Ocean is generally divided in zones in which each layer is usually distinguished by the amount of sunlight it receives, the depths it occupies, and the degree of hydrostatic pressure found there. The zones are divided as:
1. Neritic Zone:
The neritic zone is the marine counter part of the littoral zone of the lake. It is a relatively warm, nutrient rich, shallow region. It overlies the continental shelf, a submerged extension of the continent.
The neritic zone is permanently covered with generally well-oxygenated water, receives plenty of sunlight and has low water pressure; moreover, it has relatively stable temperature, pressure, light and salinity levels, making it suitable for photosynthetic life.
The nutrients of the neritia zone are supplied primarily by upwelling and sedimentary discharges of streams. Sunlight normally penetrates to the ocean bottom, permitting considerable photosynthesis activity in a large population of floating and rooted plants. Animal populations are rich and varied. Sea weeds and algae are the producers, forming the base of the food chains and webs. Tiny zooplanktons feed on the producers and in turn are consumed by fish, crabs and whales. The top consumers are the sea birds, sharks, tuna, fish, whales, and humans. The bottom-dwellers, which cling to rocks are buried in the mud, include clams, snails, crabs, and bacteria. They feed on detritus.
The total amount of biomass in the neritic zone is greater per unit of volume of water than in any other part of the ocean.
2. Euphotic Zone:
The euphotic zone is the open-water zone of the ocean which has sufficient sunlight to support photo-synthesis and a considerable population of phytoplankton. In turn, the phytoplankton supports a host of tiny grazing herbivores such as the small crustaceans. The total energy available to animal food chains from euphotic phytoplankton is much greater-than-that produced by plants of the neritic zone; the lower limit of the euphotic zone is 200 meters, beyond which sunlight cannot penetrate.
3. Bathyal Zone:
This zone lies beneath the euphatic zone. It is a semi-dark region. Photosynthesis cannot occur here and so no producer organism can survive.
4. Abyssal Zone:
This zone lies beneath the bathyal zone and immediately above the ocean floor. It is cold, dark water zone of the ocean depths. Animal life is rather sparse because of low light, intense cold, extremely low levels of dissolved oxygen, water pressure, and scarcity of food. In many areas, the sediment of the abyssal zone in rich in nutrients which come from the decaying bodies of organisms. Because of the absence of photosynthesis and herbivorous animals in the abyssal zone, most consumers are either predators or detritus feeders.