Several hundred thousand kinds of plants and animals live in the ocean. Some organisms are found near the ocean surface, others prefer middle depths, and still others are found only on the ocean floor.
But depth is not the only thing that determines where ocean life is found. Such factors as ocean size, temperature, currents, and water pressure influence where certain marine plants or animals can live.
The abundance of nutrients, the amount of light, and the characteristics of the seafloor are also important.
To classify and better understand the different regions in the ocean that support life, scientists have divided the ocean into habitat zones. These zones are based on such features as water depth, temperature, and light penetration. Marine organisms are then classified according to the zones in which they live.
The amount of sunlight able to penetrate the water determines the photic and aphotic habitat zones. The photic (lighted) zone includes the upper ocean layers where there is enough light for plant growth. The permanently dark water mass below the photic zone is termed the aphotic (unlighted) zone.
The photic zone usually extends to depths ranging from 160 to 330 feet (50 to 100 meters). The depth depends on conditions that affect the amount of light able to pass through seawater. Light-scattering sources, such as suspended material or dense patches of microscopic plants, limit light penetration. Therefore the photic zone extends much deeper in clear tropical waters than in murky coastal waters.
The ocean environment can be further subdivided both vertically and horizontally. Vertically, the ocean consists of two broad environments: the pelagic or water environment and the benthic or bottom environment. Pelagic organisms are those that live in the open water away from the sea bottom, while benthic organisms live on or in the sea bottom.
The pelagic environment is made up of two zones: 1) the neritic zone, which includes all waters over the continental shelves, and 2) the oceanic zone, which includes waters beyond the shelves.
The benthic environment is divided into three zones according to depth of the ocean floor. Ocean bottoms encompassing the continental slope down to about 13,000 feet (4,000 meters) represent the bathyl zone. Between 13,000 and 20,000 feet (4,000 and 6,000 meters) are the broad abyssal plains of ocean basins that make up the abyssal zone. The hadal zone includes the trench areas and ocean bottoms below 20,000 feet (6,000 meters). These three zones in the benthic environment constitute most of the aphotic areas of the ocean about which little is known.