Chapter II — 11 — The Marine Environment 



dimensions. The walls of submerged valleys are usually not nearly as 

 steep as those of canyons. 



A shelf is a gently sloping seaward extension of the land around conti- 

 nents and continental islands from lowest tide level to the contour where the 

 angle of descent increases rather sharply toward great depths. The water 

 depth at the outer margin of the continental shelf ranges from loo to 300 

 meters. Although there is considerable overlapping, the continental shelf 

 marks the boundaries of a fairly distinctive biotic zone. 



The continental slope is beyond the outer edge of the shelf where the 

 inclination downward is greater than 1°, or more than one meter in 60 

 meters. The declivity of the slope is usually much greater than 1°. The 

 lower limit of the slope is even more arbitrarily defined than its upper limit 

 but, in general, it is marked by the termination of the steep gradient. An 

 abyss is the bottom of the deep sea. 



A bank is a fairly extensive, relatively fiat-topped, submarine elevation 

 approaching the surface. The water over it is usually sufficiently deep 

 for navigation. The ecological significance of banks is indicated by the 

 fact that some of the world's best fishing grounds are over banks. A reef 

 is a coral or rocky, elongated elevation, part of which may actually extend 

 above the water level 



Biotic zones : — The marine environment is divided into biotic zones 

 according to the nature of the plant and animal life inhabiting each. This 

 is a function of such environmental factors as water temperature, salinity 

 or osmotic pressure, depth, proximity to shore, etc. Marine organisms 

 may be classified according to habitat into the categories of benthic or 

 pelagic. Benthic or benthonic organisms, also called benthos, inhabit the 

 sea floor. Those which live in the water and are not confined to the bot- 

 tom are pelagic. 



The distinction between the neritic and oceanic zones is less sharply 

 defined than that between the benthic and pelagic zones. Literally neritic 

 denotes the zone of shallow water which is relatively near land, and 

 oceanic refers to organisms which live in the open ocean. However, 

 marine ecologists usually employ the term neritic to include the zone out- 

 lined by the continental shelf, the oceanic zone embracing the open ocean 

 beyond these boundaries. According to Ekman (1935), the 200 meter 

 depth contour roughly separates the neritic from the oceanic zone, this 

 approximating the mean depth of water at the edge of the continental 

 shelf. It is also the depth to which little or no sunlight penetrates. 



Some workers define the littoral zone as that near the coast. Others 

 define it as being synonymous with the intertidal zone or the area between 

 high- and low-tide water marks, but Ekman (1935) considers the littoral 

 zone as the floor of the continental shelf. 



The euphotic zone receives enough sunlight for the photosynthetic 

 processes of plants. It is the zone of primary productivity in the sea. 

 The depth of the euphotic zone varies with the factors which influence the 

 penetration of light, but on the average it extends from the surface to a 

 depth of around 100 meters. Below the euphotic zone is the aphotic zone, 

 a lightless region in which photosynthetic plants cannot grow. The ani- 

 mal life in this zone, which comprises about 95 per cent of the sea, con- 

 sists almost exclusively of carnivores and detritus feeders. Bacterial life 

 abounds in the aphotic zone, particularly on the sea floor where organic 

 matter, raining down from the euphotic zone, accumulates. Some work- 



