Introduction to The Geography of Oceans
Ocean, the continuous body of saltwater that covers about 70 per cent of the earth's surface. The term “ocean” also refers to any of the four major subdivisions of this body. These are, in order of size, the Pacific, Atlantic, Indian, and Arctic oceans. The Pacific Ocean contains about half of the total ocean area and is larger in area than all the world's land combined. The oceans account for 97 per cent of the earth's water area.
The Equator divides the Atlantic and Pacific oceans into North and South Atlantic and North and South Pacific. Occasionally, but incorrectly, the southernmost part of the Atlantic, Pacific, and Indian oceans is called the Antarctic Ocean.
“Sea” is a term commonly used to designate (1) the ocean, (2) a subdivision of the ocean, or (3) a salt lake lacking an outlet to the ocean (for example, the Dead Sea and the Caspian Sea). In this article the word “sea” means the same as “ocean” unless otherwise specified.
Under international law a nation owns its territorial (coastal) waters, which extend 12 nautical miles beyond its coast. (One nautical mile is equal to 1.151 statute [land] miles or 1.852 kilometers.) All ships may move freely outside that area. Each nation also has exclusive rights to all marine life in waters extending 200 nautical miles beyond its shores. The use and exploitation of the ocean is governed by the Law of the Sea Treaty (1982).
Oceanography (or oceanology) is the science covering all aspects of ocean study and exploration. It draws on the sciences of botany, zoology, meteorology, physics, chemistry, geology, fluid mechanics, and applied mathematics. Marine biology, biological oceanography, is the branch of biological science concerned with plant and animal life in the sea. Many of the problems associated with the ocean are so complex that they must be studied by a team of specialists in various sciences.
Importance of the Ocean
Since ancient times, the ocean has served as a source of such food as fish and shellfish. Rock strata beneath the ocean floor are an important source of petroleum and natural gas. Seawater is a source of sodium chloride and other salts. Seawater from which salt has been removed is used for drinking and for agricultural and industrial purposes.
The ocean plays an important role in the water cycle, the constant movement of water from the atmosphere to the earth's surface and back. The ocean and its currents influence the climate of many areas.
A wide variety of organisms, including algae, jellyfish, lobsters, fish, sea turtles, and whales, live in the ocean. Many of these organisms are threatened by pollution. Among the causes of this pollution are the discharge of municipal and industrial wastes, and the accidental spilling of oil from tankers and offshore oil rigs. A number of laws and international agreements ban or restrict the disposal of various pollutants into the ocean.
Properties of Seawater
Seawater is ordinary water (H2O) containing dissolved mineral salts in an average concentration of 3.5 per cent by weight. Seawater also contains numerous organic and inorganic particles in suspension (undissolved). The mineral salts come from eroded rocks. Much of the rock debris resulting from erosion is ultimately carried to the sea by rivers, glaciers, and winds, and thus several billion tons of salt are added to the sea each year.
About 68 per cent of the salt in seawater is sodium chloride, or ordinary table salt. There are lesser amounts of magnesium chloride (14 per cent), sodium sulfate (11 per cent), calcium chloride (3 per cent), and other salts.
The amount of dissolved salt in seawater, expressed in parts per thousand by weight, is referred to as its salinity. The average salinity of seawater is 35. The freezing point of seawater having this salinity is 28.6° F. (-1.9° C.) compared to 32° F. (0° C.) for freshwater.
The density of seawater varies somewhat with salinity, temperature, and depth. At normal atmospheric pressure, seawater has a density that is typically about 3 per cent greater than that of freshwater. This greater density makes it easier to float and swim in seawater than in freshwater.
A person should not try to drink seawater, because the dissolved salts will increase, rather than decrease, thirst. Since the body can excrete only a small excess of salt, most of the salt accumulates in the body, withdrawing water from the blood and tissues. Large quantities can make a person violently ill and can cause death through dehydration (drying out of the body tissues).
Rainwater, which comes primarily from the ocean surface by evaporation, contains no dissolved salts because the salts do not evaporate; they remain in the ocean.
Seawater samples are obtained using various devices, such as the Nansen bottle, a cylindrical container with a lid at each end. Both lids are open during the time the bottle is being lowered on a cable, so that the seawater flows through the bottle. When the Nansen bottle has reached the desired depth, a weight is sent down the cable. When this weight reaches the bottle, it strikes a tripping mechanism that closes the lids and thus traps the seawater inside. Some types of sampling containers have lids that are closed by remote control. An apparatus called a rosette has several such sampling containers; each container can sample water at a different depth. A rosette also has sensors for measuring salinity, depth, and temperature. Seawater samples are analyzed for their free-oxygen content, the species of phyto-plankton present, and various other characteristics.
The pressure exerted by seawater increases with depth. At the ocean's surface, the pressure equals the pressure of the atmosphere above it, normally 14.7 pounds per square inch (101 kPa). The pressure increases by about this amount with every 33 feet (10 meters) of depth. At a depth of 66 feet (20 meters), for example, the total pressure is 14.7 + 29.4, or 44.1, pounds per square inch (304 kPa).
Experienced divers have descended unaided to depths of about 350 feet (107 m), and to about 435 feet (133 m) with tanks of compressed air strapped to their backs. The record for a diver in a helmeted diving suit is 728 feet (222 m). In 1960, the U.S. Navy bathyscaphe Trieste, with two men inside, reached a record depth of 35,802 feet (10,912 m), where the pressure was about 8 tons per square inch (110,000 kPa).
Depths
Ocean depths are expressed in fathoms (1 fathom = 6 feet or 1.8 m) as well as in meters or feet. Large sections of the sea are 5,000 to 15,000 feet (1,524 to 4,572 m) deep, with the average near 12,500 feet (3,810 m). The floor of some ocean basins may lie 18,000 to 20,000 feet (5,486 to 6,096 m) or more beneath the surface. The greatest depths, however, occur not in the central portion of the ocean but in trenches—long, narrow, deep cracks in the ocean bottom that are usually found near continents and on the seaward side of island chains.
The greatest known depth of any ocean is in the Challenger Deep of the Mariana Trench in the Pacific Ocean, about 250 miles (400 km) southwest of the island of Guam. It was discovered in 1951 by the British survey ship Challenger, which reported a depth of 35,760 feet (10,900 m). Later expeditions from various countries have reported even greater maximum depths. Recorded echo soundings indicate a maximum depth of about 36,000 feet (10,976 m). It was in these same waters that the Trieste made its record descent in 1960.
Submarine Topography
The shape of the ocean floor is known to scientists as submarine topography. The floor contains mountains, plains, canyons, plateaus, basins, and other topographic features that are found on land. Usually, the ocean bottom is divided into three major zones: the continental margin, the ocean-basin floor, and the mid-ocean ridge.
Continental MarginImmediately offshore from the continents and some large islands is the continental shelf. It is a slightly inclined platform under shallow seas and is composed primarily of sediments. The shelf, which varies greatly in width, terminates on its seaward side with a sharp decline, called the continental slope. At the slope's base lies the continental rise, an area where the decline is much less pronounced. In some areas, such as the west coast of Central and South America, there is no continental rise; instead, there is a trench.
Other topographic features are also found along the continental margin. There are submarine canyons, deep V-shaped depressions found mainly in the continental slope, and marginal plateaus, steplike formations at the edge of the continental shelf. Also present, mainly on the shelf, are barely submerged features, such as banks, shoals, and reefs.
Ocean-Basin FloorBeyond the continental margin, as far as the mid-ocean ridge, is the ocean-basin floor. It lies generally at great depths and is marked by various topographic features. Ridges, resembling low mountain ranges, and rises, elongated portions of the ocean floor with smooth, gentle slopes, are widespread. Abyssal plains are large, almost perfectly flat areas, usually near a continental rise. Adjoining many of the plains are expanses of abyssal hills, submerged volcanic peaks that rise less than 3,000 feet (914 m) above the ocean floor. Seamounts are submerged volcanic peaks that rise more than 3,000 feet above the ocean floor. Similar peaks with flat tops are called guyots, or tablemounts. Volcanic islands are peaks that rise above the surface of the ocean.
Mid-Ocean RidgeBeneath the sea, in some places about midway between the continents, is a vast mountain chain that is almost continuous around the world. It is some 40,000 miles (64,000 km) long, up to 2,500 miles (4,000 km) wide, and generally 6,000 to 12,000 feet (1,829 to 3,658 m) high. In a few places, such as the Azores, volcanic peaks along the ridge protrude above the surface as islands. Numerous fracture zones cut across the ridge, creating unusually irregular topography. One deep fracture, called the rift valley or mid-ocean rift, runs virtually the entire length of the ridge along its crest. According to the theory of plate tectonics, the rift forms the boundary between sections of the earth's crust that are moving apart. New sea-floor material is gradually formed along the rift as the plates diverge.
IslandsThe ocean contains thousands of islands. Continental islands, such as the British Isles, Vancouver Island, and Sri Lanka, once were joined by land to nearby continents. Oceanic islands, or islands that rise from the ocean-basin floor rather than from the continental margins, are usually of volcanic origin. Mauna Loa, an active volcano in Hawaii, is taller than Mount Everest if its height from submerged base to peak is considered. It rises 13,680 feet (4,170 m) above sea level, and its undersea height is about 18,000 feet (5,486 m).
Marine SedimentsThe ocean bottom is covered with sediments. Sediments are derived from four kinds of materials: rock, decomposed sea creatures, minerals in sea-water, and debris from meteors. In some places the sediment layer is 6,000 feet (1,800 m) thick. Sediments composed mainly of the remains of microscopic sea organisms are called oozes.
By studying marine sediments, oceanographers can obtain information on the history of the earth, including the nature of its climate millions of years ago. Sediment samples are obtained with coring devices, which are driven into the ocean bottom by long drills suspended from surface ships. Under-sea robots with mechanical arms that can scoop up sediment samples have also been developed.
SoundingThe measuring of ocean depth is known as sounding. By taking a number of soundings over a given region of ocean, oceanographers can identify ocean-bottom features. An early method of sounding consisted of tossing overboard a line with a weight on its end and noting the length of line that was payed out before bottom was reached.
Today, sounding is usually done with an instrument called an echo sounder, a form of sonar. The echo sounder contains a transmitter that sends sound waves to the ocean bottom and a receiver that intercepts the sound waves reflected from the bottom. The depth is equal to half the distance sound travels in the time interval between the sending and receiving of the sound waves. The receiver is usually connected to a continuous recorder, so that, as the ship proceeds, an uninterrupted record of depth is traced on a paper chart.
A laser sounder is similar to an echo sounder, but it uses light waves instead of sound waves. It transmits laser beam pulses toward the ocean bottom and detects light that is reflected from the bottom.
Bottom PhotographyPhotographs of the ocean bottom can be taken with cameras that are installed in submersible craft or suspended by cable from surface vessels. Waterproof television cameras can also be lowered into the ocean by cable, and the closed-circuit television pictures viewed on a screen aboard ship.
Transmission of Energy In the Ocean
LightWhen sunlight (which contains all colors) strikes the ocean surface, the water molecules scatter the light rays. This scattering causes the water to appear blue because the blue light in the sun's rays is scattered more than light of other colors.
The appearance of the water is also affected by plankton. If there is enough plankton near the surface of the water, the combined effect of the water molecules and plankton will cause the water to appear green, gray, or even yellow. Coastal waters, which contain relatively large amounts of plankton, are usually green, while mid-ocean waters are blue. A species of alga that lives near the surface of the Red Sea occasionally gives a red color to the surface of that body of water.
Nearly all of the sunlight that strikes the ocean is absorbed within 600 feet (183 m) of the surface. A small amount of blue-green light penetrates considerably deeper—down to about 2,000 feet (610 m) if the water is sufficiently clear. Beyond the maximum depth of sunlight penetration, the only illumination is that produced by luminescent sea creatures.
HeatThe Persian Gulf, which has an average depth of only 200 feet (61 m), contains the world's warmest ocean water. Surface temperatures of 96° F. (36° C.) have been recorded there. Over most of the ocean, however, the surface temperature ranges from about 86° F. (30° C.) at the Equator to about 29° F. (-1.7° C.) near the poles. This general decrease from Equator to poles is greatly modified by ocean currents, however. Both the Arctic and Antarctic oceans contain icebergs and sea ice throughout the year.
The heat rays (infrared radiation) from the sun are absorbed by seawater much more rapidly than the light rays. Only a very small amount of solar heat radiation penetrates more than three feet (90 cm) below the surface. Usually, the water temperature decreases gradually with depth. At depths below 10,000 feet (3,048 m) the water temperature is only a few degrees above the freezing point, even in the tropics.
When water near the ocean's surface absorbs a large amount of heat from the sun, a thermocline may form between this surface water and the water below. A thermocline is a layer of water in which the temperature decreases with depth more rapidly than it does in the water above or below. A thermocline acts as a barrier to the mixing of the warmer water above with the cooler water below. In temperate areas, thermoclines usually occur only during summer; in tropical areas, thermoclines are usually permanent.
Water temperatures down to depths of about 900 feet (274 m) are usually obtained with a bathythermograph. This instrument provides a continuous, permanent record of water temperature in relation to depth while it is being lowered on a cable. The record is scratched on a smoked-glass slide by a temperature-sensitive stylus. At depths below 900 feet, temperatures are obtained with electrical and mercury-in-glass thermometers.
SoundThere are many sources of sound in the ocean, including marine animals, wave action, and ship engines and propellers. In general, sound waves travel through ocean water more readily than light waves do. For this reason sound is useful in detecting and locating objects underwater. In addition to its use in echo sounders, sound is used for navigation, locating schools of fish, and detecting submerged submarines. The methods (and equipment) that make it possible to use sound for these purposes are known as sonar.
The speed of sound in the ocean is approximately 4,990 feet (1,520 m) per second, about 4 1/2 times the average speed of sound in air. The exact speed depends on the temperature and pressure of the seawater, and therefore varies with depth.
The minimum speed of sound in the ocean occurs around a depth of 3,280 feet (1,000 m). The layer of water at this depth acts as a channel for sound waves, because sound waves leaving the layer are refracted (bent) back towards it. Loud sounds of low frequency produced in this channel have been detected by hydrophones (underwater microphones) halfway around the world.
Ocean Currents
The ocean is an enormous body of about 317,000,000 cubic miles (1,321,000,000 km 3) of water, all of which is more or less in motion. Chief among the sea's movements are the currents. They occur as seasonal and permanent streams flowing horizontally at the surface and at deep levels; they also occur as vertical movements, with water upwelling to the surface or surface water sinking toward the ocean bottom. The currents best understood are the surface currents, many of which have long been studied and used, especially in navigation.
Causes of CurrentsMany interacting forces cause currents. One of the prime causes particularly of surface currents, is wind.
As the wind blows over the sea, part of its energy is transferred to the water, which is dragged along as a current. As a result of the earth's rotation, however, such a current does not move directly downwind. The earth's rotation produces an effect known as the Coriolis force, which deflects winds, ocean currents, and moving bodies to the right of a straight-line course in the Northern Hemisphere and to the left in the Southern Hemisphere. This effect is greatest at high latitudes and weakest in the tropics.
Ocean currents are generally deflected more than the winds that cause them, because the ocean currents have less speed. Thus the paths taken by the surface currents are only roughly similar to those of the prevailing winds.
Unequal heating of the sea by the sun is another major cause of the ocean's circulation, particularly in the deep layers. In polar regions, especially during winter, extremely cold, salty, and dense water sinks far below the surface and moves toward the Equator in very slowly moving layers. At the same time, warm and less dense tropical water in the ocean's upper layers moves toward the poles. Other factors also influence the ocean's circulation. Among them are the shape and relative positions of the continents; the presence of barrier-like island chains; and local winds.
For such varying reasons, some of the currents are strong and relatively steady, while others are weak and erratic. To varying degrees, they all wax and wane with the seasons, mainly because of the shifting of the winds. Currents also meander and shift their courses from year to year.
Major Surface CurrentsExcept in polar areas, the sea's main surface currents flow in giant swirls north and south of the Equator. (See map, Major Surface Currents.) They turn clockwise in the Northern Hemisphere and counterclockwise in the Southern. The waters near Antarctica move clockwise in a wide band around the ice-covered continent as the West Wind Drift. In the Arctic Ocean, the water swirls and moves mainly into the North Atlantic around Greenland after crossing from the Siberian coast.
Numerous currents make up the swirls; only the major ones are considered here. In the equatorial waters of the Pacific, Atlantic, and Indian oceans are two westward-moving currents—the North Equatorial and the South Equatorial currents. Between them and flowing in the opposite direction is the Equatorial Countercurrent. Although they differ greatly, all three currents are relatively strong in the Atlantic and particularly strong in the Pacific. In the Indian Ocean, they are seasonally disrupted by monsoons.
At the western edge of each of the oceans, the equatorial currents turn poleward and flow along adjacent continents. Three of the flows intensify and become exceptionally strong, often attaining speeds up to four miles per hour (6 km/h). They are the Gulf Stream in the Atlantic, the Kuroshio (Japan Current) in the Pacific, and the Agulhas Current in the Indian Ocean. The others, the Brazil and East Australia currents, are relatively weak.
The currents veer from the coasts and move eastward into the open ocean. Here, all of the flows become sluggish and are often called drifts rather than currents. In the north, these slow-moving flows include the North Atlantic and North Pacific currents and their branches, the Norwegian and Alaska currents. Converging with them are cold currents from the Arctic, notably the East Greenland and Labrador currents in the Atlantic and the Oyashio (Okhotsk, or Kamchatka, Current) in the Pacific. In the Southern Hemisphere, which at comparable latitudes is nearly all water, the currents merge with the broad West Wind Drift and have no individual names.
The currents veer again and return to the Equator by flowing along the west coasts of continents. In the Atlantic are the Canaries and Benguela currents; in the Pacific, the California and Peru (Humboldt) currents; and in the Indian Ocean is the West Australia Current.
Because of the circular flows of the currents, large eddies of relatively quiet water, such as the Sargasso Sea in the Atlantic, exist at the center of the loops. Beneath the surface currents, at varying depths and moving opposite to them, are various subsurface currents. Examples include the Cromwell Current in the Pacific and the Gulf Stream Countercurrent in the Atlantic.
Effects of CurrentsBy their constant movement of great masses of water, both warm and cold, the currents play a major role in determining the climate of coastal areas. Warm currents off the coasts of western Europe, southern Alaska, and Japan, for example, temper what would otherwise be a harsh, inhospitable climate. Cold currents off the coasts of Peru, Ecuador, and the western United States have a definite cooling effect on shore.
Aridity, heavy precipitation, and much foggy weather are in some areas consequences of offshore currents. Currents also bring icebergs from polar regions into shipping lanes, where they menace ships.
Besides influencing climate, currents affect the fertility of the sea—its ability to support marine life. The basic food of fish is plankton, which depends mainly on nutrients from the ocean's depths. Regions of strongly upwelling water, such as those off the western coast of South America, abound in fish. Where waters are relatively quiet and warm, there is little marine life.
Ocean Waves and Tides
Winds that blow over the ocean create surface disturbances called waves. The greater the wind speed, the larger the wave. Waves 50 feet (15 m) high are not unusual during severe storms.
Surface waves are of two types: sea waves, choppy waves driven by the wind; and swell, uniform waves occurring in areas where there is no wind. After sea waves pass out of a windy area, they develop into swell.
Swell can travel a great distance from the area of the ocean where it originated. (As in all other types of wave motion, it is the disturbance, rather than any material substance, that travels.) The breakers and surf occurring near coastlines are formed from swell.
Waves that result from dislocations of the ocean floor are called tsunamis. These waves are long and can cause great destruction in coastal areas. They are sometimes called tidal waves, although they have no relation to tides.
In addition to currents and waves, a third important type of ocean-water movement is the tide. Tide is the alternate rising and falling of the ocean surface due to the gravitational pull of the moon and the sun. Gulfs, bays, and rivers connected with the open ocean are also subject to tides.
Marine Life
Types of Marine LifeWhat living organisms are found in any one place in the ocean is determined by the environment at that place: temperature, pressure, and the amounts of light, salt, and turbulence in the water. Marine biologists have classified ocean-dwelling organisms into three main types: plankton, those that do not swim, or swim weakly; nekton, those that swim; and benthos, those that are bottom-dwelling.
Plankton and nekton live in the pelagic zone; that is, away from the ocean bottom and shore. Most pelagic organisms are found over the continental shelves, where food is plentiful. The pelagic zone is also the habitat of certain birds, such as the albatross and storm petrel, and the small group of marine insects that includes the marine water strider.
Benthos live on the bottom in zones determined by depth; in the intertidal, or littoral, zone (from the high-tide line to the low-tide line); in the sublittoral zone (from the low-tide line to about 650 feet [200 m] deep); in the bathyal zone (from about 650 feet deep to about 9,840 feet [3,000 m] deep); in the abyssal zone (from about 9,840 feet deep to about 19,680 feet [6,000 m] deep); and in the hadal zone (below 19,680 feet).
Organisms living in the intertidal zone are adapted to being repeatedly exposed to air and to the action of waves. Most marine life is found in the sublittoral zone on the continental shelves, where there is enough light for photosynthesis. In deeper zones there is little or no light. Animals in these zones have such adaptations as light-producing organs, high sensitivity to pressure and vibrations (especially in blind forms), and very sensitive eyes.
PlanktonThese organisms are drifters, carried from place to place by wind, waves, and current. Phytoplankton consists mainly of single-celled algae, such as diatoms, peridinians, and coccolithophores. These organisms make their food by photosynthesis.
Zooplankton includes a wide variety of microscopic organisms—protozoans, such as foraminifers and radiolarians; crustaceans, such as copepods, cyclops, and water fleas; and rotifers. Zooplankton also includes such larger organisms as argonauts.
Organisms that are zooplankton their entire life are called holoplankton. Those that are zooplankton for only a part of their life are called meroplankton. The eggs and larval forms (young) of many animals, including sponges, tunicates, and fish, live as plankton for the first few weeks or months of their lives and then develop into nekton or benthos.
Most zooplanktonic organisms obtain their food by filtering other plankton or organic debris from the water.
NektonAmong the nekton are large, complex animals. Fish of all varieties are a major part of this group. Other members are reptiles, such as sea turtles and sea snakes; and mammals, such as seals, whales, and porpoises. Invertebrates, such as jellyfish, sea worms, squids, shrimps, and scallops, complete the group. Animals of the nekton feed on other animals and on seaweeds, plankton, and floating organic matter.
BenthosBottom-dwelling organisms form the most varied group in the ocean. Because areas of rocky bottom afford a greater variety of living places than do flat, sandy areas of the ocean floor, there are many more species living among rocks than in flat, sandy areas.
Many types of seaweeds are found in the benthos. They are most numerous in shallow water, where light and nutrients are readily available. Most seaweeds grow attached to rocks. Seaweeds found floating in the ocean or washed up on beaches generally have been torn from rocks by waves.
There are four main groups of benthic animals: burrowing animals, sessile (attached) animals, crawling animals, and swimming animals.
Burrowing Animals live in the sand or mud. Many types of sea worms and a few mollusks are burrowers. These animals feed chiefly by creating currents of water through their bodies. They filter small organisms and organic debris from the water as it passes through.
Sessile Animals form a varied group. Among the more common sessile animals are sponges and corals. Other sessile animals are bryozoans (the moss animals), mollusks, barnacles, and tunicates. They are largely dependent upon currents and waves to bring them food. The sessile life is concentrated in shallow water, on the upper portions of submerged reefs, and in places where currents constantly renew the food supply.
Crawling Animals include snails, chitons, and sea anemones, all of which slide along on a foot. Starfish and sea urchins walk on tube feet. Lobsters and crabs walk on legs. Other crawling animals, such as clams, pull themselves along through the sand with a foot. Animals of this group catch other animals eat algae or debris, or filter their food from the water.
Swimming Animals, or Nektobenthos, live on, but are not confined to, the bottom of oceans. In this group are fish and octopuses, which live among seaweeds, coral, and other sessile organisms. Many of these swimmers have bright coloration that blends in with the surroundings. These animals catch other animals or eat algae or organic debris. Demersal zooplankton are animals, such as opossum shrimp, that are bottom-dwellers part of the day and drifters or swimmers the rest.
The Food WebThe survival of any organism in the ocean depends on its interrelationships with other organisms in regard to food. Such interrelationships form what is known as the food web. There are four primary parts in a food web:
- Nonliving Components, including sunlight and the oxygen, nitrogen, and organic compounds suspended in the water.
- Producers, consisting of phytoplankton and seaweeds. They produce their own food from the nonliving components through the process of photosynthesis.
- Consumers, the animals that feed on the producers.
- Decomposers, the organisms, such as bacteria and fungi, that break down the dead producers and consumers and return the raw materials to the nonliving parts of the food web.
The relationships of ocean organisms to each other can also be likened to a pyramid. At the bottom of the pyramid are the multitudes of microscopic organisms that provide food for a smaller number of larger organisms, which in turn provide food for a stillmaller number of yet larger organisms. At the top of the pyramid are the large fish, such as sharks, and other large sea-dwelling animals, such as whales.
Origin of Marine LifeAt the beginning of the Paleozoic Era, about 570,000,000 years ago, all of the phyla of the invertebrates were present. Fossil evidence shows that they had developed extensively in saltwater before this time. The earliest vertebrates, which were types of jawless fish, lived in saltwater about 500,000,000 years ago.
The most widely distributed group of fishes, the bony skeletoned fishes, is primarily marine. Among this group are the main food and game fishes, such as trout, mackerel, herring, and perch; and many oddshaped fish, such as marine sunfishes, globefishes, flying fishes, sea horses, and eels.
The fish ancestors of marine reptiles and mammals were freshwater fish that became adapted to drought conditions. These adaptations eventually led to the development of land animals. Turtles and snakes that lived on land are the ancestors of sea turtles and sea snakes. The remote ancestors of whales and porpoises are ancient land herbivores. Seals and walruses have evolved more recently from land carnivores. Sea cows (dugongs and manatees) share a common ancestor with the modern elephant.
Food From the OceanAs a source of food the ocean has been invaluable to humans for thousands of years. The animals that constitute the main sources of food are the ones that are most readily available: those that live on or near the shore and over the continental shelves.
In recent centuries, fishing for pelagic animals has become increasingly important. The bony fishes, such as cod, salmon, tuna, and flounder, are the most important food fishes. Other food animals are mollusks, such as clams and oysters; and crustaceans, such as crabs, lobsters, and shrimps. Food and food additives are made from seaweeds such as kelp, agarweed, laver, Irish moss, and dulse.
History of Ocean Research
Ocean investigations in ancient times were of two types. One was concerned with marine life, as exemplified by Aristotle's writings on natural science. The other was interested in exploration for trade and colonization; the activity of the Phoenicians is an example. Through the Middle Ages, ocean study was hampered by fear of rumored monsters and of the unknown, and by technical difficulties in navigation.
The voyages of Ferdinand Magellan (1519–22), Henry Hudson (1607–11), James Cook (1768–78), and other early explorers were primarily made for geographical discoveries. Any information obtained on currents, sea ice, and other physical and biological phenomena was incidental.
The early explorers, whose sounding lines were relatively short, thought the oceans were bottomless. Columbus in 1492 used a 400-fathom (730-m) line in an unsuccessful attempt to sound the Atlantic floor. Magellan in 1521 failed to touch bottom in the Pacific using a line of similar length.
Serious study of marine life was resumed in the 18th century. Chevalier de Lamarck devised a system of classification of animals, including marine animals, that became the basis for modern classification. Baron Georges Cuvier, the founder of comparative anatomy, is noted for his detailed studies of fish and mollusks. His system of classification of fish families was the basis for the modern classification of fish.
Sir James Clark Ross, using a crude hemp line and a lead weight, in 1840 successfully sounded the bottom of the South Atlantic, to 16,063 feet (4,896 m). The early deep ocean soundings were time-consuming and sometimes inaccurate. Gradually, however, technological developments in sounding equipment, especially the introduction of pianowire sounding lines around 1873, improved the accuracy of ocean soundings and the speed with which they could be made.
Also in the 19th century, Michael Sars and Johannes Muller, two of the founders of marine biology, became the first to collect live specimens from the seashore and to dredge the ocean for deep-sea organisms. Edward Forbes studied and classified many new species and formulated a theory of life zones in the ocean. Several nations, including Germany, Great Britain, Norway, Russia, and the United States, sponsored sea expeditions for the study of marine life, currents, and the ocean bottom. Among the most famous of these expeditions are those HMS Beagle and HMS Challenger.
During his historic voyage in the Beagle (1831–36), Charles Darwin determined the nature of coral atolls, discovered many marine animals and plants, and made observations that he later used in developing his theory of evolution. The scientific expedition of the Challenger (1872–76) was the most ambitious venture of its kind up to that time. Much information about the ocean bottom and undersea life was obtained by its team of scientists.
In 1911 the United States electrical engineer Reginald Fessenden invented the echo sounder, and this device came into extensive use in the 1920's. Research ships designed to carry a variety of instruments for oceanographic research were developed. The need for a more stable vessel from which to conduct experiments was filled in 1962 by FLIP (Floating Instrument Platform). This long ship could be flooded to float vertically, making it almost motionless.
Meanwhile, there were developments in working underwater. The invention of the aqualung, a portable breathing system, by Jacques-Yves Cousteau and Émile Gagnan during World War II freed divers from the restricted range of the deep-sea diving suit.
The first humans to venture below the zone of sunlight were naturalist William Beebe and engineer Otis Barton. In Barton's hollow steel bathysphere they descended by cable more than half a mile (800 m) in 1934. The bathyscaphe, a free-diving craft, was invented in the late 1940's by Auguste Piccard. In 1960 his son Jacques and U.S. Navy Lieutenant Don Walsh took the bathyscaphe Trieste down to a record depth of 35,802 feet (10,912 m).
The development in 1959 by Cousteau of the first tiny mobile manned submersible marked a new era in oceanography. Unmanned submersibles were designed to carry electronic equipment to depths beyond those that many of the manned vehicles could reach. In 1962 experiments were begun in underwater living and working, with American and French groups under Edwin A. Link and Cousteau. By 1970 similar programs were being conducted in various countries. Official United States projects included Sealab and Tektite.
Since the early 1970's, most ocean research has been concerned with the ocean's effect on the world's climate, pollution of the oceans, life in the oceans, and the evolution of the ocean floor. Artificial satellites have been very important in much of this research. Satellites carrying such equipment as cameras, radar, and infrared sensors are used to study various phenomena, including the circulation of seawater, the topography of the ocean floor, and the distribution of phytoplankton.
In 1977, volcanic fissures were discovered along the mid-ocean ridge. These fissures, called hydrothermal vents, spew forth water as hot as 750° F. (400° C.). The water contains hydrogen sulfide, methane, and other chemicals that help provide nourishment for many types of unusual organisms, including giant clams and worms, that live around the vents.
In 1994, marine biologists discovered that there is life far below the ocean floor. Bacteria were found more than 1,600 feet (500 m) below the floor of the Pacific Ocean.