Oceans and Coastlines

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Oceans and Coastlines


Water covers around two thirds of Earth’s surface. Its depth ranges from seven mi (11 km) at the greatest depth of the oceans to just a few inches (centimeters) near the line of the coast. Oceans and coastlines vary in their temperature, oxygen content, and access to sunlight. These factors create a wide range of different ecosystems containing thousands of invertebrate species, as well as fish, birds, and whales.

Oceans and coastlines are important zones for human activity for economic reasons, such as fishing, and for recreation and sport. However, both oceans and coastlines face severe threats. Oceans are at risk from overfishing because fish supplies are finite if not managed as a renewable resource, and the fishing industry has expanded greatly in recent years. Coastlines are close to land, and so are threatened by development and pollution from sewage, agriculture, and dumping rubbish. Some important ecosystems like mangrove swamps and coral reefs now face destruction because of the impact of human activities.

Historical Background and Scientific Foundations

Oceans and seas differ only in their size, with the oceans comprising the world’s five great bodies of water, namely the Pacific, Antarctic, Indian, Atlantic, and Arctic. Seas are smaller bodies of water, like the North Sea or Barents Sea. Both are composed of saltwater of varying temperatures from 104°F (40°C) in the tropics to frozen water near the Poles. Surface waters may be warmed by the sun, but deeper waters are cold. Near the shore, which is the part of an ocean closest to land, the water is naturally shallow and cloudy so the sun can only penetrate a few feet. The waters of the open ocean are clearer and sunlight may penetrate to a depth of up to 300 ft (91 m).

The type of ecosystem found in the ocean depends largely upon the availability of sunlight, oxygen and nutrients, and the temperature. The surface layer where sunlight penetrates is called the euphotic zone, and is where photosynthesis is carried out by phytoplankton. Phytoplankton act as food for larger species, forming the basis for marine food webs. The bottom layers are nutrient rich, because dead organisms sink and provide food for scavengers. There is, however, a certain amount of mixing of the upper and lower layers of the oceans driven by surface currents and deeper circulation patterns. This provides oxygen to lower layers and nutrients to upper layers. These currents lead to some particularly productive marine biomes called upwellings where the mixing is particularly pronounced. One such upwelling is found along the equator in the Pacific Ocean, another along the Antarctic coast, which provides food supplies in the form of krill for both baleen whales, which are filter feeders, and large seabird populations.

The open ocean is not just a featureless mass of water. It exists in distinct layers. The column of water extending down from the surface to a depth of about 2.5 mi (4 km) is called the pelagic zone and is composed of the epipelagic (top), mesopelagic (middle or twilight), and bathypelagic (bottom) zones. Below this, extending to about 4 mi (6 km) are the abyssal and hadal zones. Hadal comes from the Greek word for unseen, abyssal from the Greek word for bottomless. Marine ecosystems are stratified too, with respect to the vertical layer they can adapt to. For example, there is no light in the layers close to the ocean floor save for that emitted from the organisms themselves. Thousands of species, including bacteria, squid, and fish, exhibit bioluminescence, which is the ability to emit flashes of light to distract predators and locate prey.

The oceans have the same salt composition as living cells so, in that sense, it is a viable environment and indeed life probably evolved in the oceans. Compared to the land, however, the marine environment is not so bio-diverse. But thousands of different invertebrates do exist in the oceans, some of which live on the shore. These include sponges and echinoderms, such as starfish and cnidarians, including jellyfish and corals. Among vertebrates, whales are the most important mammals living in the ocean, but there are many birds and fish there, too. Around 1,000 fish species occupy the open ocean, inhabiting all of its different vertical zones.

The coast is the margin of land next to an ocean, or sea. It gives way to the beach, the very edge of the water, which may regularly be covered with water by the tides. The ocean or sea floor slopes gradually down from the coast to the deeper water and may be soft or rocky. The area nearest to the sea is called the littoral zone and many fish and shellfish are found here. For instance, molluscs, which are found on both land and sea, include the bivalves such as clams, mussels, cockles, oysters, and scallops, which are economically important. So too are the crustaceans, which include crabs, lobsters, and shrimps.

A rocky-bottomed beach encourages epifauna, which are animals that are attached to the solid surface and tend to filter feed, such as mussels. A soft bottom is generally populated by infauna, which live within the mud. Some are scavengers, like small shrimp and worms, while others, like tube worms, are filter feeders. The sand on a beach is created by erosion, which shapes a coastline over time. Not all coastlines have a beach and some


BIOME: A well-defined terrestrial environment (e.g., desert, tundra, or tropical forest) and the complex of living organisms found in that region.

EPIFAUNA: Animals that live attached to rocks.

EUTROPHICATION: The process whereby a body of water becomes rich in dissolved nutrients through natural or man-made processes. This often results in a deficiency of dissolved oxygen, producing an environment that favors plant over animal life.

FILTER FEEDERS: Animals that obtain their food from filtering passing water.

INFAUNA: Animals living within the material, such as mud or sand, at the bottom of an ocean or sea, or on the beach.

PHYTOPLANKTON: Microscopic marine organisms (mostly algae and diatoms) that are responsible for most of the photosynthetic activity in the oceans.

UPWELLING: The vertical motion of water in the ocean by which subsurface water of lower temperature and greater density moves toward the surface of the ocean.

coastlines occur inland. Barrier islands and sea cliffs are important features of some coastlines. The nature of a

coastline depends on geological processes originating on land and in the ocean or sea.

Mangrove swamps and coral reefs are among the specialized biomes found in coastal areas. Mangroves are salt-resistant trees that support fish or shrimp. Coral reefs, like Australia’s Great Barrier Reef that stretches for around 1,200 mi (1,930 km) along the eastern coast of Australia, consist of animals called stony corals, each of which is a polyp with tentacles that can trap organisms. Coral reefs tend to occur in warm, shallow, clear waters where they provide a home for a diverse community of fish, worms, and crustaceans, protecting the small fish from larger predatory fish.

Impacts and Issues

Although the oceans are vast, they are not inexhaustible. Overfishing is perhaps the greatest threat they now face. Global fish harvesting has increased 5 times during the last 50 years or so. The oceans can support a fish harvest of about 100 million tons of fish per year and this limit is close to being reached. Not only will this hit the fishing industry, but it also impacts on marine ecosystems.

Nearer to the coast, there are a number of ecosystem threats caused by human activities. For instance, the coral reefs are among the world’s most threatened biomes. They are affected by a range of factors, including destructive fishing practices, pollution, and sewage, while global warming is beginning to destroy them through bleaching. Mangrove swamps are similarly at risk.

Shore areas are also at risk from pollution, which can cause eutrophication by raising nutrient levels in the water because of sewage outlet and agricultural runoff. As oxygen levels fall, fish and other organisms begin to die off. Eutrophication also causes an overgrowth of algae in the water, often visible as a red, yellow, or green scum on the surface. They are a visible sign of pollution, as are found in many areas around the world, such as the Mediterranean and the east coast of the United States. Meanwhile, many beaches are rendered unsightly by rubbish such as plastic bags, which can strangle wildlife.

See Also Coastal Zones; Marine Ecosystems; Ocean Tides; Reef Ecosystems



Cunningham, W.P., and A. Cunningham. Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.

Kaufmann, R., and C. Cleveland. Environmental Science. New York: McGraw-Hill International Edition, 2008.

Web Sites

Office of Naval Research. “Habitats: Beaches.”http://www.onr.navy.mil/focus/ocean/habitats/beachesl.htm (accessed April 3, 2008).

Susan Aldridge