Oceans
Oceans
Oceans are huge bodies of salt water that are extremely deep and which cover about 71 percent of Earth's surface. As the largest home for life on Earth, oceans contain a diverse set of environments that support a tremendous variety of animals and plants. All of the world's oceans are linked together, although they do not makeup one uniform environment.
Nearly three-quarters of Earth's surface is covered by oceans or sea-water. To a scientist, an ocean is different from a sea. To qualify as a sea, a body of water must be large enough to have an effect on its environment, and usually be landlocked. An area is only considered an ocean if it is at least 6.562 feet (2,000 meters) deep. According to this, there are, therefore only six oceans: the Arctic, the North Atlantic, the South Atlantic, the North Pacific, the South Pacific, and the Indian Ocean.
All of the Earth's oceans are connected to one another, meaning that a person could actually go completely around the world and never touch land. All oceans are salty because seawater is made up of dissolved inorganic substances, of which sodium and chloride are the two largest, forming about 85 percent. Together, sodium and chloride form salt. All oceans also have tides (the regular rise and fall of their water level). These are caused mostly by the gravitational pull of the moon that moves around Earth. All oceans also have currents or a stream of water moving in one direction both at their surface and deeply below it. The currents usually are caused by the wind and the differences in water temperature. Oceans lose water by evaporation but regain it by precipitation (rain) and from rivers that drain into them.
PARTS OF AN OCEAN
The marine or ocean environment is divided into two major parts. The pelagic environment makes up all of the ocean water, and the benthic environment makes up the ocean floor. The pelagic environment is further subdivided into what are called provinces. The neritic province is from the shoreline out to open ocean of about 656.2 feet (200 meters) deep. The oceanic province is the ocean at a depth greater than 200 meters. No ocean is uniform in any of its features, and there are enormous differences between its several different zones. For example, in coastal areas close to shore, the primary producer, or the first level on the food chain (the transfer of energy in an ecosystem), is the phytoplankton (singlecelled algae that live by photosynthesis). On the surface of the open ocean, however, the primary producers are the macroscopic algae, also known as seaweeds. Differences in the amount of light, as well as pressure and temperature, affect what will grow or thrive in which zone.
The Littoral Zone. Starting at the shoreline, or the littoral zone, where the land and ocean come together, high levels of light and nutrients make this a productive area. Since the water is so shallow, most organisms lie on or in the seabed, which may be rocky, sandy, or muddy. Rocky shores allow seaweeds to anchor themselves firmly and also provide protection for tiny animals.
The Continental Shelf. Extending outward from the littoral zone is the continental shelf, which is really just an underwater extension of the land. This area is by far the most productive part of any ocean. The ocean above the continental shelf, which extends from shore anywhere from 40 to 200 miles (64.4 to 321.8 kilometers), is fairly shallow compared to the rest of the ocean. It is usually in this shallower part of the shelf that tuna, porpoises, and sharks are found because that is where most of the fish they eat are found. The richness of the shelf is due to the light it receives as well as to the nutrients that rivers often pour into it. As the shelf gradually slopes away from shore, it eventually falls off rather sharply. This eventually leads to the abyssal zone that can reach an incredible depth of 7 miles (11.3 kilometers) in some places. Since light cannot penetrate below a depth of about 650 feet (198.1 meters), this depth is where the aphotic (no light) zone begins.
The Aphotic Zone. In such a deep ocean habitat, there is little life because there is no sunlight, the pressure is enormous, and the temperatures are constantly cold (barely above freezing). The few fish that do inhabit this dark zone have adaptations like huge jaws that allow them to swallow another fish whole and are often able to generate some light of their own. It was once believed that nothing could live below 1,800 feet (548.6 meters), but the fairly recent discovery of activity around the hydrothermal vents showed this not to be true. During the 1970s, new underwater technology allowed scientists to penetrate to great depths and they came upon "vent communities," or areas around underwater volcanoes that were teeming with life. This heated water was loaded with minerals that kept special bacteria as well as clams, crabs, and fish alive.
As with many other natural habitats, oceans also are threatened by humankind. Pollution of the oceans is usually a direct result of human activities, as oceans have long been a dumping ground for waste and land runoff. Recently, contamination from oil spills has become the most severe threat to their health. Overfishing also is becoming more of a threat. Human technology has created floating fish factories that in the long run may seriously endanger entire species. If something is not done to protect the oceans, people may soon lose the largest and most fascinating environment on Earth.