Ocean Pelagic Zone

Amy Halstead and Hannah Basham

Ocean Pelagic Zone

The Ocean Pelagic Zone, sometimes referred to as the Open Ocean Zone, is comprised of nearly the entire ocean beyond the costal region. The word pelagic comes from the Greek word pélagos, meaning open sea. This region is purely comprised of saltwater, contrasting it from the freshwater zone. Although warm water currents run through the Pelagic Zone, the water is generally cold: The average temperature of the warmest subzone, the Epipelagic Zone, is 17°C, or 68°F.

Subzones

The Pelagic Zone is divided into four subzones:1) The Epipelagic Zone2) The Mesopelagic Zone3) The Bathypelagic Zone4) The Abyssopelagic Zone

I. Pelagic - (1) Epipelagic (2) Mesopelagic (3) Bathypelagic (4) AbyssopelagicII. Benthic - (5) Littoral, Sub-littoral (6) Bathyal (7) Abyssal (8) Ultra-abyssal | Note: depths are in meters

Epipelagic Zone

The Epipelagic Zone is called the Sunlit Zone because it is the topmost layer, comprised of the surface and waters a small depth below; therefore, the sun penetrates the water enough for photosynthesis to occur. Because of the vastness of the Ocean Pelagic Zone, about half of the world’s photosynthesis occurs in this zone. The presence of sunlight in this zone and the abundance of plankton causes most of the life found in the Ocean Pelagic Zone to be found in this subzone.

Mesopelagic Zone

The Mesopelagic Zone, also called the Twilight Zone, extends from about 200 meters below the surface to 1,000 meters. Vision in this subzone is limited: Little sunlight penetrates through to this zone, an insufficient amount for photosynthesis. In this depth range, oxygen is completely depleted from the water, but organisms have adapted with highly efficient gills and minimal movement. At this level, many of the organisms are able to produce light themselves, or a bioluminescent.

Bathypelagic & Abyssopelagic ZoneThe Bathypelagic Zone (Midnight Zone) is completely

dark. No plant life can exist here and many of the organisms that live here eat falling detritus from above subzones or prey on other organisms in this subzone. Some of the larger organisms are prey to deep diving sperm whales.

The Abyssopelagic Zone (Lower Midnight Zone) was once thought to be bottomless. Few organisms can successful survive in the harsh environment, and many of those that can have adapted to be transparent and eyeless due to the extreme darkness.

Bathypelagic & Abyssopelagic Zone

Pelagic PlantsThe primary plant in the pelagic zone is phytoplankton,

a microalgae, fuelled by photosynthesis. Other plant types are:

• Diatoms, single-celled phytoplankton• Dinoflagellates, microscopic, single-celled algae• Seaweeds, these plants float across the surface of

the ocean providing nutrients to other organisms.

Temperature & Pressure

The temperature in the Pelagic Zone varies greatly depending on the depth, with the Epipelagic Zone reaching up to 26°C and the Bathypelagic Zone falling to temperatures as low as 2°C. Because of the depth and water, pressure increases by great amounts in each zone. Because of the variability of the atmospheric conditions in the Ocean Pelagic Zone, many of the organisms cannot travel between subzones due to the environmental conditions.

LightThe amount light in each subzone varies because

sunlight is unable to penetrate the waters deep enough to reach all levels.

• The Epipelagic Zone is the only subzone in which enough light is available for photosynthesis.

• Some sunlight reaches the Mesopelagic Zone and allows certain organism to hunt if their eyesight is strong enough.

• The Bathypelagic and Abyssopelagic Zones are too deep for any sunlight to reach; however, some light may be available from bioluminescent organisms.

Depth & Location

The Ocean Pelagic Zone reaches down to a depth of 11 km (6.8 mi) at the lowest point and has an average (mean) depth of about 3.68 km (2.29 mi). The depth plays an important role: As depth increases, the abundance of life decreases.

Water covers over 70% of Earth’s surface, with the largest contributions to this fact being the oceans. Because of this, the Ocean Pelagic Zone can be found almost anywhere where there are not masses of land.

Water TypeThe water in the Ocean Pelagic Zone is always salty.

The salinity of the water ranges from 6 – 40 ppt. The range can be attributed to evaporation rates, sea ice formation, and freshwater supply rates.

Nutrients

Certain vital nutrients in the Ocean Pelagic Zone are in small supply. The following elements, called limiting nutrients, are recycled through marine plants:

• Nitrogen• Phosphorus• Calcium• Silicon• Iron• Copper

• Magnesium• Zinc

Animals

There are five main types of animals that live in the Ocean Pelagic Zone:

• Birds• Invertebrates• Mammals• Fish• Reptiles

Pelagic Birds

Pelagic birds are birds that live on the open sea rather than land or waters close to land. These birds feed on crustaceans, forage fish, and squid.

• Atlantic Puffin• Macaroni Penguin• Sooty Terns• Shearwaters• Procellariiformes

Pelagic Invertebrates

Pelagic invertebrates play a huge role in the Ocean Pelagic food chain. Photosynthetic zooplankton are included in this group which are base of the food cycle. Other invertebrates include:

• Jellyfish• Crustaceans (crabs, lobsters, prawns)

Pelagic Fish & Reptiles

Fish are by far the most abundant type of animal in the Pelagic Zone. A majority of the fish live relatively near the surface.

Pelagic reptiles include sea turtles and some species of sea snakes.

Pelagic Mammals

The Pelagic Zone is home to many mammals, one of which is the blue whale—the largest mammal on earth. The three main types of mammals found in the Pelagic Zone are whales, dolphins, and porpoises. Because the oxygen concentration is much higher near the surface, all Pelagic mammals live in the upper region.

Animal Adaptations

The animals that live in the Pelagic Zone face conditions that are very different from many other parts of the world, and therefore have adapted to this environment:

• Mammals lungs collapse so they can dive down to greater depths.• Deep sea fish have lung-like swim bladders to control their

buoyancy.• Fish have developed specialized functions to keep their bodies from

equalizing saltwater through osmosis.• Fish have blubber to protect them from the cold, as well as a

chemical in their blood to keep it following• Organisms that live in the deep sea have adapted so they can with

stand the pressure, darkness, and cold—as well as the scarcity of resources.

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http://www.untamedscience.com/biology/world-biomes/pelagic-biome• "The Structures and Adaptations to Marine Living - MarineBio.org". MarineBio Conservation Society. Web. Saturday,

October 12, 2013.• Woodward, Susan L. "Life Zones in the Ocean." Biomes of the World. Radford University, 2012. Web. 13 Oct. 2013.