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Oceans cover about 70% of the Earth's surface, containing 97% of our water supply. This massive amount of water located in one place affects the weather and temperature of the entire Earth. The waters absorb sunlight, keeping the earth much cooler than it would be otherwise. The Earth's oceans are all connected to one another. Until 2000, there were four recognized oceans: the Pacific, Atlantic, Indian, and Arctic. In Spring 2000, the International Hydrographic Organization delimited a new ocean, the southern Ocean (it surrounds Antarctica and extends to 60 degrees latitude). There are also many seas, which are smaller branches of an ocean and can be partly enclosed b land. The largest seas are the South China Sea, the Caribbean Sea, and the Mediterranean Sea.

The ocean appears blue because it reflects the blue color of the sky. On a cloudy, gray day, the ocean appears gray. The Red Sea often looks red because of red algae that live in this sea. The Black Sea looks almost black because it has a high concentration of hydrogen sulfide (which appears black).

Coral reefs are warm, clear, shallow ocean habitats that are rich in life. The reef's massive structure is formed from coral polyps, tiny animals that live in colonies; when coral polyps die, they leave behind a hard, stony, branching structure made of limestone. The coral provides shelter for many animals in this complex habitat, including sponges, nudibranchs, fish (like Blacktip Reef Sharks, groupers, clown fish, eels, parrotfish, snapper, and scorpion fish), jellyfish, anemones, sea stars (including the destructive Crown of Thorns), crustaceans (like crabs, shrimp, and lobsters), turtles, sea snakes, snails, and mollusks (like octopi, nautilus, and clams). Birds also feast on coral reef animals.

Coral reefs develop in shallow, warm water, usually near land, and mostly in the tropics; coral prefer temperatures between 70 and 85 ° F (21 - 30 °C). There are coral reefs off the eastern coast of Africa, off the southern coast of India, in the Red Sea, and off the coasts of northeast and northwest Australia and on to Polynesia. There are also coral reefs off the coast of Florida, USA, to the Caribbean, and down to Brazil.

Fringing reefs are reefs that form along a coastline. They grow on the continental shelf in shallow water. Barrier reefs grow parallel to shorelines, but farther out, usually separated from the land by a deep lagoon. They are called barrier reefs because they form a barrier between the lagoon and the seas, impeding navigation. Coral Atolls are rings of coral that grow on top of old, sunken volcanoes in the ocean. They begin as fringe reefs surrounding a volcanic island; then, as the volcano sinks, the reef continues to grow, and eventually only the reef remains.

The intertidal area (also called the littoral zone) is where the land and sea meet, between the high and low tide zones. This complex marine ecosystem is found along coastlines worldwide. It is rich in nutrients and oxygen and is home to a variety of organisms. Much of this inhospitable environment is washed by the tides each day, so organisms that live here are adapted to huge daily changes in moisture, temperature, turbulence (from the water), and salinity. The littoral zone is covered with salt water at high tides, and it is exposed to the air at low tides; the height of the tide exposes more or less land to this daily tide cycle. Organisms must be adapted to both very wet and very dry conditions. The turbulence of the water is another reason that this area can be very difficult one in which to survive - the rough waves can dislodge or carry away poorly-adapted organisms. Many intertidal animals burrow into the sand (like clams), live under rocks, or attach themselves to rocks (like barnacles and mussels). The temperature ranges from the moderate temperature of the water to air temperatures that vary from below freezing to scorching. Depressions on the shores sometimes form tide pools, areas that remain wet, although they are not long-lasting features. The salinity of tidepools varies from the salinity of the sea to much less salty, when rainwater or runoff dilutes it. Animals must adapt their systems to these variations. Some fish, like sculpin and blennies, live in tide pools.

Spray Zone: Also called the Upper Littoral, the Supralittoral Fringe, the Splash Zone, and the Barnacle Belt. This area is dry much of the time, but is sprayed with salt water during high tides. It is only flooded during storms and extremely high tides. Organisms in this sparse habitat include barnacles, isopods, lichens, lice, limpets, periwinkles, and whelks. Very little vegetation grows in this area. High Tide Zone: Also called the Upper Mid-littoral Zone and the high intertidal zone. This area is flooded only during high tide. Organisms in this area include anemones, barnacles, brittle stars, chitons, crabs, green algae, isopods, limpets, mussels, sea stars, snails, whelks and some marine vegetation. Middle Tide Zone: Also called the Lower Mid-littoral Zone. This turbulent area is covered and uncovered twice a day with salt water from the tides. Organisms in this area include anemones, barnacles, chitons, crabs, green algae, isopods, limpets, mussels, sea lettuce, sea palms, sea stars, snails, sponges, and whelks. Low Tide Zone: Also called the Lower Littoral Zone. This area is usually under water - it is only exposed when the tide is unusually low. Organisms in this zone are not well adapted to long periods of dryness or to exteme temperatures. Some of the organisms in this area are abalone, anemones, brown seaweed, chitons, crabs, green algae, hydroids, isopods, limpets, mussels, nudibranchs, sculpin, sea cucumber, sea lettuce, sea palms, sea stars, sea urchins, shrimp, snails, sponges, surf grass, tube worms, and whelks.

Animals that live in the littoral zone have a wide variety of predators who eat them. When the tide is in, littoral organisms are preyed upon by sea animals (like fish). When the tide is out, they are preyed upon by land animals, like foxes and people. Birds (like gulls) and marine mammals (like walruses) also prey on intertidal organisms extensively.

The salinity (salt content) of ocean water varies. The oceans and seas contain roughly 5 x 10 ¹⁶ tons of salts. One cubic foot of average sea water contains 2.2 pounds of salt. The oceans are about 3.5% salt (by weight). Salinity is generally reported in terms of parts per thousand (abbreviated o/oo), the number of pounds of salt per 1,000 pounds of water; the average ocean salinity is 35 o/oo. The saltiest water is in the Red Sea and in the Persian Gulf, which have a salinity of about 40 o/oo (due to very high evaporation rates and low fresh water influx). The least salty seas are in the polar regions, where both melting polar ice and a lot of rain dilute the salinity.

The Hydrologic Cycle is the journey water takes as it circulates from the land to the sky and back again. The Sun's heat provides energy to evaporate water from the Earth's surface (oceans, lakes, etc.). Plants also lose water to the air (this is called transpiration). The water vapor eventually condenses, forming tiny droplets in clouds. When the clouds meet cool air over land, precipitation (rain, sleet, or snow) is triggered, and water returns to the land (or sea). Some of the precipitation soaks into the ground. Some of the underground water is trapped between rock or clay layers; this is called groundwater. But most of the water flows downhill as runoff (above ground or underground), eventually returning to the seas as slightly salty water.

As water flows through rivers, it picks up small amounts of mineral salts from the rocks and soil of the river beds. This very-slightly salty water flows into the oceans and seas. The water in the oceans only leaves by evaporating (and the freezing of polar ice), but the salt remains dissolved in the ocean - it does not evaporate. So the remaining water gets saltier and saltier as time passes.

The winds cause waves on the surface of the ocean (and on lakes). The wind transfers some of its energy to the water, through friction between the air molecules and the water molecules. Stronger winds (like storm surges) cause larger waves. You can make your own miniature waves by blowing across the surface of a pan of water.

Waves of water do not move horizontally, they only move up and down. You can see a demonstration of this by watching a floating buoy bob up and down with a wave; it does not, however, move horizontally with the wave. Tsunamis are different from surface waves; they are usually caused by underwater earthquakes, volcanic eruptions, or landslides.

Tides are periodic rises and falls of large bodies of water. Tides are caused by the gravitational interaction between the Earth and the Moon. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side). Since the earth is rotating while this is happening, two tides occur each day. Isaac Newton was the first person to explain tides scientifically.

Spring tides are especially strong tides (they do not have anything to do with the season Spring). They occur when the Earth, the Sun, and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides. Spring tides occur during the full moon and the new moon. The Proxigean Spring Tide is a rare, unusually high tide. This very high tide occurs when the moon is both unusually close to the Earth (at its closest perigee, called the proxigee) and in the New Moon phase (when the Moon is between the Sun and the Earth). The proxigean spring tide occurs at most once every 1.5 years. Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with respect to the Earth). Neap tides occur during quarter moons.