[Geography Notes] on Pelagic Zone Pdf for Exam

“The Pelagic zone is the region of the ocean outside the coastal areas and is also known as the open ocean”

Pelagic Waters

The Pelagic waters refer to open and free waters in the oceanic body. It stretches between the ocean surface and ocean bottom. Any marine life residing in the pelagic zone can swim freely without any boundary constraints. 

Pelagic zone, an ecological realm that covers the entire ocean water column. Of the earth’s inhabitants, the pelagic zone has the largest volume. It consists of the water column of the open ocean. It can be further divided into regions by depth. The Pelagic zone or water column can go from the surface of the sea to the entire bottom. As the water column changes with respect to the distance from the surface, its pressure increases, and in turn the temperature and light decrease. It results in a change of salinity, amount of dissolved oxygen, and micronutrients such as iron, magnesium, and calcium.

In addition to changes in the oceanic Pelagic zone, marine life is affected by underwater topography and by the surface of the seafloor or shoreline. Not only the discussed factors are responsible for marine life variations, instead, but the boundary between the ocean and atmosphere is also equally important. It brings the light for photosynthesis but can also bring predation from up and water currents.

Depth and Layers of Pelagic Zone

Depending upon the depth of the sea, the pelagic zone can extend up to five vertical regions in the ocean. These are as follows:

1. Epipelagic – This is the lighter zone where enough light is present for photosynthesis. All primary production in the water occurs here. Plants and animals are concentrated in this layer. Examples of pelagic zone organisms residing here are plankton, seaweed, jellyfish, tuna, and dolphins.

2. Mesopelagic – The pelagic zone organisms residing in the mesopelagic zone are heterotrophic bacteria. Examples of organisms that live here are swordfish, squid, wolffish, and species of cuttlefish.

3. Bathypelagic – At this level of Pelagic water, the ocean is pitch black. No living plant exists here apart from occasional bioluminescent organisms such as anglerfish. Organisms living here survive by consuming detritus falling from the above zone. This phenomenon is also known as marine snow. The inhabitants living in this zone are giant squid, dumbo octopus.

4. Abyssopelagic – This zone starts from around 4000 meters. Very few creatures can reside in cold temperatures, high pressure, and complete darkness. Species found in this zone are several species of squid, swimming cucumber, sea pig, and sea spider. Most of the species living here are transparent and eyeless. The reason being this evolution is the total lack of light.

5. Hadopelagic – This is the deepest part of the ocean lying within oceanic trenches. The zone is found from a depth of 6000 to 11000 meters. Most of the hadal habitat is found in the Pacific ocean. 

Pelagic Environment

The Pelagic environment is based on phytoplankton. They manufacture their own food using photosynthesis. They need sunlight, they inhabit the upper zone. The biodiversity reportedly decreases in the deeper zones. The pelagic zone organisms range from tiny planktons to large mammals. Phytoplanktons provide oxygen for humans and food for many organisms. 

Zooplanktons are found in this zone. It includes heterotrophic planktons consisting of most of the micro and microorganisms.

Invertebrates like squids, jellyfish, octopus, and krill are found in the pelagic zone. Large ocean vertebrates such as crustaceans, sharks, bluefin tuna, sea turtles migrate through the pelagic zone. Pelagic birds live on open seas rather than inland. They feed on planktonic crustaceans, squids, and forage fish. The Pelagic environment consists of the following:

1. Pelagic Invertebrates – Low latitudes invertebrates tend to produce a large number of eggs whereas high latitude invertebrates produce fewer and larger eggs and larger offsprings. 

2. Pelagic Fish – These live in the coastal ocean and lake waters but not on or near the bottom of the sea. These fishes are migratory forage fish. They feed on planktons.

3. Pelagic Reptiles – The pelagic sea snake is the only reptile of one of the 65 species of marine snake to spend its entire life in the pelagic zone. It is helpless on land. Many species of sea turtles spend the first year of life in the pelagic zone and on reaching maturity they move closer to the shore.