Pelagic Zone Definition
“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.
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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:
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.
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.
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.
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.
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:
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.
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.
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.
FAQs on Pelagic Zone
1. What is the pelagic zone?
The pelagic zone refers to the vast expanse of open water in an ocean, sea, or large lake that is not near the coast or the bottom. Originating from the Greek word 'pélagos' meaning 'open sea', it encompasses the entire water column. Organisms living in this zone are known as pelagic organisms and do not come into contact with the seabed or shore during their life cycle.
2. How is the pelagic zone divided into different layers based on depth?
The pelagic zone is vertically divided into five main layers, primarily based on the penetration of sunlight and increasing depth:
- Epipelagic Zone (Sunlight Zone): From the surface to 200 metres. It receives enough sunlight for photosynthesis.
- Mesopelagic Zone (Twilight Zone): From 200 to 1,000 metres. Sunlight is faint, and photosynthesis is not possible.
- Bathypelagic Zone (Midnight Zone): From 1,000 to 4,000 metres. This zone is in complete darkness and has immense water pressure.
- Abyssopelagic Zone (The Abyss): From 4,000 to 6,000 metres. It is near-freezing, pitch-black, and has extreme pressure.
- Hadopelagic Zone (The Trenches): Below 6,000 metres, found in the deepest oceanic trenches.
3. What are the key characteristics of the pelagic zone?
The main characteristics of the pelagic zone include its vast three-dimensional volume, making it the largest habitat on Earth. It is generally a nutrient-poor environment (oligotrophic), especially compared to coastal waters, because nutrients tend to sink. Key physical factors like temperature, light, and pressure change dramatically with depth, creating distinct layers or sub-zones, each with uniquely adapted life forms.
4. What types of plants and animals are found in the pelagic zone?
Life in the pelagic zone is dominated by organisms that can swim (nekton) or drift (plankton).
- Plants and Producers: Plant life is restricted to the sunlit epipelagic zone and consists mainly of microscopic phytoplankton, such as diatoms and dinoflagellates. These form the base of the marine food web.
- Animals: The zone is home to a diverse range of animals, including zooplankton, jellyfish, squid, and large free-swimming animals like tuna, sharks, dolphins, and whales. In deeper, darker zones, many animals exhibit bioluminescence.
5. What is the main difference between the pelagic zone and the benthic zone?
The primary difference lies in their location. The pelagic zone is the entire column of open water, from the surface to just above the seafloor. In contrast, the benthic zone is the ecological region at the very bottom of the water body, including the sediment surface and sub-surface layers. Pelagic life swims or drifts, while benthic life lives on, in, or attached to the seafloor.
6. How does the pelagic zone in a lake differ from that in an ocean?
While both refer to the open water habitat away from the shore and bottom, there are key differences. The pelagic zone of a lake is much smaller in scale, has less extreme pressure and temperature variations, and is influenced by the surrounding land in terms of nutrient input. The organisms are also different; freshwater fish, plankton, and invertebrates inhabit a lake's pelagic zone, whereas an ocean's pelagic zone supports marine life adapted to saltwater and greater depths.
7. Why is the deeper pelagic zone often nutrient-poor compared to coastal waters?
The deeper pelagic zone is often considered a 'biological desert' because it lacks a consistent supply of nutrients. Unlike coastal areas which receive nutrient runoff from land, the open ocean relies on the cycling of organic matter. When marine organisms in the upper layers die, they sink, taking their nutrients down to the benthic zone. This continuous downward flow, combined with the lack of sunlight for photosynthesis in deeper layers, limits the overall productivity of the deep pelagic waters.
8. How do animals adapt to survive in the extreme conditions of the deep pelagic zones (e.g., bathyal and abyssal)?
Animals in the deep pelagic zones have developed remarkable adaptations to cope with high pressure, total darkness, and scarce food. Common adaptations include:
- Bioluminescence: The ability to produce light, used for attracting prey, deterring predators, or finding mates.
- Slow Metabolism: Conserving energy in a low-food environment by having a slower metabolic rate.
- Physical Features: Many creatures have soft, watery bodies to withstand immense pressure, along with large mouths and expandable stomachs to consume any prey they encounter, regardless of size.
- Enhanced Senses: Since vision is limited, many organisms have highly developed senses to detect pressure waves and chemical traces in the water.
