The world’s oceans cover almost three-quarters of the Earth’s surface. Over 97 percent of the Earth’s water is salt water. The oceans may seem mysterious and inaccessible, but scientists explore the oceanic zone using numerous tools. As the oceans' secrets are discovered, scientists describe the oceans in a variety of ways.
Types of Ocean
Unlike the artificial subdivision of the ocean into the “seven seas,” modern oceanographers consider the ocean as one body of water. This change in thinking developed as researchers learned more about the great conveyor belt, a large current that moves water around the Earth. This current, driven by density differences due to variations in salinity and temperature, travels through the deep and surface waters, ultimately circumnavigating the globe through every oceanic zone. People now realize that, rather than different types of ocean, there is only one world ocean.
Dividing the Ocean
The ocean can be divided into zones based on different sets of characteristics. For example, the ocean can be divided into three zones based on density changes resulting from temperature and salinity variations. The three zones in that classification are the surface or mixed zone, the pycnocline and the deep ocean. Another system describes the neritic or shallow zone, then separates the open ocean or pelagic zone from the ocean bottom or benthic zone. These two zones are then subdivided based on depth. Another way of subdividing the ocean considers how deep light penetrates the ocean.
Oceanic Zones Based on Light
Epipelagic Zone Facts
The surface zone where sunlight penetrates is called the epipelagic zone. The epipelagic zone extends to a depth of approximately 650 feet. This zone, sometimes called the sunlight zone, absorbs most of the visible light that penetrates the ocean. Photosynthesis, which depends on sunlight, only takes place in the epipelagic zone. Phytoplankton are microscopic oceanic plants that use photosynthesis to produce food. Phytoplankton form the base of the food chain for most marine life. Phytoplankton also produce much of the oxygen in the atmosphere, making them a critical factor for all animal life.
The epipelagic zone tends to be the warmest layer of the ocean. Swimming, fishing, beach combing and other activities let people interact with the plants and animals in this zone. Familiar epipelagic plants and animals include corals, kelp, manatees, jellyfish, crabs and lobsters. Fish with lunate or crescent-shaped tails tend to live in the epipelagic zone. Many animals in the epipelagic zone are fast moving, transparent or small, all adaptations to avoid being eaten.
Because the epipelagic zone is accessible, people tend to consider the entire ocean based on epipelagic zone facts. The deeper layers, however, hold their own fascinating secrets.
Mesopelagic Zone Facts
The second layer of the ocean is the mesopelagic or twilight zone. The mesopelagic zone extends from the bottom of the epipelagic, about 650 feet, down to about 3,300 feet. Water temperature in this zone doesn't change much with the seasons but ranges from 70 °F to near freezing, depending on latitude and depth. Some sunlight penetrates this zone, but not enough for photosynthesis. About 20 percent of the food production from the epipelagic zone drifts down into the mesopelagic zone. Food in the mesopelagic layer is scarce, however. Some organisms in the mesopelagic zone exhibit bioluminescence, meaning living light. Some bioluminescent structures are used as lures for food while others seem to be used to communicate and for mating rituals. Some familiar animals found in the mesopelagic zone include angler fish and swordfish.
Bathypelagic or Aphotic Zone Facts
From about 3,300 feet down to about 12,000 feet is the bathypelagic or aphotic (no light) zone, sometimes referred to as the midnight zone. No light reaches this zone so there are no plants other than bits and pieces that may float down. However, only 5 percent of the material produced in the epipelagic zone reaches the bathypelagic zone. This zone's temperature remains cold, barely above freezing. The pressure from the overlying water column means people require special equipment to visit this zone. Animals in the bathypelagic zone tend to have more water in their tissues, less developed muscles and softer bones. Bioluminescent features are common. Only about 1 percent of the ocean's animals live here. Residents of the bathypelagic zone include giant squid, vampire squid, anglerfish, deep water corals and slime stars.
Abyssopelagic Zone Facts
Below the bathypelagic zone lies the abyssopelagic zone. This zone extends from about 13,000 to 19,700 feet. In much of the ocean this zone reaches the ocean floor. This deep ocean environment is permanently dark. The pressure in the abyssopelagic zone ranges from 401 atmospheres at the top to 601 atmospheres at the bottom. Like the bathypelagic zone, the temperature remains just above freezing, at about 39 °F. Despite these extreme conditions, life exists in the abyssopelagic zone. Crabs, worms and flatfish can be found where the abyssopelagic zone encompasses the ocean floor.
Hadopelagic Zone Facts
The deepest portion of the ocean lies in the deep trenches: the hadopelagic zone, also called the hadalpelagic zone. This zone lies below 19,700 feet. The pressure in the deepest hadopelagic zone, at the bottom of the Marianas Trench, is over 1,000 times the atmospheric pressure at sea level. The hadalpelagic zone temperature hovers just above freezing. Even so, life still can be found there. At undersea vents, an ecosystem based on chemosynthesis abounds with crabs, tube worms, bacteria and fish. Elsewhere, crabs, worms and demersal fish are among the residents of the deepest trenches.
Food and Migration
Scarcity of food in the lower levels of the ocean means that some organisms move vertically between zones every day. This is called diel migration. Other organisms move freely horizontally and vertically, feeding where convenient. The blue whale, the largest animal ever known, eats tiny krill in the epipelagic zone, feeding in the colder, krill-rich waters near the poles before migrating to warmer waters to give birth. Some organisms, however, are so well-adapted to their oceanic zone that they can never leave.