The Marine Environment

Compared with freshwater and land, the marine environment is relatively uniform. Although oxygen is available and salinity of the open ocean is relatively constant, light and temperature varies greatly as a consequence of depth.

Thus, life is not uniformly distributed throughout the depth and breadth of the world's oceans which cover 71% of the the earth's surface. The margins of the continents extend seaward in the form of shelves to depths of 150 to 200 m and steep more steeply to depths of 3000 m or more.

The floor of the sea basin is called the abssyal plain, ranges from 3000 to 5000 m in depth and may be marked by features such as sea mounts, trenches and ridges.

Waters over the continental shelves constitute the neritic zone while those beyond the shelf constitute the oceanic zone. The edge of the sea which falls and rises with the tide is the intertidal (littoral) zone. The region above is the supratidal (suppralittoral) zone and that below is the subtidal (sublittoral) zone.

The continental slopes form the bathyal zone, the abyssal plains form the abyssal zone, which the trenches form the hadal zone.

Vertical distribution of marine organisms is largely controlled by the depth of light penetration. Light sufficient for photosynthesis penetrate only the depth of 200 m (the euphotic zone) below the surface depending on the turbidity of water. Below this zone is a transition zone where some photosynthesis may occur. Below this zone to the ocean floor, total darkness prevails. This region is known as the aphotic zone.

Permanent inhabitants in the apothic and transition zone are carnivores, suspension or detritus feeders and depend indirectly on the photosynthetic activity of the microscopic algae in the upper, lighted regions.

The suspended or swimming animals of the ocean waters constitute the pelagic fauna, and those that live on the bottom compose of benthic fauna. Bottom dwellars may live on the surface (epifauna) or beneath the surface (infauna) of the ocean floor and usually reflect the character of the substratum, whether it is hard bottom of coral or rock or a soft bottom of sand or mud.

Many animals are adapted for living in the spaces between sand grains and compose the interstitial fauna or meiofauna.