There are several life zones in the ocean. A life zone is a region that contains specific organisms that interact with one another and with their environment. These zones are determined by the depth of water and the amount of sunlight the water receives. Look at the zones described below.
Zones by water depth
- Pelagic zone – Together, the neritic and oceanic zones make up the largest marine life zone. This zone is also referred to as the water column.
- Neritic zone – lies above the continental shelf, the shallow part of the seafloor that surrounds the continents. It is the main area of commercial fishing. Part of the pelagic zone between the high tide line and ocean bottom; less than 600 feet deep.
- Intertidal zone: is above the high-tide mark, receives wave splash; blue-green bacteria and algae grow on the moist rocks; when algae die, they stain the rocks black; periwinkle snails and limpets graze on the algae.
- Upper intertidal zone: an area of the upper beach that gets a fine mist of salt spray from the crashing waves. Has beach plants, including grasses, shrubs, and trees.
- Mid-intertidal zone: is inhabited by barnacles, mussels, and seaweeds; barnacles attach to the rocks by means of a natural glue; mussels produce sticky byssal threads that adhere to the rocks; rockweeds adhere to rocks by means of a holdfast attachment; at high tide, barnacles filter feed on plankton; at low tide, barnacles close their shells to avoid drying out.
- Lower intertidal: is dominated by seaweeds; at low tide, spaces between the rocks retain water, forming small living communities (of algae, invertebrates, small fish), called tide pools. When the tide is in, sea stars, sea urchin, and fish invade this zone to feed.
- Subtidal zone: the area below the intertidal zone. This zone includes the surf zone, an area of wave turbulence. Fish, crabs, sea stars, and sea urchins are typical inhabitants of this zone.
- Oceanic zone – extends beyond the Neritic zone and includes most of the open ocean. Includes water deeper than 600 feet.
- Benthic zone – includes the entire ocean floor, from the intertidal zone to the ocean basin. Organisms that live on the seafloor are called benthos.
Zones by sunlight
The ocean can be divided into three zones based on the amount of light received, the ephotic zone, disphotic zone, and aphotic zone.
The euphotic zone is the layer nearest the surface of the ocean. It is also referred to as the sunlit zone. Because this zone receives the most light, this is where most photosynthesis takes place. Free floating photosynthetic organisms live in this zone and provide the basis for the food chain. Because of their presence, more than 90 percent of all marine life lives in the euphotic zone!
Only a small amount of light can penetrate the water at this depth. As the water becomes deeper, the pressure increases. Lack of light and increased pressure does not allow many plants to grow here. Only animals that have adapted to little light survive. The disphotic zone is also known as the twilight zone. This murky part of the ocean begins at about 600 feet under the water and extends to the darkest part, which begins about 3000 feet down. Some squid and fish can use their bodies to make light. These creatures are said to have bioluminescence.
Ninety percent of the ocean is in the midnight zone. It is entirely dark—there is no light. The water pressure is extreme and the temperatures are near freezing. The aphotic zone is also called the midnight zone.
The living things found here live close to cracks in the Earth’s crust. These cracks give off mineral-rich materials from the Earth itself. Special forms of bacteria utilize hydrogen sulfide from the cracks for energy to make food. All other living things in the midnight zone are nourished by these bacteria.
Zones of marine organisms
Plankton are any floating or feebly swimming organisms that live close to the surface of the ocean. In fact, the term plankton comes from the Greek word planktos which means “wanderer.” These organisms include animals, plants, bacteria, or any other organisms which can not resist the ocean current. Phytoplankton are plankton that are autotrophic and can produce their own food; whereas zooplankton are plankton that rely on other organisms for food. Additionally, there are bacterioplankton that resemble bacteria and archea and help recycle minerals and decaying organisms in the water column. Some plankton live their entire lives as plankton. These organisms are referred to as holoplankton. In contrast, there are organisms that live only part of their lives as plankton. These organisms are referred to as meroplankton.
Nekton are strong swimming organisms that live in open water. Nektonic creatures, however, begin their lives as plankton. Most of the nekton are vertebrates such as fish, marine mammals, and marine reptiles. Squid are the only invertebrates that are strong enough to be considered nekton. Although the nekton can swim freely, they are confined by invisible barriers created by changes in the temperature, salinity, and density of the ocean.
Benthos are organisms that live on, in, or near the ocean floor. The term benthos means “depths of the sea” but this does not necessarily mean these organisms must live deep within the ocean. Benthic organisms can live in the tide pools along the foreshore or along the continental shelf. Some benthic organisms have unique adaptations that allow them to live within the harsh conditions of the ocean floor. Examples of benthic organisms include polychaete worms, coral, seaweed, copepods, crabs, and bivalves.