Biological Zones Found in the Open Sea

Oceanographers divide the ocean into distinct zones, depending on their existing physical and biological characteristics. The pelagic zone encompasses all open ocean zones, and is divided into other regions determined by depth and various aquatic conditions. The vertical component of an aquatic system is called the water column. The physical characteristics involving an aquatic system include temperature, pressure, mineral nutrients, sunlight and oxygen. The distribution of these features varies with depth, and the organisms inhabiting certain zones is determined by how well these organisms have adapted to a given aquatic habitat.

Oceans cover nearly 71% of the Earth’s surface, and are the habitat of about 240,000 known animal and plant species; however, because the depths of the ocean have remained unexplored, it is believed that more than two million species might exist in its profundities. Oceanographers have divided the ocean into different biological zones depending on various factors. Pressure, temperature, and the distribution of oxygen, sunlight and nutrients varies significantly along the water column, and the variations of these elements along the water column determines the kind of marine species that inhabit a particular biological zone.

The pelagic zone

The pelagic zone (open-ocean zone) includes all open ocean regions that are not close to the shore or the bottom of the ocean. The pelagic zone comprises a volume of 1,370,000,000 km³ (330,000,000 mile³), and a vertical range of 11 km (6.8 miles). Biodiversity is found throughout the entire reaches of the pelagic zone, although the number of species decreases with increasing depth. Life in the regional and vertical components of the pelagic zone are determined by the abundance of dissolved oxygen, nutrients, water temperature, sunlight, salinity, pressure and the topographic barriers present in the submarine environment.

Epipelagic zone

The epipelagic zone is the region at the surface of the ocean where sunlight penetrates. At this zone, there is sufficient light for photosynthesis. The surface temperatures may vary from 30 °C (86 °F) in the tropics to temperatures below the freezing point at the poles. The epipelagic zone comprises the zone from the surface to approximately 200 meters (650 ft.) down the water column. There are plenty of plants and animals in this zone, including floating seaweed, plankton, tuna, jellyfish, dolphins and sharks. The epipelagic zone is rich in oxygen and nutrients. The majority of primary production occurs in this zone.

Mesopelagic zone

This zone lies at a range depth of 200-1,000 meters (656-3,300 ft.). Some light penetrates this zone, although, it is not sufficient to permit the process of photosynthesis. In addition, at approximately 500 meters (1640 ft.) oxygen begins to be scarce. This is a problem for some mid-water species which have implemented biochemical and behavioral adaptations in order to survive. Animals living in this zone include the wolfish, the swordfish, squid and some mollusks. Many organisms have developed bioluminescence. Some marine animals will swim to the epipelagic zone at night to feed.

Bathypelagic zone

This zone occupies the region ranging from 1,000-4,000 meters (3,300-13,100 ft.). At this depth, the ocean is completely dark. Animals inhabiting this zone include the Dumbo octopus, marine hatchetfish, the giant squid and some bioluminescent organisms, such as the lanternfish. Most animals in this zone consume organic material (detritus) falling from the upper zones. The sperm whale, which is the deepest diving mammal, may dive as deep as 3 km (1.8 miles) to hunt for the giant squid in the bathypelagic zone. From time to time the dead bodies of large animals, including whales or sharks descend to this zone.

Abyssopelagic zone

This zone is situated at 4,000 meters (13,000 ft.) to just above the ocean floor. Few marine species survive in this zone due to extreme water pressure, cold temperatures and total darkness. Among the few species that can survive in this zone are a few marine arthropods, such as the sea spider, the basket star, swimming cucumber, the sea pig and some species of squid. Many marine animals in this zone have developed special adaptations in order to survive in this zone, including transparency and the lack of eyes as a result to the total darkness which characterizes this biological zone.

Hadopelagic zone

This is the zone lying at the very bottom of the ocean floor, and includes the ocean trenches. It is estimated that only 2% of this zone has been explored by humans, making this zone almost entirely unknown. Life is limited in this zone by the complete darkness, cold freezing temperatures and the lack of food; however, deep ocean explorations have discovered life organisms thriving in hydrothermal vents near volcanic active regions. The water pressure in the hadopelagic zone is enormous. For every 10 meters (33 ft.) down the ocean, one atmosphere of pressure is added.

Open ocean life is diverse. The biological zones host a variety of animal and plant species, although this diversity decreases by increasing depth, and is limited by a number of factors such as sunlight, pressure, temperature, dissolved oxygen and nutrients. According to, the amount of light reaching through sea water and down to the ocean floor is an important factor in the behavior and distribution of plant and animal life.