Introduction to Oceans and Marine Life
From the window of an airplane or the deck of a ship, the ocean appears to be almost featureless. Beneath the rolling blue waves, however, is a seemingly endless world of mountains, valleys, and diverse organisms engaged in life-and-death struggles. Scientists have learned much about the features and life forms of this vast realm.
The world ocean is divided into three major bodies of water: the Pacific, Atlantic, and Indian oceans. The Pacific and Atlantic are further subdivided into North and South Pacific and North and South Atlantic. The Pacific, Atlantic, and Indian oceans are connected at their southern boundaries by the Antarctic, or Southern, Ocean. Lastly, the Arctic Ocean, which many oceanographers consider to be part of the Atlantic, lies at the northern end of the Earth. Together, the seven ocean divisions--the “Seven Seas”--cover more than 70 percent of Earth's surface and contain about 97 percent of all the water on the planet.
The Pacific is the largest of the three main oceans, with a surface area of 181 million square kilometers (70 million square miles) and an average depth of 3,940 meters (12,900 feet). The basin of the Pacific (the rocky depression that forms the bottom and sides of the ocean) is the most geologically active of the ocean basins. It contains numerous volcanic vents, high seamounts (isolated underwater mountains), and deep trenches (long, narrow valleys).
The Atlantic Ocean has a surface area of 94 million square kilometers (36 million square miles) and an average depth of 3,580 meters (11,700 feet). The floor of the Atlantic basin is dominated by a mountain chain called the Mid-Atlantic Ridge, which runs down the basin's central part.
The Indian Ocean, which lies between Africa, Indonesia, and Australia, is the smallest of the major oceans. It has an area of 74 million square kilometers (29 million square miles) and an average depth of 3,840 meters (12,600 feet).
Ocean Currents
Over time, the waters of the world ocean circulate within each basin and from one basin to another. Ocean waters move in shallow currents, which are driven by the wind, and deep currents, which are produced by the force of gravity. Wind-driven currents usually move horizontally within the uppermost 100 to 200 meters (330 to 660 feet) of water. Along continental coasts, however, upwellings (movements of water from the deep sea to the surface) occur as winds cause surface waters to move farther offshore. Colder, deeper water then rises to the surface to replace the original surface waters.
Wind-driven surface currents move across ocean basins in huge circular patterns called gyres. In the Northern Hemisphere, gyres in subtropical regions move clockwise, while in the Southern Hemisphere, subtropical gyres move counter-clockwise. The direction of rotation is determined by a phenomenon called the Coriolis effect, which results from Earth's rotation. The Coriolis effect causes moving air masses to be deflected in a sideways direction--toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere. The curved paths of the air masses, in turn, drive the circular motion of the gyres.
Ocean currents take on different characteristics depending on their position within the gyres. On the western edges of subtropical gyres, the currents, called western boundary currents, are warm and swiftly moving as they carry water from the tropics or subtropics poleward. The Gulf Stream is an example of a western boundary current. It flows from Florida to the North Atlantic Ocean, where it splits up into several currents, a few of which flow toward Europe.
On the eastern edges of subtropical gyres, water from subpolar regions circulates in cool, sluggish currents called eastern boundary currents. These currents carry water toward the tropics. One of the major Pacific eastern boundary currents is the California Current, which carries cool water southward along the west coast of the United States.
Deep currents, also called thermohaline currents, are movements of water from the surface to the bottom and back. These currents result from changes in water density, which is determined by the water's temperature and salinity (saltiness). Cold water in polar regions is dense, and thus heavier, than warm equatorial waters, partly because water molecules move closer together and become more compacted as water cools. Furthermore, the weight of polar water is increased by salt crystals, which are forced out of sea ice and into the surrounding water as ice forms in winter. This cold, heavy water sinks under the force of gravity. After the polar water sinks, it spreads out slowly toward the equator. As it warms along the way, the water becomes less dense and gradually rises back to the surface in some areas.
The Sea Floor and Seawater
Beneath the ever-moving water lies the seemingly motionless ocean floor. If the basins of the world ocean could somehow be drained, the ocean bottom would look like a rugged, varied landscape, sculpted by volcanoes, earthquakes, erosion, and plate tectonics. Plate tectonics is a geological process in which the rigid pieces--called tectonic plates--that make up Earth's crust gradually shift in position over eons. Thus, even the sea floor moves, if ever so slowly.
As the plates move, they alter the topography (surface features) of the ocean floor. In the deep ocean, at sites where plates move apart from each other, magma (liquefied rock) oozes from cracklike rifts on the sea floor. The magma hardens into volcanic rocks called basalts, which form new oceanic crust on the sea floor. The build-up of new crust produces large, rocky ridges on both sides of the rift. The ridges form an almost continuous underwater mountain range that extends for approximately 60,000 kilometers (37,000 miles) across the Pacific, Indian, and Atlantic basins. The Mid-Atlantic Ridge is a segment of this feature.
Although ridges, trenches, and seamounts mark much of the deep ocean's landscape, extensive parts of the ocean floor are flat, featureless, and covered in a thick layer of sediment. These areas are called the abyssal plains. The sediment builds up from minerals and other material eroded from the continents and from the shells and other remains of tiny sea organisms.
Another sea-floor region lies along the continental margins, the submerged extensions of the continents. A continental margin consists of the continental shelf, the upper edge of the continent; the continental slope, the side of the continent; and the continental rise, a thick layer of sediment at the bottom of the slope. Deep underwater canyons mark the continental slope in a number of places.
The ocean's topography and currents make the sea a unique place, but the characteristics of seawater itself are also important in distinguishing the ocean from lakes and other bodies of fresh water. Seawater differs from fresh water because it contains salts. The most common salt in seawater is sodium chloride (table salt).
Salts alter the basic physical and chemical properties of water. Besides making saltwater heavier than fresh water of the same temperature, salts also lower the freezing point of water. This allows seawater to remain in a liquid state when its temperature is as low as -2 degrees C (28 degrees F).
The Diversity of Sea Life
The sea contains countless life forms found nowhere else on Earth. Biologists classify ocean life into three main groups: the plankton (floaters); the nekton (swimmers); and the benthos (creatures on the sea floor). Plankton drift passively with ocean waves and currents. Many organisms of the plankton are microscopic, including a diverse group of tiny algae known as phytoplankton. These microbes carry out photosynthesis, the use of energy from the sun to combine carbon dioxide and water to make nutrients called carbohydrates.
Small drifting or weakly swimming animals called zooplankton are the primary consumers of phytoplankton. Microscopic shrimplike crustaceans (shelled animals with jointed legs), tiny worms, small jellyfish, and various larvae (immature forms) of larger animals, such as clams, are some of the many types of zooplankton.
The nekton consists of strong swimmers that can propel themselves against currents, waves, and tides. Nektonic animals include most of the marine species that people usually associate with the ocean, including fish, whales, and seals. Most animals of the nekton are streamlined and muscular, with bodies adapted for swimming and diving.
The creatures of the benthos burrow into sediments at the bottom of the sea, attach themselves to rocks and reefs, or move about on or near the ocean floor. Sea anemones, barnacles, oysters, lobsters, snails, sole, and halibut are all part of the benthos. Scientists divide the benthic realm into two subdivisions--the littoral (shallow) and deep-sea benthos.
The ocean's diversity of life is particularly rich in littoral waters, such as tide pools. Many animals in these shallow waters, including sea anemones and barnacles, are passive feeders, organisms that take in food particles carried to them by currents.
In most parts of the deep-sea floor, life is scarce, except for some small types of fish and a few simple organisms, including sponges and crinoids (sea lilies). However, there is a variety of deep-sea benthic life near hydrothermal vents, chimneylike structures along mid-ocean ridges from which hot water and minerals flow. At these sites, microorganisms use the ejected minerals to grow and reproduce, while other creatures eat the microbes or feed on nutrients produced by them. These oases of life in the otherwise barren desert of the deep consist of such animals as worms up to 3.7 meters (12 feet) long that live in tubes and clams as large as 30 centimeters (1 foot) across.
The Ocean Food Web
The benthos, nekton, and plankton are connected via a food web, a web of complex interrelationships in which one organism consumes another. In most instances, the rule is: The higher up the food web, the larger the predator.
At the base of the food web are the tiny phytoplankton, the primary producers of carbohydrates. At the next level are the zooplankton, which eat the phytoplankton and--next step up--are themselves eaten by small fish, crustaceans, and other marine animals. At the top of the food web are the large predators of the nekton, such as sharks and killer whales. Sponges, crinoids, and certain other animals at the bottom of the sea occupy their own niche in the web, feeding on the broken-down remains of dead organisms and other small food particles raining down from upper waters. And in a reverse process that brings everything full circle, upwellings transport waste products and other animal debris from the bottom to the surface, where they serve as nutrients for the phytoplankton.
The Ocean's Bounty
Both the biological and geological features of the ocean provide many valuable resources for humans. Foods, medicines, industrial products, fuel, sand, gravel, minerals, and drinking water are all obtained from the sea.
Fish and shellfish are among the sea's most important commodities, feeding people around the globe. Worldwide, the proteins in fish and shellfish provide up to 17 percent of the total animal protein consumed by the human population.
But marine animal life provides much more than food. For example, scientists are increasingly looking to ocean animals as sources of medicines. A number of chemical compounds derived from marine animals were being tested for medicinal value in 2001. Among these were two substances with anticancer potential--one extracted from a small, soft-bodied animal called a sea squirt, the other from a sponge.
The algae, or seaweeds, of the ocean are also a valuable resource. Consumers in Japan eat more than 20 varieties of seaweed. In many countries, calcium and other minerals are extracted from seaweed for dietary supplements. Seaweeds are also the source of alginates, compounds added to ice cream, beer, salad dressings, cosmetics, paints, and ink to improve their texture.
Many of the seas' resources are nonliving. For example, vast amounts of crude oil and gas come from deposits deep below the sea floor. The Persian Gulf, North Sea, and Gulf of Mexico are prime sources of offshore oil. According to the National Oceanic and Atmospheric Administration, a government agency responsible for managing ocean resources, more than half of the United State's undiscovered deposits of oil and gas lie buried in the country's outer continental shelf.
Besides fuel, the most important geological resources obtained from the ocean are sand and gravel dredged from ocean sediment. Both of these materials are widely used in the construction of buildings, roads, and other structures.
The sea is also an immense potential source of many other valuable minerals. For example, copper, nickel, cobalt, and manganese are buried in seamounts, while iron, silver, gold, and platinum can be found around ocean vents and in ridges. However, the technology required to mine these metals efficiently and profitably had not yet been developed as of 2001.
Yet another potential resource from the ocean is drinking water. Facilities for the desalination (salt removal) of seawater from the Red Sea and Persian Gulf already provide much of Saudi Arabia's drinking water. Although desalination is a very expensive process, some scientists expected that it would become more common in the future as countries in the Middle East and other regions struggle with growing populations and shortages of fresh water.
When one considers the size and importance of the sea to the world, it seems that Earth may have been misnamed. There is much more ocean than land on the planet. The global ocean is an enormous resource, and humanity will undoubtedly continue to depend upon its bounty for centuries to come.