressures and very little food.
The sea bottom and the seashore together make up the Benthic Division which comprises three major zones, the Littoral, the Sublittoral and the Deep Sea Zones. The Littoral Zone includes the greater part of the seashore together with the wave-splashed region above high tide level (see Section 8.6). The Sublittoral Zone is the shallow sea bottom extending from the lower part of the shore to the continental edge. The Deep Sea Zone lies below the continental shelf, and can be subdivided into Bathybenthic and Abyssobenthic Zones. The Bathybenthic zone lies between the continental edge and a depth of about 4000 m, comprising mainly the continental slope. The Abyssobenthic Zone is the bottom below 4000 m, including the continental rise, abyssal plain and deeper parts of the sea floor.
The deepest parts of the ocean within the trenches below some 6000 to 7000 m are termed the Hadopelagic and Hadobenthic Zones.
Subdivisions of the marine environment with respect to temperature and light are mentioned later (see Sections 4.2.3 and 4.6).
Organisms of the Pelagic Division comprise two broad categories, plankton and nekton, differing in their powers of locomotion. The plankton consist of floating plants and animals which drift with the water, and whose swimming powers, if any, serve mainly to keep them afloat and adjust their depth level rather than to carry them from place to place. A brief account of the constituents of marine plankton appears in Chapter 2. The nekton comprises the more powerful swimming animals, vertebrates and cephalopods, which are capable of travelling from one place to another independently of the flow of the water.
The populations of the Benthic Division, the sessile and attached plants and animals and all the creeping and burrowing forms, are known collectively as benthos.
The term benthopelagic refers to animals, mainly fish, which live very close to, but not actually resting on, the bottom. Hovering slightly above the sea floor, they are well placed for taking food from the bottom.
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