ocean. (Image by Flickr user PoppyW, CC)


Water: No Longer Taken For Granted The Columbia Encyclopedia, 6th ed.UXL Encyclopedia of ScienceWorld of Earth Science Further reading


Oceans and Estuaries

Chapter 6
Oceans and Estuaries


A view of the earth from a satellite shows an azure planet composed almost entirely of water. According to Tom S. Garrison, in Oceanography: An Invitation to Marine Science (2005), the ocean covers over two-thirds of the earth's surface to an average depth of 12,451 feet (or almost 2.3 miles). The U.S. Geological Survey (USGS), in Where Is Earth's Water Located? (August 28, 2006, http://ga.water.usgs.gov/edu/earthwherewater.html), notes that the earth contains 97% of the planet's water. It has a profound influence on the earth's environment. The ocean has subdivisions that are recognized as the Atlantic Ocean, the Pacific Ocean, the Indian Ocean, and the Arctic Ocean.

The terms ocean and sea are sometimes used interchangeably, but they are terms used out of tradition. According to Garrison, the ocean is "the vast body of saline water that occupies the depressions of the Earth's surface" and includes all the oceans and seas. For convenience sake, the ocean is subdivided into artificial compartments and given names that include the Atlantic, Pacific, and Indian oceans and the Mediterranean, Red, and Black seas.

Origin of the Ocean

Research suggests that the ocean is about four billion years old and that both the atmosphere on Earth and the ocean were formed through a process called outgassing of the earth's deep interior. According to this scientific theory, the ocean originated from the escape of water vapor from the melted rocks of the early Earth. The vapor rose to form clouds surrounding the cooling planet. After the earth's temperature had cooled to a point below the boiling point of water, rain began to fall and continued falling for millions of years. As this water drained into the huge hollows of the planet's cracked surface, the oceans were formed. The force of gravity kept this water on Earth. The oceans are still forming today at an extremely slow pace. According to Garrison, about 0.025 cubic miles is added to the ocean each year.

Why Is the Ocean So Salty?

The salinity (saltiness) of the ocean is due to the presence of a high concentration of dissolved inorganic solids in water, primarily sodium and chloride (the components of table salt). Table 6.1 shows the principal constituents of ocean water. Early in the life of the planet, the ocean probably contained little of these substances. However, since the first rains descended on the young Earth billions of years ago and ran over the land, the rain has eroded the soil and rocks, dissolving them and transporting their inorganic solids to the ocean. Rivers and streams also carry dissolved inorganic solids and sediments and discharge them into the ocean. In Why Is the Ocean Salty? (1993, http://www.palomar.edu/oceanography/salty_ocean.htm), Herbert Swenson estimates that U.S. rivers and streams discharge 225 million tons of dissolved solids (salts) and 513 million tons of suspended sediment into the ocean each year. Throughout the world, rivers annually transport about four billion tons of dissolved salts to the ocean.

Over billions of years the ocean has become progressively more salty. The activity of the hydrological cycle concentrates the ocean salts as the sun's heat evaporates water from the surface of the ocean, leaving the salts behind. (See Figure 6.1.) There is so much salt in the ocean that if it could be taken out and spread evenly over the earth's entire land surface, it would form a layer more than five hundred feet thickabout the height of a forty-story building. (See Figure 6.2.)

Swenson notes that the salinity of the ocean is currently about thirty-five pounds per thousand pounds of ocean water, or thirty-five parts per thousand (ppt). This is similar to having a teaspoon of salt added to a glass of drinking water. By contrast, the U.S. Environmental Protection Agency (EPA), in In Condition of the Mid-Atlantic Estuaries (November 1998, http://www.epa.gov/emap/html/pubs/docs/groupdocs/estuary/assess/cond_mae.pdf), indicates that freshwater has less than 0.5 ppt. Salinity in estuaries varies from slightly brackish (0.5 to 5 ppt) at the freshwater end to moderately brackish (5 to 18 ppt), to highly saline (19 ppt or more) near the ocean. An estuary is a body of water where a river meets the ocean and fresh- and saltwater mix.

Principal constituents of seawater
Chemical constituent Content (parts per thousand)
Source: Herbert Swenson, "Principal Constituents of Seawater," in Why Is the Ocean Salty? U.S. Geological Survey, 1993
Calcium (Ca) 0.419
Magnesium (Mg) 1.304
Sodium (Na) 10.710
Potassium (K) 0.390
Bicarbonate (HCO3) 0.146
Sulfate (SO4) 2.690
Chloride (Cl) 19.350
Bromide (Br) 0.070
   Total dissolved solids (salinity) 35.079

Ocean as Controller of Earth's Climate

The ocean plays a major role in the earth's weather and long-term climate change. It has a huge capacity to store heat and can affect the concentration of atmospheric gases that control the planet's temperature. In Environmental Health: Ecological Perspectives (2005), Kathryn Hilgenkamp indicates that the top eight feet of the ocean hold as much heat as the entire atmosphere, making the ocean's ability to distribute heat an important factor in climate changes. For example, the occurrence of a southward-flowing current of warm water off the coast of western South America (El Niño), which is caused by a breakdown of trade wind circulation (steady winds blowing from east to west above and below the equator) can disrupt global weather patterns.

The ocean plays a crucial role in the cycle of carbon dioxide, a process affecting global warming. For example, the National Oceanic and Atmospheric Administration (NOAA) reports in "After Two Large Annual Gains, Rate of Atmospheric CO2 Increase Returns to Average, NOAA Reports" (NOAA Magazine, March 31, 2005), that the ocean stores some of the seven billion tons of carbon dioxide added each year to the atmosphere by natural sources and humankind's burning of fossil fuels. The ocean, trees, plants, and the soil serve as reservoirs for about half of all the human-produced carbon dioxide emitted each year since the Industrial Revolution, whereas the other half is accumulating in the atmosphere.


According to the Population Reference Bureau (2006, http://www.prb.org/Content/NavigationMenu/PRB/Journalists/FAQ/Questions/Coastal_Population.htm), approximately 3 billion people worldwide, about half the world's population, live within 200 kilometers (about 125 miles) of a coastline. This living preference places huge segments of the world population at risk from coastal hazards, such as hurricanes, tidal waves, and flooding and increases pollution of both the ocean and estuaries.

Leading states in coastal population growth, 19802003
State Total change (million persons) State Percent change
Source: Kristen M. Crossett et al., "Table 2. Leading States in Coastal Population Growth, 19802003," in Population Trends Along the Coastal United States: 19802008, National Oceanic and Atmospheric Administration, National Ocean Service, September 2004, http://marineeconomics.noaa.gov/socioeconomics/assessment/population/pdf/2_national_overview.pdf (accessed February 5, 2007)
California 9.9 Florida 75
Florida 7.1 Alaska 63
Texas 2.5 Washington 54
Washington 1.7 Texas 52
Virginia 1.6 Virginia 48
New York 1.6 California 47
New Jersey 1.2 New Hampshire 46
Maryland 1.2 Delaware 38
Michigan 0.8 Georgia 35
Massachusetts 0.7 South Carolina 33

In Population Trends along the Coastal United States: 19802008 (September 2004, http://marineeconomics.noaa.gov/socioeconomics/assessment/population/pdf/1_front_matter_intro.pdf), Kristen M. Crossett et al. state that in 2003 an estimated 53% of the U.S. population (occupying only 26% of the total U.S. land mass) lived in coastal counties. From 1980 to 2003 the total coastal population of the United States increased by 27%, which is consistent with the nation's rate of increase as a whole. However, some coastal communities increased in population much more than others. Table 6.2 shows the leading states in coastal population growth, from 1980 to 2003. California had the greatest number of people move into the state, but Florida had the highest percentage growth.

Coastal Storms

The most common coastal hazard is the threat of the huge ocean storms that come ashore, generally during the warmer months of the year, and cause devastating damage to property and human life. These storms go by different names in different parts of the world. They are called hurricanes or tropical storms in the North Atlantic, the eastern North Pacific, and the western South Pacific. Typhoon is used for storms in the China Sea and the western North Pacific, whereas cyclone is used for storms in the Arabian Sea, the Bay of Bengal, and the South Indian Ocean.

In August 2005 Hurricane Katrina hit the U.S. Gulf Coast, causing widespread devastation in cities such as New Orleans, Louisiana; Mobile, Alabama; and Gulfport, Mississippi. Katrina was one of the strongest storms to reach the U.S. coast in the last 100 years with sustained winds during landfall of 125 miles per hour. Since 1900, it was the third deadliest hurricane (killing 1,833 people) and was by far the costliest hurricane. (See Table 6.3 and Table 6.4.)

The deadliest mainland United States hurricanes, 19002006
Rank Hurricane Year Category Deaths
Note: Hurricanes, or tropical cyclones, were not named until the 1950s.
aThis figure could be as high as 10,000 to 12,000.
bOver 500 lost on ships at sea, 600-900 estimated deaths.
cMoving more than 30 miles per hour.
dOf the total lost 344 were lost on ships at sea.
Source: Adapted from Jerry D. Jarrel et al., "Table 2. The Thirty Deadliest Mainland United States Tropical Cyclones 19002000," in The Deadliest, Costliest, and Most Intense United States Hurricanes from 1900 to 2000, National Oceanic and Atmospheric Administration, National Weather Service, October 2001, http://www.aoml.noaa.gov/hrd/Landsea/deadly/Table2.htm (accessed January 9, 2007)
1 Unnamed (TX, Galveston) 1900 4 8,000a
2 Unnamed (FL, Lake Okeechobee) 1928 4 1,836
3 Katrina (Eastern LA, western MS) 2005 3 1,833
4 Unnamed (FL, Keys) 1919 4   600b
5 Unnamed (New England) 1938 3c   600
6 Unnamed (FL, Keys) 1935 5   408
7 Audrey (Southwest LA, inland TX) 1957 4   390
8 Unnamed (Northeast) 1944 3c   390d
9 Unnamed (LA, Grand Isle) 1909 4   350
10 Unnamed (LA, New Orleans) 1915 4   275
11 Unnamed (TX, Galveston) 1915 4   275

Tsunamis are another coastal threat. A tsunami is an ocean wavewhich may reach enormous dimensionsproduced by a submarine (undersea) earthquake, landslide, or volcanic eruption. A December 2004 tsunami, triggered by a massive earthquake in the Indian Ocean, killed over two hundred thousand people and caused massive damage in Indonesia, Sri Lanka, India, Thailand, and many small islands in the region. The true death toll from the tsunami may never be known, and the devastation was so overwhelming that it is difficult to attach a dollar figure to it.

Much research is targeted at understanding and predicting coastal storms and tsunamis so that coastal residents can be warned of an impending event. Besides increasing the amount of property at risk, coastal population growth has created potentially life-threatening problems with storm warnings and evacuation. It has become increasingly difficult to ensure that the ever-rising numbers of residents and visitors can be evacuated and transported to adequate shelters during storm events. Sometimes hurricane evacuation decisions must be made well in advance of issuing hurricane warnings to mobilize the appropriate manpower and resources needed for the evacuation. Also, when a significant percentage of the coastal population has not experienced an event such as a hurricane, people are less likely to prepare and respond properly before, during, and after the event. However, following the 2004 tsunami, government officials and scientists began working to create a new tsunami warning system for that region.

The costliest U.S. hurricanes, 19002006
Rank Hurricane Year Category Damage in billions of dollars
aCosts normalized to 2002 dollars using gross national product (GNP) Inflation/Wealth Index.
bCosts unadjusted for inflation.
Source: Compiled in January 2007 by Sandra Alters for Thomson Gale, 2007, from Jerry D. Jarrel et al., "Table 3. Costliest U.S. Hurricanes 19002000 (unadjusted)," in The Deadliest, Costliest, and Most Intense United States Hurricanes from 1900 to 2000, National Oceanic and Atmospheric Administration, National Weather Service, October 2001, http://www.aoml.noaa.gov/hrd/Landsea/deadly/Table3.htm (accessed January 10, 2007) and "19802005 Billion Dollar U.S. Weather Disasters," in Billion Dollar U.S. Weather Disasters, National Oceanic and Atmospheric Administration, National Climatic Data Center, January 2007, http://www.ncdc.noaa.gov/img/reports/billion/disasterssince1980.pdf (accessed January 10, 2007)
1 Katrina (eastern LA, western MS coastlines)a 2005 3 $125
2 Andrew (southeast FL and LA)a 1992 5 36
3 Rita (TX-LA border coastlines)a 2005 3 16
4 Wilma (southwest FL)a 2005 3 16
5 Charley (FL, SC)a 2004 4 15
6 Ivan (AL)a 2004 3 14
7 Hugo (SC)a 1989 4 13.9
8 Frances (FL, GA, SC, NC, NY)a 2004 2 9
9 Jeanne (east central FL)a 2004 3 7
10 Agnes (FL, northeast U.S.)b 1972 1 8.6
11 Betsy (southeast FL and LA)b 1965 3 8.5
12 Camille (MS, southeast LA, VA)b 1969 5 7
13 Georges (FL Keys, MS, AL)a 1998 2 6.6
14 Floyd (mid atlantic & northeast U.S.)a 1999 2 6.5
15 Alicia (north TX)a 1983 3 5.9
16 Fran (NC)a 1996 3 5.8
17 Diane (northeast U.S.)b 1955 1 5.5
18 Isabel (NC, VA, MD)a 2003 2 5
19 Frederic (AL, MS)b 1979 3 5
20 Unnamed (New England)b 1938 3 4.7
21 Opal (northwest FL, AL)a 1995 3 3.6
22 Carol (northeast U.S.)b 1954 3 3.1
23 Juan (LA)a 1985 1 2.8
24 Carla (north & central TX)b 1961 4 2.6
25 Donna (FL, eastern U.S.)b 1960 4 2.4
26 Iniki (HI)a 1992 4 2.4
27 Elena (MS, AL, northwest FL)a 1985 3 2.4
28 Bob (NC, northeast U.S.)a 1991 2 2.1
29 Celia (south TX)b 1970 3 2
30 Dennis (FL, AL, GA, MS, TN)a 2005 3 2
31 Hazel (SC, NC)b 1954 4 1.9
32 Unnamed (FL, MS, AL)b 1926 4 1.7
33 Unnamed (north TX)b 1915 4 1.5
34 Dora (northeast FL)b 1964 2 1.5
35 Eloise (northwest FL)b 1975 3 1.5
36 Gloria (eastern U.S.)b 1985 3 1.5
37 Unnamed (northeast U.S.)b 1944 3 1.2
38 Beulah (south TX)b 1967 3 1.1
39 Bonnie (eastern NC & VA)a 1998 3 1.1


A coral reef is a submerged ridge near the surface of the water made up not only of colonies of coral animals that secrete hard skeletons but also of other aquatic organisms such as algae, mollusks, and worms. Coral reefs are among the richest marine ecosystems in terms of beauty, species, productivity, biomass (the amount of living matter), and structural complexity. They are dependent on intricate interactions between the coral, which provides the structural framework, and the organisms that live among the coral. Most reefs form as long narrow ribbons along the edge between shallow and deep waters, and their assets are many: fisheries for food, income from tourism and recreation, materials for new medicines, and shoreline protection from coastal storms.

Coral Reef Structure

Corals are simple, bottom-dwelling organisms related to the sea anemone and jellyfish. The basic building block of coral is a polyp, a tiny animal that has a common opening used to take in food and excrete wastes and is surrounded by a ring of tentacles. The weak stinging cells of the tentacles are used to capture small zoo-plankton (minute floating aquatic animals) for food. Each polyp sits in its own tiny bowl of calcium carbonate (its skeleton), which the coral constantly builds as it grows up from the ocean floor. Reef-building corals live in large colonies formed by the repeated divisions of genetically identical polyps, although many species of coral animals are represented within the reef. The colonies can take a wide variety of shapes, including branched, leafy, or massive forms, which may grow continuously for thousands of years.

Inside the sac of each coral polyp lives a single-celled algae. The algae give off oxygen and nutrients the coral uses to grow, and the coral gives off carbon dioxide and other substances the algae uses. Such a living situation is called symbiosisthe living together of two dissimilar organisms with mutual benefit.

Because of their dependence on symbiotic algae, coral reefs can grow only under conditions favoring the algae. Coral reefs are confined to tropical waters because the algae require warm, shallow, well-lit waters that are free of turbidity and pollution.

Coral ReefsEcosystems at Risk

The proximity of coral reefs to land makes them particularly vulnerable to the effects of human actions. Because they depend on light, coral reefs can be severely damaged by silt, which leads to an overgrowth of seaweed and other factors that reduce water clarity and quality. Sport diving and overfishing for food and the aquarium trade can deplete species and damage coral, resulting in disruption to the intricate interactions among reef species, as well as coral decline. (Overfishing means that so many fish are harvested that the natural breeding stock is depleted.) Introduction of exotic species through human activity can be devastating as the new predators consume the living reefs.

NOAA notes in "Major Reef-Building Coral Diseases" (January 23, 2007, http://www.coris.noaa.gov/about/diseases/) that along with becoming infected by bacteria or developing diseases of unknown causes, corals may respond to stress and damage with a condition known as coral bleaching. The coral expels the microscopic algae that normally live within its cells and provide the coral with their color, their ability to rapidly grow skeleton, and much of their food. The bleached coral turns pale, transparent, or unusual colors and then starves because it is unable to feed or reproduce. Increased bleaching is an early warning sign of deteriorating health and can be caused by extremes of light, temperature, or salinity.

Natural events, such as hurricanes, can also damage coral reefs. Healthy reefs generally recover from such damage, but unhealthy reefs often do not. In "Hazards to Coral Reefs" (January 23, 2007, http://www.coris.noaa.gov/about/hazards/), NOAA warns, "Current estimates note that 10 percent of all coral reefs are degraded beyond recovery. Thirty percent are in critical condition and may die within 10 to 20 years. Experts predict that if current pressures are allowed to continue unabated, 60 percent of the world's coral reefs may die completely by 2050."

Coral Reefs in the United States

The EPA notes in the 2000 National Water Quality Inventory (August 2002, http://www.epa.gov/305b/2000report/chp4.pdf) that coral reefs are found in only three places in the United States: Florida (primarily in the Florida Keys), throughout the Hawaiian archipelago, and in the offshore Flower Gardens of Texas. The Florida reef system is part of the Caribbean reef system, the third largest barrier-reef ecosystem in the world. Five U.S. territoriesAmerican Samoa, Guam, the Northern Mariana Islands, Puerto Rico, and the U.S. Virgin Islandsalso have lush reef areas. Figure 6.3 shows that the northwestern Hawaiian Islands make up 69% of the country's coral reef areas, by far the largest percentage in the United States and its territories.

Many U.S. coral reefs have been designated as marine sanctuaries with varying degrees of protection. The full extent and condition of most of these coral reefs is only beginning to be studied as a special area of focus.

In September 2002 NOAA released the first national assessment of the condition of coral reefs in the United States. The report, The State of Coral Reef Ecosystems of the United States and Pacific Freely Associated States: 2002 (http://www.nccos.noaa.gov/documents/status_coralreef.pdf), was prepared under the auspices of the U.S. Coral Reef Task Force and established a baseline that is used for biennial reports on the health of coral reefs in the United States. NOAA also released the report A National Coral Reef Action Strategy (September 2002, http://www.coris.noaa.gov/activities/actionstrategy/) to Congress outlining specific action to address thirteen major goals, including the continuation of mapping and monitoring, to protect coral reefs.

According to the State of Coral Reef Ecosystems of the United States and Pacific Freely Associated States, there are an estimated 7,607 miles of U.S. reefs and a range of 4,479 to 31,470 miles of reefs off the Freely Associated States. (The United States and Freely Associated States refer to fourteen jurisdictions that contain coral reef ecosystems, which are listed in Figure 6.4.) The report notes that an estimated 27% of the world's shallow water coral reefs may already be beyond recovery, and about 66% are severely degraded. The report also indicates that in all areas some coral reefs in the United States are in good to excellent health. However, every reef system is suffering from both human and natural disturbances. These reefs suffer from the same problems as do reefs all over the world, especially those resulting from rapidly growing coastal populations. The report states that 10.5 million people now live in U.S. coastal areas next to shallow coral reefs, and every year about 45 million people visit the areas.

Florida and the U.S. Caribbean were considered to be in the most unfavorable condition, mainly because of nearby dense populations and the effects of hurricanes, disease, overfishing, and a proliferation of algae. The NOAA report indicates that live coral cover in the Florida Keys has declined 37% since 1997. Since 1982, white-band disease has killed nearly all the elkhorn and staghorn corals off the coasts of St. Croix (U.S. Virgin Islands), Puerto Rico, and southeastern Florida.

In August 2005 NOAA released the second national assessment of the condition of coral reefs in the United States, The State of Coral Reef Ecosystems of the United States and Pacific Freely Associated States: 2005, (http://ccma.nos.noaa.gov/ecosystems/coralreef/coral_report_2005/CoralReport2005_C.pdf), which was edited by Jenny E. Waddell. Waddell notes progress in building a mapping and monitoring system. She also indicates that many local action strategies have been developed to counter threats to coral reefs, especially in priority threat areas. In separate chapters, each reporting area describes in quantitative detail the state of the coral reefs in their area. This differs from the more general qualitative (descriptive) nature of the first report.

A summary of a comparison between the 2002 and 2004 perceived levels of threat to coral reef ecosystems is shown in Figure 6.4. The rows compare each jurisdiction for all threat categories for 2002 and 2004. In Florida, for example, the threat level in each threat category remained the same from 2002 to 2004, except for the "other" category. Thus, the composite trend rose by one point from 2002 to 2004, as shown in the far right column, resulting in a increase of threat overall (arrow).

The columns compare each threat category for all jurisdictions for 2002 and 2004. For example, climate change, diseases, tropical storms, and fishing increased in threat level over those two years across all jurisdictions. However, eight of fourteen threat categories were perceived to have decreased in severity since 2002.


Nearshore waters are shallow waters a short distance from the shore in lakes, rivers, estuaries, and the ocean. Depending on the size of the water body, the nearshore waters may be minimal in size (a small lake) or large (the coastal waters of the Atlantic Ocean). They reflect the conditions and activities within the watershed. A watershed is an area in which water, sediments, and dissolved materials drain to a common outlet, such as a lake, river, estuary, or the ocean.

Whether marine, estuarine, or fresh, nearshore waters serve a variety of functions. They are the prime recreational waters, providing opportunities for swimming, boating, diving, surfing, snorkeling, and fishing. Nearshore waters are intimately linked with wetlands and sea grasses and provide a unique habitat for a variety of plants and animals. According to the EPA, in "Nearshore Waters and Your Coastal Watershed" (July 1998, http://www.epa.gov/owow/oceans/factsheets/fact3.html), these waters are the source of food and shelter for many species of fish and shellfish and provide habitat for 80% of the fish species in the United States. Nearshore waters also provide many opportunities for education and research for students, naturalists, and scientists.

Because of their proximity to the shoreline, nearshore waters are particularly vulnerable to pollution. As a result, water quality in most confined waters and some nearshore waters is deteriorating, which in turn affects the plant and animal life. Besides pollution, nearshore waters are vulnerable to the everyday (and to all appearances, harmless) activities of people. For example, swimming has been restricted in some shallow lagoons with coral reefs and beautiful beaches because heavy use by swimmers resulted in chemical concentrations of suntan oil and sunblock lotion in the water that was high enough to kill or impair the coral reefs. Wakes from recreational powerboats in high-use areas have been shown to increase wave action, resulting in increased shoreline erosion. Increased pollutant levels from boat paints, spills during refueling, and leaks of gas and oil from recreational boat engines in areas of high recreational use affect both plants and animals. Private pier and boathouse construction result in shading of water, which contributes to sea grass decline. Balancing the need to accommodate the public's desire to enjoy water-related activities and ownership of waterfront property and the need to protect nearshore waters is a difficult management issue.


Estuaries are places of transition, where rivers meet the sea. An estuary is a partially enclosed body of water formed where freshwater from rivers flows into the ocean, mixing with the salty seawater. As mentioned earlier, salinity in estuaries varies from slightly brackish (0.5 to 5 ppt) at the freshwater end to moderately brackish (5 to 18 ppt), to highly saline (19 ppt or more) near the ocean. Although influenced by the tides, estuaries are protected from the full force of ocean waves, winds, and storms by reefs, barrier islands, or fingers of land, mud, or sand that make up their seaward boundary. Estuaries come in all shapes and sizes. Examples include the Chesapeake Bay, Puget Sound, Boston Harbor, San Francisco Bay, and Tampa Bay. In "The National Estuary Program: A Ten Year Perspective" (October 4, 2006, http://www.epa.gov/owow/estuaries/aniv.htm), the EPA states that there are approximately 130 estuaries in the United States.

The tidal sheltered waters of estuaries support unique communities of plants and animals that are specially adapted for life under a wide range of conditions. Estuarine environments are incredibly productive, producing more organic matter annually than any equal-sized area of forest (including rain forests), grassland, or cropland. A wide range of habitats exists around and in estuaries, including shallow open water, tidal pools, sandy beaches, mud and sand flats, freshwater and salt marshes, rocky shores, oyster reefs, mangrove forests, river deltas, wooded swamps, and kelp and sea grass beds.


The Water Quality Act of 1987 created the National Estuary Program (NEP) to help achieve long-term protection of living resources and water quality (the basic "fishable/swimmable" goal of the Clean Water Act) in estuaries. To improve an estuary, the NEP brings together community members to define program goals and objectives, identify estuary problems, and design action plans to prevent or control pollution, while restoring habitats and living resources such as shellfish. This approach results in the adoption of a comprehensive conservation and management plan for implementation in each estuary. This integrated watershed-based, stakeholder-oriented, water resource management approach has led to some significant local environmental improvements since its founding. The EPA notes in "National Estuary Program" that in 1987 the NEP consisted of six estuary programs located throughout the nation; in 2006 there were twenty-eight estuary programs in eighteen states and Puerto Rico.

In "National Estuary Program Success Stories" (December 12, 2006, http://www.epa.gov/owow/estuaries/success.htm), the EPA mentions several NEP successes. Two examples of environmental improvement resulting from the NEP can be found in the Leffis Key and Corpus Christi projects. The Leffis Key restoration project in Sarasota Bay, Florida, resulted in thirty acres of productive intertidal habitat being created and planted with more than fifty thousand native plants and trees at a cost of $315,000. In Corpus Christi Bay, Texas, treated biosolids were applied to a twenty-five-acre plot of aluminum mine tailings, resulting in plant growth promotion, wildlife habitat, and improved water quality. Biosolids are composed of sewage sludge that has been properly treated and processed to make a nutrient-rich material that can be safely recycled and applied as fertilizer.


According to the Chesapeake Bay Program Office (August 19, 2003, http://www.chesapeakebay.net/info/bayfaq.cfm#big), the Chesapeake Bay (the Bay) is the largest estuary in North America and one of the most productive estuaries in the world. It has a sixty-four-thousand-square-mile watershed that encompasses six states and the District of Columbia. Its watershed is home to more than fifteen million people and thirty-six hundred species of plants and animals. The Bay has over 11,600 miles of shoreline and averages 21 feet deep, with hundreds of thousands of acres of shallow water. It is two hundred miles long and thirty-five miles wide at its widest point.

The first estuary in the United States to be targeted for restoration and protection, the Bay is protected under its own federally mandated program, separate from the NEP. The Chesapeake Bay Program (2007, http://www.chesapeakebay.net/) began in 1983 with a meeting of the governors of Maryland, Pennsylvania, and Virginia; the mayor of the District of Columbia; and the EPA administrator. These individuals signed the Chesapeake Bay Agreement committing their states and the District of Columbia to prepare plans for protecting and improving water quality and living resources in the Chesapeake Bay. The Chesapeake Bay Program evolved as the institutional mechanism to restore the Bay and to meet the goals of the Chesapeake Bay Agreement.

Although great progress has been made in the Chesapeake Bay restoration, much remained to be done by early 2007. The editorial "A Cleaner Bay, a Step at a Time" (Salisbury Daily Times, February 8, 2007) notes that increased population growth in the Chesapeake Bay area is contributing to ongoing problems in the estuary. The editorial sums up the work still needed to meet a 2010 cleanup deadline:

  • About 1 million failing septic tanks across the regional watershed need to be dug up and repaired.
  • Farmerssome 80,000 of themwould have to make expensive and radical changes to the way they manage their land and their fertilizer application.
  • Hundreds of municipal and private sewage plants would have to be overhauled, costing millions of dollars each.
  • In all, about $28 billion would have to be spent, or double the $14 billion already being devoted toward the restoration effort.


The EPA, NOAA, the USGS, the U.S. Fish and Wildlife Service, coastal states, and the National Estuary Programs coordinate efforts to produce the National Coastal Condition Report, which describes the ecological and environmental conditions in U.S. coastal waters, including estuaries. The most recent report is the National Coastal Condition Report II (2005) (December 2004, http://www.epa.gov/owow/oceans/nccr/2005/downloads.html). Figure 6.5 is a summary graphic from this report and shows the overall national coastal condition for estuaries.

The report uses five indicators of estuarine condition. (See Figure 6.5.) For each indicator, researchers assessed the condition at 2,073 estuary sites in the "lower 48" states between 1997 and 2000. The second step was to assign a regional rating. The water quality index consists of indicators such as nitrogen and phosphorus levels, water clarity, and level of dissolved oxygen. The sediment quality index refers to the level of contamination of the sediment with toxic chemicals. The benthic index refers to organisms that live at the bottom of estuaries. A good benthic index is one in which a wide variety of benthic species are found, of which there are few pollution-tolerant species and several pollution-sensitive species that are found. The coastal habitat index is an assessment of the loss of wetland areas (the terrestrial-aquatic interface) of estuarine ecosystems. The fish tissue index refers to levels of chemical contaminants within fish.

The National Coastal Condition Report II states that "the overall condition of estuaries in the United States is fair." The report also notes that estuaries in the Northeast have poorer water quality conditions than those in other regions of the country. The sediment quality index is poor in the estuaries of the Great Lakes, the Northeast, and Puerto Rico. The benthic index is also poor in the Northeast and Puerto Rico. For estuaries along the northeastern coast, only the coastal habitat index has a good rating.

Figure 6.6 shows percentages of estuary area in the United States (excluding the Great Lakes) that is impaired, threatened, or unimpaired for human or aquatic life uses. Impaired human use means that fish are contaminated in those waters and are not acceptable to eat. Impaired aquatic life use means that the benthic index is poor. Threatened use correlates with a fair condition. Twenty-one percent of estuaries are unimpaired for human and/or aquatic life uses. Forty-four percent is threatened for both uses. Fifteen percent are impaired for both uses.


The EPA, states, tribes, and other federal agencies are collaborating on a new process to monitor the nation's waterways. Following the publication of the 2000 National Water Quality Inventory, the EPA entered a transition period in the gathering and analysis of water quality data in nationally consistent, statistically valid assessment reports. The EPA article "Schedule for Statistically Valid Surveys of the Nation's Waters" (December 5, 2005, http://www.epa.gov/owow/monitoring/guide.pdf) details its new reporting schedule.

In the 2000 National Water Quality Inventory, the EPA reports that fourteen of the twenty-seven coastal states and territories had rated the water quality of some of their coastal waters in 2000. The states had assessed 14% of the 22,618 miles of national coastline excluding Alaska, or 5.5% (3,221 miles) of ocean shoreline (including Alaska's 36,000 miles of coastline). Of the 14% of ocean waters assessed, 79% fully supported their designated uses, 14% were impaired, and 7% were supporting uses but threatened. Designated uses (such as fishing and drinking water supply) are the beneficial water uses assigned to each water body by a state as part of its water quality standards.

The EPA reports in the 2000 National Water Quality Inventory that bacteria (pathogens, which are disease-causing organisms) were identified as the leading contaminants of ocean shoreline waters, followed by oxygen-depleting substances and turbidity (cloudiness) in 2000. (See Figure 6.7.) Bacteria provide evidence of possible fecal contamination that may cause illness. States use bacterial indicators to determine if oceans are safe for swimming or secondary contact recreation, such as waterskiing. Figure 1.5 in Chapter 1 shows the pathways of bacteria to surface waters. The most common sources of bacteria are urban runoff, inadequately treated human sewage, and runoff from pastures and feedlots (nonpoint sources), all of which were identified by several states as leading sources of ocean shoreline impairment. (See Figure 6.8.)

Turbidity, which is a measure of the relative clarity of water, is caused by suspended matter or other impurities that make the water look cloudy. These impurities may include clay, silt, finely divided organic and inorganic matter, plankton, and other microscopic organisms. It interferes with the transmission of light to underwater grasses and other plant life in need of this light. If the transmission of light is reduced because of heavy silt in the water, this can smother bottom-dwelling organisms such as oysters. Turbidity was responsible for more than 10% of the impaired ocean shoreline miles reported to the EPA in 2000. (See Figure 6.7.) Three of the leading sources of ocean impairment are also contributors to turbidity: runoff from highly developed urban areas, agricultural activities (nonpoint sources), and construction projects. (See Figure 6.8.)

The EPA reports in the 2000 National Water Quality Inventory that most of the ocean waters assessed supported the five general-use categories shown for estuaries: aquatic life support, fish consumption, shellfishing, primary contact, and secondary contact. These categories represent summaries of the designated uses and their achievement provided by the states to the EPA. Waters that either support their designated uses only part of the time or do not support their uses at all are considered impaired. Good water quality supports primary contact (swimming without risk to public health) in 85% of the assessed ocean waters (the same percentage for use support in estuaries) and fish consumption (fish safe to eat) in 91% (compared with 52% of use support in estuaries).

In 94% of the waters assessed, the water was considered of good quality and capable of supporting aquatic life (suitable habitat for protection and propagation of desirable fish, shellfish, and other aquatic organisms). In the shellfish harvesting summary (water quality supports a population of bivalves free from toxicants and pathogens that can pose a health risk to people who eat them), 86% of the ocean waters assessed had good water quality that supported this use. In addition, good water quality in 91% of the ocean waters assessed supported secondary contact recreation (people can perform water-based activities such as waterskiing and boating without risk of adverse human health effects).


Beach closings take place hundreds of times each year to protect the public from possible exposure to pathogens. The bacteria that cause the closings are generally harmless, but they are present in large numbers in human and animal sewage. Their presence indicates the possible presence of disease-causing organisms.

The most common problem caused by swimming in contaminated water is gastroenteritis, which is contracted by swallowing water while swimming and can result in diarrhea, nausea, vomiting, and cramps. Even though gastroenteritis is generally not harmful to healthy adults, it can cause serious illness in children, the elderly, and people with autoimmune diseases, such as the human immunodeficiency virus and the acquired immunodeficiency syndrome.

The EPA established the Beaches Environmental Assessment and Coastal Health (BEACH) Program in 1997 to help reduce the risk of waterborne illness at the nation's beaches and recreational waters through improvements in water protection programs and risk communication. Three years later, the BEACH Act of 2000 was signed into law. This law was an amendment to the Clean Water Act and required:

  1. The EPA to issue new or revised water quality criteria for pathogens and pathogen indicators.
  2. Coastal states to adopt these new or revised water quality standards.
  3. The EPA to award grants to states and local governments to develop and implement beach monitoring and assessment programs.

The BEACH Act also required the EPA to prepare a progress report for Congress every four years. The first report was Implementing the BEACH Act of 2000: Report to Congress (October 2006. http://www.epa.gov/water science/beaches/report/full-rtc.pdf).

In Implementing the BEACH Act, the EPA determines that the major pollution sources responsible for beach closings and advisories in 2002 included runoff of storm water following rainfall (21%), sewage spills or overflows from various sources (13%), and unknown sources (43%). (See Figure 6.9.)

Table 6.5 shows the number of beaches surveyed for closings and advisories from 1997 through 2004. The number of beaches in the survey grew voluntarily from 1,021 in 1997 to 2,823 in 2002. Beginning in 2003, however, coastal states were required to report beach information to the EPA. Thus, the number surveyed in 2004 grew to 3,574.

Table 6.5 shows that from 1997 through 2004 approximately 26% of U.S. beaches were affected by advisories or closings. Nevertheless, the EPA notes in Implementing the BEACH Act that most of the advisories or closings lasted only one or two days. In 2004, for example, only 4% of possible open beach days were lost to advisories or closings.

As a result of the BEACH Act, the EPA has improved its Beach Advisory and Closing On-line Notification Web site (http://oaspub.epa.gov/beacon/beacon_national_page.main), which provides data to the public on beach advisories and closings. Along with collecting more comprehensive data and strengthening water quality standards, the EPA is also working to improve pollution control efforts at the nation's beaches.

Numbers and percentages of beaches affected by advisories or closings, 19972004
Voluntary survey Required reporting
1997 1998 1999 2000 2001 2002 2003 2004
*Incomplete data from 11 states; Environmental Protection Agency is working to complete data set.
Source: "Table 3.2. National Health Protection Survey of Beaches Trends, 19972004," in Implementing the BEACH Act of 2000 Report to Congress, U.S. Environmental Protection Agency, October 2006, http://www.epa.gov/waterscience/beaches/report/full-rtc.pdf (accessed February 6, 2007)
Number of beaches 1,021 1,403 1,891 2,354 2,445 2,823 1,857* 3,574
Number of beaches affected by advisories or closings 230 353 459 633 672 709   395 942
Percentage of beaches affected by advisories or closings 23 25 24 27 27 25    21 26


Any number of human-made materials or excessive amounts of naturally occurring substances can adversely affect marine and estuarine waters and their inhabitants. Because water is such an effective solvent and dispersant, it is difficult to track and quantify many pollutants known to have been discharged into marine and estuarine waters, and in many cases the source of pollution may be unknown. Some pollutants, such as oil spills, are easily detected the moment they enter the water. Others, such as toxic chemicals, are less obvious, and their presence may remain undetected until they cause extensive damage.

Oil Spills

Oil is one of the world's most important fuels. Its uneven distribution on the planet, however, forces its transport over the ocean, through pipelines, and over land. This inevitably results in accidents, some massive and some small, during drilling and transporting. In March 1967 the 118,285-ton supertanker Torrey Canyon, carrying oil from Kuwait, caused the world's first massive marine oil spill off the coast of England.

Oil spills are a dramatic form of water pollutionvisible, immediate, and sometimes severe. The sight of dead and dying otters and birds covered with black film arouses instant sympathy, and the bigger the spill, the more newsworthy it is. Even though it is true that oil can have a devastating effect on marine life, the size of the spill itself is often not the determining factor in the amount of damage it causes. Other factors include the amount and type of marine life in the area and weather conditions that can disperse the oil. Despite the drama that tanker spills create, worldwide pollution from them is a relatively minor source of marine pollution. Tanker spills represent a small fraction of the oil released to the environment worldwide when compared with industry sources, nontanker shipping releases, and oil seepage from natural sources.

According to the EPA (March 9, 2006, http://www.epa.gov/oilspill/exxon.htm), when the supertanker Exxon Valdez ran into a reef in Prince William Sound, Alaska, in March 1989, more than eleven million gallons of oil spilled into one of the richest and most ecologically sensitive areas in North America. The Exxon Valdez Oil Spill Trustee Council (2007, http://www.evostc.state.ak.us/History/PWSmap.cfm) states that a slick (spill area) of approximately eleven thousand square milesthe size of Rhode Island and Maryland combinedthreatened fish and wildlife. Otters died by the thousands, despite efforts by trained environmentalists and local volunteers to save them. Oil-soaked birds lined the shores, only to be eaten by larger predator birds, which then succumbed to dehydration and starvation because the ingested oil destroyed their metabolic systems.


In response to the Exxon Valdez disaster, Congress passed the Oil Pollution Act of 1990 (OPA). Most of the OPA provisions were targeted at reducing the number of spills and reducing the quantity of oil spilled. Among its provisions were the creation of a $1 billion cleanup-damage fund (the money comes from a tax on the petroleum industry), advance planning for controlling spills, stricter crew standards, and the requirement that new tankers have double hulls. When the exterior hull of a double-hulled tanker is punctured, the interior hull holding the oil may still remain intact. (The Exxon Valdez was not double-hulled.) The law requires older tankers to be fitted with double hulls by 2010. The OPA also:

  • Compels the use of escort tugboats in certain harbors to assist tankers.
  • Requires standards for tank levels and pressure-monitoring devices to detect leaks in cargo tanks.
  • Requires the U.S. Coast Guard to establish minimum standards for overfill devices to prevent overfill oil spills. (An overfill oil spill is the result of too much oil being pumped into a tanker during a transfer from a facility to a tanker or between two tankers. On occasion, overfill spills have involved large quantities of oil.)


Declines in oil spills are being seen on a global scale. The International Tanker Owners Pollution Federation reports that between 1970 and 1979 the average incident rate for large spills (over 700 metric tons or approximately 215,000 gallons) from the worldwide tanker industry was 25.2 spills per year. (See Figure 6.10.) Between 1980 and 1989 the average rate dropped to 9.3 spills per year. From 1990 to 1999 the global spill rate declined further to 7.8 spills per year and from 2000 through 2006 to 3.7 spills per year. Larger spills, such as these, are most often caused by collisions and groundings. Smaller spills most often occur because of routine operations, such as loading in ports or at oil terminals.

The decline in oil spills in U.S. waters is even more dramatic. Figure 6.11 shows the number of oil spills over one thousand gallons in U.S. waters from 1973 to 2004. Most spills are not shown on this graph because they are smaller than one thousand gallons. According to the report U.S. Coast Guard Polluting Incident Compendium: Cumulative Data and Graphics for Oil Spills 19732004 (2006, http://www.uscg.mil/hq/gm/nmc/response/stats/Summary.htm), 88% of all spills from 1973 to 2004 were between one and one hundred gallons. Spills over one hundred thousand gallons have not occurred in U.S. waters since 1996, and before that, since 1990.

Although oil tanker spills are highly visible cases of pollution entering the ocean, the U.S. Department of the Interior's Minerals Management Service reports in OCS Oil Spill Facts (September 2002, http://www.mms.gov/stats/PDFs/2002OilSpillFacts.pdf) that the largest input of oil into marine environments is natural seepage (naturally occurring oil in the ground that moves through the soil and into the water). In North America natural seepage contributes 63% of the total marine oil input. Twenty-two percent of the oil found in the marine waters off the coast of North America is due to municipal and industrial waste and runoff. Marine transportation is responsible for only 3%, although worldwide, it is responsible for 33%.

Marine Debris

NOAA (March 27, 2007, http://marinedebris.noaa.gov/whatis/welcome.html) defines marine debris as "any man-made object discarded, disposed of, or abandoned that enters the coastal or marine environment. It may enter directly from a ship, or indirectly when washed out to sea via rivers, streams and storm drains." The effects of marine debris can be both costly to coastal communities and dangerous to humans and aquatic life. Certain types of marine debris, such as broken glass and medical waste, can pose a serious threat to public health, causing beach closures and swimming advisories and robbing coastal communities of significant tourism dollars.

In December 2006 the Marine Debris Research, Prevention, and Reduction Act became law. The purpose of the act was to create a Marine Debris Research, Prevention, and Reduction Program within NOAA and the Coast Guard to help identify, determine sources of, assess, reduce, and prevent marine debris and its adverse impacts on the marine environment and navigation safety.


Laurie Asseo reports in "Cruise Line Fined Millions for Dumping" (Milwaukee Journal Sentinel, July 22, 1999) that in 1999 the Royal Caribbean, one of the world's largest cruise lines, pleaded guilty in federal court to dumping oil and hazardous chemicals in U.S. waters and lying about it to the Coast Guard. It agreed to pay a record $18 million fine for polluting waters. This was besides the $9 million in criminal fines the company agreed to pay in a previous plea agreement. Asseo notes that six other cruise lines have pleaded guilty to illegal waste dumping since 1993 and have paid fines of up to $1 million. These cases focus attention on the difficulties of regulating the expanding cruise line industry, because most major ships sailing out of U.S. ports are registered in foreign countries.

In "Cruise Ship Water Discharges" (March 23, 2007, http://www.epa.gov/owow/oceans/cruise_ships/), the EPA reports that there are more than 230 cruise ships operating worldwide. Feeding and housing thousands of people on each of these vessels means that a great deal of waste is generated while at sea. The EPA notes that "some of the waste streams generated by cruise ships include bilge water (water that collects in the lowest part of the ship's hull and may contain oil, grease, and other contaminants), sewage, graywater (waste water from showers, sinks, laundries and kitchens), ballast water (water taken onboard or discharged from a vessel to maintain its stability), and solid waste (food waste and garbage)."

The EPA is conducting a variety of activities to address the problem of cruise ship waste. Among these activities the agency is assessing the need for additional standards for waste discharge from cruise ships operating in Alaska and is preparing a cruise ship waste assessment report (September 14, 2006, http://www.epa.gov/owow/oceans/cruise_ships/disch_assess.html). The assessment report will characterize cruise ship waste, analyze existing programs for managing those wastes, and determine whether better management of those wastes is needed.


Plastics such as bags, containers, bottle caps, and beverage carriers are dumped daily from oceangoing vessels, commercial and recreational fishing boats, offshore oil and gas platforms, and military ships. Other types of plastic debrisfactory wastes, sewer overflows, illegal garbage dumping, and human litteringcome from land sources. Thousands of seabirds and marine animals die each year as a result of ingesting or becoming entangled in this plastic.

Another concern is commercial fishing nets. Once made of natural materials, these nets are now made mainly of durable, nondegradable plastic. When they are lost or discarded in the ocean, they pose a floating hazard to seals, dolphins, whales, and diving birds, which can become entangled in the nets. In 1988 thirty-one nations ratified an agreement making it illegal for their ships to dump plastic debris, including fishing nets, into the ocean. As part of that agreement, the United States enacted the Marine Plastics Pollution Research and Control Act, which went into effect in 1989. Among other regulations, the act imposes a $25,000 fine for each violation.

Plastic pellets are the raw materials that are melted and molded to create plastic products. According to Paul Watson, in "The Plastic Sea" (July 24, 2006, http://www.seashepherd.org/editorials/editorial_060724_1.html), sixty billion pounds of resin pellets are manufactured in the United States annually. The two primary ways that these pellets enter the ocean are direct spills during cargo handling operations at ports or spills at sea, and storm water discharges that carry the pellets from industrial sites. Plastic pellets may persist in the water environment for years, depending on the resin type, the amount and types of pellet additives, and how the pellets react to sunlight, wave action, and weathering. Although pellets have been found in the stomachs of wildlife, primarily seabirds and sea turtles, their effects have not been clearly demonstrated to be harmful.

Since 1991 the Society of Plastics Industries (SPI), the major national trade association for manufacturers who make plastic products in the United States, has been working with the EPA to identify and minimize the sources of plastic pellet entry into the ocean. In July 1991 the SPI instituted Operation Clean Sweep (http://www.opcleansweep.org/), an industry-wide education campaign to encourage members to adopt the SPI 1991 Pellet Retention Environmental Code and the 1992 Processor's Pledge aimed at committing the U.S. plastics industry to total pellet containment.


Another important problem is ghost fishing. This is the entrapment of fish and marine mammals by lost or abandoned nets, pots, fishing line, bottles, and other discarded objects. When marine creatures are entangled in old six-pack beverage binders or caught in abandoned fishing nets, they suffer and may die.

Ocean Dumping

In 1972 Congress enacted the Marine Protection, Research, and Sanctuaries Act (also known as the Ocean Dumping Act) to prohibit the dumping into the ocean of material that will unreasonably degrade or endanger human health or the marine environment. The act applies to waters within two hundred miles of the U.S. coast and was amended in 1988 to prohibit dumping industrial waste and sewage sludge into the ocean. As a result, the only ocean dumping allowed as of early 2007 was dredged material from the bottom of water bodies to maintain navigation channels and berthing areas. To dump dredged material in the ocean, a permit must be obtained from the EPA.


Algae are plantlike organisms that manufacture their own food via photosynthesis. Most algal species in U.S. coastal waters are not harmful and serve as the energy producers at the base of the food chain. However, sometimes algae may grow fast and bloom, creating dense, visible patches near the water surface. Red tide is a common name for events in which certain algae containing reddish pigments bloom so that the water appears to be red. Often, these particular species are toxic to humans and wildlife, but not all species of algae are toxic, nor do all species impart color to the water during blooms.

Eutrophication and Hypoxia

Algal blooms often occur because an abundance of plant nutrients, such as nitrates and phosphates, have entered the water. Thick layers of algae block the sunlight from reaching the algae and other plant life below, so those organisms die. (See Figure 6.12.) When the nutrients run out, much of the rest of the algae die. Bacteria and other decomposers feed on the dead algae, using oxygen in the water as they break down the tissues. This process, in which an overenrichment of a water body with nutrients results in an excessive growth of organisms and a resultant depletion of oxygen concentration, is called eutrophication. The deficiency of oxygen in the water is called hypoxia, and it is a condition that can have severe effects on local ecosystems.

Hypoxia kills most of the sessile (permanently attached) bottom-dwelling benthic organisms in a body of water, such as oysters and clams; aquatic animals that swim or crawl, such as fish, shrimp, and crabs, either leave the area or die as shown in Figure 6.12. For this reason, areas where hypoxic conditions exist are frequently referred to as dead zones. Hypoxia is a worldwide problem that often occurs where rivers carrying large amounts of agricultural runoff empty into lakes, estuaries, and the ocean.

In 1998 Congress passed the Harmful Algal Bloom and Hypoxia Research Act. The act requires the formation of a federal multiagency task force to investigate the problem and report back to Congress with a plan and recommendations to address harmful algal blooms and hypoxia. The Harmful Algal Bloom and Hypoxia Amendments Act of 2004 reauthorized the 1998 act.


One location in the United States where hypoxia occurs is the Gulf of Mexico, off the Louisiana coast. According to the USGS in the fact sheet "Restoring Life to the Dead Zone: Addressing Gulf Hypoxia, a National Problem" (June 2000, http://www.nwrc.usgs.gov/factshts/016-00/016-00.htm), the Gulf's hypoxic zone is comparable to the largest hypoxic areas in the world, such as those in the Black and Baltic seas. The Gulf of Mexico hypoxic zone is approximately six to seven thousand square miles of water where the oxygen level is below two ppm. Under normal conditions dissolved oxygen levels would be five to six ppm.

The zone is caused by harmful algal blooms that are believed to be the result of the discharge of nutrients from the Mississippi River watershed into the Gulf of Mexico. The nutrients (nitrogen and phosphorus) come from fertilizers, animal waste, and domestic sewage. The nitrate-nitrogen level in the main stem of the Mississippi River, which drains thirty-one states, has doubled since the 1950s. Figure 6.13 shows the Mississippi Basin watershed and the states whose rivers drain into it.

To correct the situation and as a requirement of the Harmful Algal Bloom and Hypoxia Research Act, the EPA, six other federal agencies, nine states, and two Native American tribes developed an action plan to reduce nutrient loads reaching the Gulf: the Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico (January 2001, http://www.epa.gov/msbasin/taskforce/pdf/actionplan.pdf). It has the goal of reducing the size of the hypoxic zone by over 68% no later than 2015. The plan also calls for implementation of nutrient management strategies to achieve a 30% reduction in the amount of nutrients reaching the Gulf of Mexico. Reducing nutrients in the water, particularly nitrogen and phosphorus, will help reverse the hypoxia. This is accomplished primarily by implementing farming practices that reduce fertilizer runoff, restoring wetland areas, and restoring riverbanks. Information generated through the research and monitoring portions of the plan will be used to modify future goals and actions as necessary. As of early 2007, the Basin Expert Review report was expected to be completed in the spring or summer of 2007, with a revised action plan due in 2008 (January 22, 2006, http://www.epa.gov/msbasin/taskforce/pdf/timeline_process01_06.pdf).


Exotic species are plants, animals, and microbes that have been carried from one geographic region to another, either intentionally or unintentionally. Unintentional introduction includes transport in ballast water of ships or as pests on imported fruits, vegetables, and animals or animal products. Before modern times, movement from one geographical region to another was infrequent and slow, allowing time for the ecology to absorb and counterbalance the newcomers.

However, because of rapid transport, organisms can now move across continents in a matter of hours or days. Once removed from their natural ecological system, where eons of evolution have established predator-prey relationships, competitive species, and other devices to maintain balance, exotic species may reproduce unchecked in their new locations because they have no natural competitors or predators.

Both estuarine and ocean habitats have suffered from exotic species introduction. In the Chesapeake Bay, MSX (Haplosporidium nelsoni ) and Dermo (Perkinsus marinus ), two oyster diseases that have ravaged oyster populations, came to the Bay with oysters introduced from other regions. The coral reefs in the Northern Mariana Islands are being decimated by the introduction of the crown-of-thorns starfish. The green crab introduced from the Baltic Sea to the shores of New England occurs in such high numbers that the crab is believed to be eating young scallops and other valuable seafood.

Passage of the Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990 was a first step in attempting to prevent species migration. This legislation authorized the Fish and Wildlife Service and NOAA to adopt regulations to prevent the unintentional introduction of aquatic nuisance species. In 1999 the Invasive Species Council was created by presidential executive order to oversee efforts to control unwanted exotic species. The council is chaired jointly by the secretaries of interior, agriculture, and commerce. Council members include the secretaries of state, treasury, and transportation, and the administrator of the EPA. To date, a variety of laws have been enacted and legislation has been introduced to help research this problem and find ways to abate and control it. The Northeast Midwest Institute, in "Biological Pollution" (February 20, 2007, http://www.nemw.org/biopollute.htm), provides a listing of the proposed and enacted legislation.

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ocean, interconnected mass of saltwater covering 70.78% of the surface of the earth, often called the world ocean. It is subdivided into four (or five) major units that are separated from each other in most cases by the continental masses. See also oceanography.

The World Ocean

Of the major units that comprise the world ocean, three—the Atlantic, Indian, and Pacific oceans—extend northward from Antarctica as huge "gulfs" separating the continents. The fourth, the Arctic Ocean, nearly landlocked by Eurasia and North America and nearly circular in outline, caps the north polar region. The Southern Ocean (also called the Antarctic Ocean) is now often considered a fifth, separate ocean, extending from the shores of Antarctica northward to about 60°S. The major oceans are further subdivided into smaller regions loosely called seas, gulfs, or bays. Some of these seas, such as the Sargasso Sea of the North Atlantic Ocean, are only vaguely defined, while others, such as the Mediterranean Sea or the Black Sea, are almost totally surrounded by land areas. Large and totally landlocked saltwater bodies such as the Caspian Sea are actually salt lakes.

The boundaries between oceans are usually designated by the continental land masses bordering them or by ridges in the ocean floor, which also serve as geographic boundaries. Where these features are absent (such as the ill-defined northern boundary of the Antarctic Ocean), the boundary is somewhat arbitrarily fixed by fluctuating zones of opposing currents that act as partial barriers to the mixing of waters between the two adjacent oceans.

The oceans are not uniformly distributed on the face of the earth. Continents and ocean basins tend to be antipodal, or diametrically opposed to one another, i.e., continents are found on the opposite side of the earth from ocean basins. For example, Antarctica is antipodal to the Arctic Ocean; Europe is opposed by the South Pacific Ocean. Furthermore, over two thirds of the earth's land area is found in the Northern Hemisphere, while the oceans comprise over 80% of the Southern Hemisphere.

The world ocean has an area of about 361 million sq km (139,400,000 sq mi), an average depth of about 3,730 m (12,230 ft), and a total volume of about 1,347,000,000 cu km (322,280,000 cu mi). Each cubic mile of seawater weighs approximately 4.7 billion tons and holds 166 million tons of dissolved solids. One of the most unique and intriguing aspects of ocean water is its salinity, or dissolved salt content. The measurement of salinity is essentially the determination of the amount of dissolved salts in 1 kg of ocean water and is expressed in parts per thousand (‰). Ocean salinities commonly range between 33 ‰ to 38 ‰, with an average of about 35 ‰. Thirty-five parts per thousand salinity is equivalent to 3.5% by weight. Six elements (chlorine, sodium, magnesium, sulfur, calcium, and potassium) constitute over 90% of the total salts dissolved in the oceans. Pressure in the ocean waters increases with increasing depth due to the weight of the overlying water. The pressure increases at the rate of 1 atmosphere for every 10 m (33 ft) of depth (1 atm=15 lb per sq in. or 1,016 dynes per sq cm). The average temperature of the oceans is 3.9°C (39°F).

It now appears that the waters making up the present oceans (and the gases that make up the present atmosphere) were not of cosmic origin, i.e., were not present in the primordial atmosphere. Instead, they were derived from the interior of the earth sometime in the first one or two billion years after the earth's formation. It is now also generally accepted that a new ocean crust has been forming more or less continuously for at least the past 200 million years through a process of volcanic activity along the midocean ridge system (see seafloor spreading), which consists of a series of underwater mountains. On the basis of present knowledge it seems highly probable that all ocean waters and atmospheric gases were gradually released by the separation of these volatile components from the silicate rocks of the crust and upper mantle through volcanic activity. (Molten lava is known to contain appreciable amounts of water and other volatiles that are released upon solidification.) With the passage of time, water released by volcanic activity gradually filled oceanic depressions.

Continental Shelves, Slopes, and Rises

Virtually all continents are surrounded by a gently sloping submerged plain called the continental shelf, which is an underwater extension of the coastal plain. The continental shelves are the regions of the oceans best known and the most exploited commercially. It is this region where virtually all of the petroleum, commercial sand and gravel deposits, and fishery resources are found. It is also the locus of waste dumping. Changes in sea level have alternatingly exposed and inundated portions of the continental shelf. Continental shelves vary in width from almost zero up to the 1,500-km-wide (930-mi) Siberian shelf in the Arctic Ocean. They average 78 km (48 mi) in width. The edge of the shelf occurs at a depth that ranges from 20 to 550 m (66 to 1,800 ft), averaging 130 m (430 ft). The shelves consist of vast deposits of sands, muds, and gravels, overlying crystalline rocks or vast thicknesses of consolidated sedimentary rocks. Although there is a great variation in shelf features, nonglaciated shelves are usually exceptionally flat, with seaward slopes averaging on the order of 205 m per km (10 ft per mi), or less than 1° of slope. The edge of the shelf, called the shelf break, is marked by an abrupt increase in slope to an average of about 4°.

The continental slopes begin at the shelf break and plunge downward to the great depths of the ocean basin proper. Deep submarine canyons, some comparable in size to the Grand Canyon of the Colorado River, are sometimes found cutting across the shelf and slope, often extending from the mouths of terrestrial rivers. The Congo, Amazon, Ganges, and Hudson rivers all have submarine canyon extensions. It is assumed that submarine canyons on the continental shelf were initially carved during periods of lower sea level in the course of the ice ages. Their continental slope extensions were carved and more recently modified by turbidity currents—subsea "landslides" of a dense slurry of water and sediment.

Many continental slopes end in gently sloping, smooth-surfaced features called continental rises. The continental rises usually have an inclination of less than 1/2°. They have been found to consist of thick deposits of sediment, presumably deposited as a result of slumping and turbidity currents carrying sediment off the shelf and slope. The continental shelf, slope, and rise together are called the continental margin.

Trenches, Plains, and Ridges

One of the most surprising findings of the early oceanographers was that the deepest parts of the oceans were not in the centers, as they had expected, but were in fact quite close to the margins of continents, particularly in the Pacific Ocean. Further exploration showed that these deeps were located in long V-shaped trenches bordering the seaward edge of volcanic island arcs. These trenches are one of the most striking features of the Pacific floor. Trenches virtually encircle the rim of the Pacific basin. The trenches have lengths of thousands of kilometers, are generally hundreds of kilometers wide, and extend 3 to 4 km (1.9–2.5 mi) deeper than the surrounding ocean floor. The greatest ocean depth has been sounded in the Challenger Deep of the Marianas Trench, a distance of 10,911.5 m (35,798.6 ft) below sea level.

The deep ocean floor begins at the seaward edge of the continental rise or marginal trench, if one is present, and extends seaward to the base of the underwater midocean mountains. Many relief features of great importance are present in this region. Vast abyssal plains cover significant portions of the deep ocean basin. Such plains are occasionally broken by low, oval-shaped abyssal hills. The abyssal plains cover about 30% of the Atlantic and nearly 75% of the Pacific ocean floors. They are among the flattest portions of the earth's crust and appear to be formed by the deposition of fine sediment carried by turbidity currents that have covered and smoothed out irregularities in the ocean floor.

One of the most significant features of the ocean basins is the midocean ridge. First discovered in the Atlantic Ocean on the Challenger expedition, its relief features were further investigated during the German Meteor expedition of 1925–26. By the early 1960s it had been confirmed that the Mid-Atlantic Ridge was only part of a continuous feature that extended 55,000 km (34,000 mi) through the Atlantic, Indian, South Pacific, and Arctic oceans. The ridge is a broad bulge in the ocean floor that rises 1 to 3 km (0.6–2 mi) above the adjacent abyssal plains. It has a variable width averaging more than 1,500 km (c.900 mi). It is crossed by a number of fracture zones (transform faults) and displays a deep rift 37 to 48 km (23–30 mi) wide and about 1.6 km (1 mi) deep at its very crest.

Relationship of the Ocean and the Atmosphere

The atmosphere affects the oceans and is in turn influenced by them. The action of winds blowing over the ocean surface creates waves and the great current systems of the oceans. When winds are strong enough to produce spray and whitecaps, tiny droplets of ocean water are thrown up into the atmosphere where some evaporate, leaving microscopic grains of salt buoyed by the turbulence of the air. These tiny particles may become nuclei for the condensation of water vapor to form fogs and clouds.

In turn, the oceans act upon the atmosphere—in ways not clearly understood—to influence and modify the world's climate and weather systems. When water evaporates, heat is removed from the oceans and stored in the atmosphere by the molecules of water vapor. When condensation occurs, this stored heat is released to the atmosphere to develop the mechanical energy of its motion. The atmosphere obtains nearly half of its energy for circulation from the condensation of evaporated ocean water.

Because the oceans have an extremely high thermal capacity when compared to the atmosphere, the ocean temperatures fluctuate seasonally much less than the atmospheric temperature. For the same reason, when air blows over the water, its temperature tends to come to the temperature of the water rather than vice versa. Thus maritime climates are generally less variable than regions in the interiors of the continents.

The relationships are not simple. The pattern of atmospheric circulation largely determines the pattern of oceanic surface circulation, which in turn determines the location and amount of heat that is released to the atmosphere. Also, the pattern of atmospheric circulation determines in part the location of clouds, which influences the locations of heating of the ocean surface.

Currents and Ocean Circulation

Surface Circulation

The surface circulation of the oceans is intimately tied to the prevailing wind circulation of the atmosphere (see wind). As the planetary winds flow across the water, frictional stresses are set up which push huge rivers of water in their path. The general pattern of these surface currents is a nearly closed system of currents, called gyres, which are approximately centered on the horse latitudes (about 30° latitude in both hemispheres). Major circulation of water in these gyres is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. In the North Pacific and North Atlantic oceans, smaller counterclockwise gyres are developed partly due to the presence of the continents. These are centered on about 50°N lat. The most dominant current in the Southern Ocean is the West Wind Drift, which circles Antarctica in an easterly direction. The northern and southern hemispheric gyres are divided by an eastward flowing equatorial countercurrent, which essentially follows the belt of the doldrums. This countercurrent is caused by the return flow of water piled up along the eastward portion of the equatorial seas, and its return flow is uninhibited by the weak and erratic winds of the doldrums. Analysis of current records shows that a number of major currents, such as the Gulf Stream, have strong fast-moving currents beneath them trending in the opposite direction to the surface current. Such undercurrents, or countercurrents, appear to be as important and pervasive as the surface currents. In 1952 the Cromwell current was found flowing eastward beneath the south equatorial current of the Pacific. In 1961 a similar current was discovered in the Atlantic. See also tide.

Thermohaline Circulation

Thermohaline circulation refers to the deepwater circulation of the oceans and is primarily caused by differences in density between the waters of different regions. It is mainly a convection process where cold, dense water formed in the polar regions sinks and flows slowly toward the equator. Most of the deep water acquires its characteristics in the Antarctic region and in the Norwegian Sea. Antarctic bottom water is the densest and coldest water in the ocean depths. It forms and sinks just off the continental slope of Antarctica and drifts slowly along the bottom as far as the middle North Atlantic Ocean, where it merges with other water. The circulation of ocean waters is vitally important in dispersing heat energy around the globe. In general, heat flows toward the poles in the surface currents, while the displaced cold water flows toward the equator in deeper ocean layers.

The Ocean as a Biological Environment

The oceans hold the answers to many important questions about the development of the earth and the history of life on earth. For instance, within the rocks and sediment of the ocean floors the geological history of the earth is recorded. Fossils in this sediment record a portion of the biological history of the earth at least back to the Jurassic period, which ended about 140,000,000 years ago. The first appearance of life on the earth is thought to have occurred in the oceans 2 or 3 billion years ago. The modern marine environment is divided into two major realms, the benthic and the pelagic, based upon the ecological characteristics and marine life associated with them. See also marine biology.

The Benthic Realm

The benthic realm refers to the floor of the oceans, extending from the high tide line to the greatest ocean depths. The organisms that live in or on the bottom are called benthos. The benthic realm is subdivided on the basis of depth into the littoral zone, which extends from high tide to a depth of about 200 m (660 ft), and the deep-sea realm. The benthic life forms are both sessile (attached) and motile (mobile). They are distributed from near-shore littoral regions to the ocean depths and play an important role in the food chain. Some benthonic life forms live by predation, others sift organic matter from the water, and others scavenge the bottom for organic debris that has settled there. Benthonic plants can live only in the euphotic zone, the uppermost 100–200 m (330–660 ft) of the ocean, where sunlight penetrates. Benthonic animals that live below the euphotic zone often must depend on the rain of organic debris from above to supply their food needs, and thus the deep regions of the benthic realm are not highly populated except in the areas around hydrothermal vents where chemosynthesis provides an alternative food source.

The Pelagic Realm

The pelagic realm consists of all of the ocean water covering the benthic realm. It is divided horizontally into the neritic, or fertile near-shore, province and the oceanic province. Vertically it is divided into the euphotic, or photic, zone and the aphotic (without sunlight) zone. Drifting, free-floating organisms, called plankton, and organisms with poor mobile ability populate the euphotic zone. Most plankton are microscopic or near-microscopic in size. Phytoplankton are photosynthetic bacteria (cyanbacteria) and floating algae, such as diatoms, dinoflagellates, and coccolithopores. Heterotrophic plankton (zooplankton) are floating animals and protozoans of the sea and rely on the phytoplankton as food sources. Foraminifera and radiolaria are the dominant protozoan zooplankton that secrete tests (shells), which become incorporated into the sediment of the ocean floor. Many juvenile forms of swimmers (such as shrimp) or bottom dwellers (such as barnacles) pass through a planktonic phase. Marine organisms capable of self-locomotion are called nektonic life forms. Fish, squid, and whales are examples of marine nekton.

Importance of the Ocean

Throughout history humans have been directly or indirectly influenced by the oceans. Ocean waters serve as a source of food and valuable minerals, as a vast highway for commerce, and provide a place for both recreation and waste disposal. Increasingly, people are turning to the oceans for their food supply either by direct consumption or indirectly by harvesting fish that is then processed for livestock feed. It has been estimated that as much as 10% of human protein intake comes from the oceans. Nevertheless, the food-producing potential of the oceans is only partly realized. Other biological products of the oceans are also commercially used. For example, pearls taken from oysters are used in jewelry, and shells and coral have been widely used as a source of building material.

Ocean water is processed to extract commercially valuable minerals such as salt, bromine, and magnesium. Although nearly 60 valuable chemical elements have been found dissolved in ocean water, most are in such dilute concentrations that commercial extraction is not profitable. In a few arid regions of the world, such as Ascension Island, Kuwait, and Israel, ocean water is desalinated to produce freshwater.

The shallow continental shelves have been exploited as a source of sands and gravels. In addition, extensive deposits of petroleum-bearing sands have been exploited in offshore areas, particularly along the Gulf and California coasts of the United States and in the Persian Gulf. On the deep ocean floor manganese nodules, formed by the precipitation of manganese oxides and other metallic salts around a nucleus of rock or shell, represent a potentially rich and extensive resource. Research is currently being conducted to explore nodule mining and metallic extraction techniques. Ocean water itself could prove to be a limitless source of energy in the event that nuclear fusion reactors are developed, since the oceans contain great quantities of deuterium.

The oceans also are important for recreational use, as each year more people are attracted to the sports of swimming, fishing, scuba diving, boating, and waterskiing. Ocean pollution, meantime, has escalated dramatically as those who use the oceans for recreational and commercial purposes, as well as those who live nearby, have disposed of more and more wastes there (see water pollution).


See also R. Carson, The Sea around Us (1961); J. Bardach, Harvest of the Sea (1968); J. R. Moore, ed., Oceanography (1971); R. Perry, The Unknown Ocean (1972); L. Paine, The Sea and Civilization (2013).

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Oceans are large bodies of salt water that surround Earth's continents and occupy the basins between them. The four major oceans of the world are the Atlantic, Arctic, Indian, and Pacific. These interconnected oceans are further divided into smaller regions of water called seas, gulfs, and bays.

The combined oceans cover almost 71 percent of Earth's surface, or about 139,400,000 square miles (361,000,000 square kilometers). The average temperature of the world's oceans is 39°F (3.9°C). The average depth is 12,230 feet (3,730 meters).

Origin of ocean water

One scientific theory about the origin of ocean water states that as Earth formed from a cloud of gas and dust more than 4.5 billion years ago, a huge amount of lighter elements (including hydrogen and oxygen) became trapped inside the molten interior of the young planet. During the first one to two billion years after Earth's formation, these elemental gases rose through thousands of miles of molten and melting rock to erupt on the surface through volcanoes and fissures (long narrow cracks).

Within the planet and above the surface, oxygen combined with hydrogen to form water. Enormous quantities of water shrouded the globe as an incredibly dense atmosphere of water vapor. Near the top of the atmosphere, where heat could be lost to outer space, water vapor condensed to liquid and fell back into the water vapor layer below, cooling the layer. This atmospheric cooling process continued until the first raindrops fell to the young Earth's surface and flashed into steam. This was the beginning of a fantastic rainstorm that, with the passage of time, gradually filled the ocean basins.

Words to Know

Continental margin: Underwater plains connected to continents, separating them from the deep ocean floor.

Fracture zone: Faults in the ocean floor that form at nearly right angles to the ocean's major ridges.

Guyot: An extinct, submarine volcano with a flat top.

Ridge: Very long underwater mountain ranges created as a by-product of seafloor spreading.

Rift: Crevice that runs down the middle of a ridge.

Seafloor spreading: Process whereby new oceanic crust is created at ridges.

Seamount: Active or inactive submarine volcano.

Cosmic rain. In mid-1997, however, scientists offered a new theory on the how the oceans possibly filled in. The National Aeronautics and Space Administration's Polar satellite, launched in early 1996, discovered that small comets about 40 feet (12 meters) in diameter are bombarding Earth's atmosphere at a rate of about 43,000 a day. These comets break up into icy fragments at heights 600 to 15,000 miles (960 to 24,000 kilometers) above ground. Sunlight then vaporizes these fragments into huge clouds, which condense into rain as they sink lower in the atmosphere.

Scientists calculate that this cosmic rain adds one inch of water to Earth's surface every 10,000 to 20,000 years. This amount of water could have been enough to fill the oceans if these comets have been entering Earth's atmosphere since the planet's beginning 4.5 billion years ago.

Ocean basin

Ocean basins are that part of Earth's surface that extends seaward from the continental margins (underwater plains connected to continents, separating them from the deep ocean floor). Basins range from an average water depth of about 6,500 feet (2,000 meters) down into the deepest trenches. Ocean basins cover about 70 percent of the total ocean area.

The familiar landscapes of continents are mirrored, and generally magnified, by similar features in the ocean basin. The largest underwater mountains, for example, are higher than those on the continents. Underwater plains are flatter and more extensive than those on the continents. All basins contain certain common features that include oceanic ridges, trenches, fracture zones, abyssal plains, and volcanic cones.

Oceanic ridges. Enormous mountain ranges, or oceanic ridges, cover the ocean floor. The Mid-Atlantic Ridge, for example, begins at the tip of Greenland, runs down the center of the Atlantic Ocean between the Americas on the west and Africa on the east, and ends at the southern tip of the African continent. At that point, it stretches around the eastern edge of Africa, where it becomes the Mid-Indian Ridge. That ridge continues eastward, making connections with other ridges that eventually end along the western coastline of South and Central America. Some scientists say this is a single oceanic ridge that encircles Earth, one that stretches a total of more than 40,000 miles (65,000 kilometers).

In most locations, oceanic ridges are 6,500 feet (2,000 meters) or more below the surface of the oceans. In a few places, however, they actually extend above sea level and form islands. Iceland (in the North Atlantic), the Azores (about 900 miles [about 1,500 kilometers] off the coast of Portugal), and Tristan de Cunha (in the South Atlantic midway between southern Africa and South America) are examples of such islands.

Running along the middle of an oceanic ridge, there is often a deep crevice known as a rift, or median valley. This central rift can plunge as far as 6,500 feet (2,000 meters) below the top of the ridge that surrounds it. Scientists believe ocean ridges are formed when molten rock, or magma, escapes from Earth's interior to form the seafloor, a process known as seafloor spreading. Rifts may be the specific parts of the ridges where the magma escapes.

Trenches. Trenches are long, narrow, canyonlike structures, most often found next to a continental margin. They occur much more commonly in the Pacific than in any of the other oceans. The deepest trench on Earth is the Mariana Trench, which runs from the coast of Japan south and then west toward the Philippine Islandsa distance of about 1,580 miles (2,540 kilometers). Its deepest spot is 36,198 feet (11,033 meters) below sea level. The longest trench is located along the coast of Peru and Chile. Its total length is 3,700 miles (5,950 kilometers) and it has a maximum depth of 26,420 feet (8,050 meters). Earthquakes and volcanic activity are commonly associated with trenches.

Fracture zones. Fracture zones are regions where sections of the ocean floor slide past each other, relieving tension created by seafloor spreading at the ocean ridges. Ocean crust in a fracture zone looks like it has

been sliced up by a giant knife. The faults in a zone usually cut across ocean ridges, often nearly at right angles to the ridge. A map of the North Atlantic Ocean basin, for example, shows the Mid-Atlantic Ridge traveling from north to south across the middle of the basin, with dozens of fracture zones cutting across the ridge from east to west.

Abyssal plains. Abyssal plains are relatively flat areas of the ocean basin with slopes of less than one foot of elevation difference for each thousand feet of distance. They tend to be found at depths of 13,000 to 16,000 feet (4,000 to 5,000 meters). Oceanographers believe that abyssal plains are so flat because they are covered with sediments (clay, sand, and gravel) that have been washed off the surface of the continents for hundreds of thousands of years. On the abyssal plains, these layers of sediment have now covered up any irregularities that may exist in the rock of the ocean floor beneath them.

Abyssal plains found in the Atlantic and Indian Oceans tend to be more extensive than those in the Pacific Ocean. One reason for this phenomenon is that the majority of the world's largest rivers empty into either the Atlantic or the Indian Oceans, providing both ocean basins with an endless supply of the sediments from which abyssal plains are made.

Volcanic cones. Ocean basins are alive with volcanic activity. Magma flows upward from the mantle to the ocean bottom not only through rifts, but also through numerous volcanoes and other openings in the ocean floor. Seamounts are submarine volcanoes and can be either active or extinct. Guyots are extinct volcanoes that were once above sea level but have since receded below the surface. As they receded, wave or current action eroded the top of the volcano to a flat surface.

Seamounts and guyots typically rise about 0.6 mile (1 kilometer) above the ocean floor. One of the largest known seamounts is Great Meteor Seamount in the northeastern part of the Atlantic Ocean. It extends to a height of more than 1,300 feet (4,000 meters) above the ocean floor.

[See also Coast and beach; Continental margin; Currents, ocean; Oceanography; Ocean zones; Plate tectonics; Tides; Volcano ]

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Oceans and Seas

Oceans and seas

Oceans are large bodies of saltwater connected together, unevenly covering 70% of the earth, and containing about 97% of Earth's water supply as salt water. The five major oceans, in descending order of size by area , are the Pacific Ocean (64 million square miles, about 165 million square kilometers), Atlantic Ocean (33 million square miles, about 85 million square kilometers), Indian Ocean (28 million square miles, about 70 million square kilometers), Southern Ocean (almost 8 million square miles, about 20 million square kilometers), and Arctic Ocean (5,000,000 square miles, almost 13 million square kilometers).

The Pacific Ocean is the largest ocean, covering about one third of the earth's surface, which is a bigger area than all of the continents combined. The Pacific Ocean is not only the home of the highest mountain on Earth (Mauna Kea, 33,476 ft, or 10,203 m), but also has the deepest trench (Mariana trench, 36,198 ft, or 11,033 m deep). The Atlantic Ocean is the second largest, covering 20% of Earth's surface. It is also the youngest among the five oceans, and it is the ocean where the most shipping occurs. The third largest ocean is the Indian Ocean, which provides an important trade route between Africa and Asia , and is home to the most expressed monsoon system. The next in size is the Southern Ocean, which surrounds Antarctica , and was officially named in the year 2000 by the International Hydrographic Organization. Finally, the Arctic Ocean is the smallest ocean of all five, but contains the widest continental shelf .

Seas are the smaller bodies of salt water connecting the oceans, which can be partially or entirely enclosed by land. Examples of seas adjacent to the oceans include the South China Sea (the largest sea), Caribbean Sea, Mediterranean Sea, Bering Sea, Gulf of Mexico , Hudson Bay, Gulf of California, Sea of Japan, and Persian Gulf. Examples of inland seas (or lakes ) are the Caspian Sea, Sea of Galilee, or Dead Sea.

Earth's oceans and seas are unique in that there is no other planet in our Solar System that has liquid water on its surface. Life on Earth began in the oceans, and today they still are the home to some of the most spectacular wildlife in the world. Most of the oceans' wildlife is located in the upper ocean layers, which contain about two percent of the oceans' volume. The oceans also play a significant role in the earth's water cycle. Oceans are a large source of water vapor for the atmosphere, which is important in heat transportation in the atmosphere in the form of latent heat. Additionally, the oceans gather water at the end of the water cycle not only from precipitation , but also from surface runoff and return flow from rivers and as groundwater flows from land. The oceans are major reservoirs in the carbon cycle. In order to double the current atmospheric carbon dioxide , it would be necessary to release only 2% of the carbon currently stored in the oceans.

Oceans produce important and widespread effects on Earth's atmosphere, weather , and climate . Oceans and land exhibit different heating and cooling properties; solar energy penetrates deeper into water than into land, and water can circulate that absorbed heat easily into deeper layers. Because the specific heat of water (the amount of heat required to raise the temperature of one gram of water by one degree Celsius) is higher than that of land, it takes about five times more energy to warm up water by one degree Celsius than to equally heat a rock . Consequently, oceans not only warm more slowly than land, but also cool more slowly. Oceans, therefore, act as a giant heat reservoir, which heats the land during winter and cools it during summer, moderating the climate of the land located next to it.

Oceans not only moderate the climate of adjacent areas by absorbing and storing solar energy, they also distribute heat between lower and higher latitudes by a global, interconnected system of ocean currents. An example of the climatic effects of oceans on lands is the Gulf Stream , which is not only part of the heat redistributing process by carrying warm waters towards higher latitudes, but also brings mild air to the British Isles and Northwest Europe , causing a significantly milder climate than it would normally have according to its latitudes.

Interesting examples of the interaction between the oceans and the atmosphere are the El Niño and La Niña phenomena patterns. Along with the Southern Oscillation, El Niño and La Niña influence not only nearby areas in the Pacific Ocean, but effect the entire global climate system, along with the ecologies and economies of many countries worldwide, from New Zealand to the United States.

As the oceans and seas are a significant part of the atmosphere and the climate system with many interactions and feedback mechanisms, there is a recent debate about their role in anthropogenic climate change, and also about the possible consequences of climate change for the oceans. Rising sea levels could occur due to both the thermal expansion of the oceans, and from the partial melting of the polar ice because of global warming . Although there is a debate between scientists about the possibility and the intensity of this prediction, if rising sea levels happen at any level, it could be potentially devastating for many coastal cities around the world.

See also El Niño and La Niña phenomena; Hydrologic cycle; Ocean circulation and currents

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ocean Continuous body of salt water that surrounds the continents and fills the Earth's great depressions. Oceans cover c.71% of the Earth's surface (more than 80% of the Southern Hemisphere), and represent c.98% of all the water on the face of the Earth. There are five main oceans, the Atlantic, Pacific, Indian, Arctic, and Antarctic. The oceans are constantly moving in currents, tides, and waves, and they form an integral part of the Earth's hydrological cycle and climate. They are a rich source of fossils and minerals, such as oil and gas. Marine fauna, such as fish and plankton, are a vital part of the food chain. Total area: 360 million sq km (138 million sq mi). Total volume: c.1.4 billion cu km (322 million cu mi). Average depth: 3500m (12,000ft). Average temperature: 3.9°C (39°F). See also Antarctica; continental drift; desalination; seafloor spreading; water pollution

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ocean †proper name of the great outer sea surrounding the mass of land of the Eastern Hemisphere XIII; any of the main regions into which the water of the globe is geographically divided XIV. ME. occean(e) — OF. occean(e) (mod. océan) — L. ōceanus — Gr. ōkeanós orig. the great river encompassing the disc of the earth and personified as a god, son of Uranus (heaven) and Gaia (earth).
So oceanic XVII. — medL. ōceanicus.

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o·cean / ˈōshən/ • n. a very large expanse of sea, in particular, each of the main areas into which the sea is divided geographically: the Atlantic Ocean. ∎  (usu. the ocean) the sea: [as adj.] the ocean floor. ∎  (an ocean of/oceans of) fig. a very large expanse or quantity: she had oceans of energy. DERIVATIVES: o·cean·ward / -wərd/ (also -wards) adv. & adj.

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Average depth

Greatest known depth

sq km

sq mi






* 7th deepest trench in the world; 8 of the deepest 10, including 1–6, are in the Pacific Ocean







Mariana Trench









Puerto Rico Trench*









Java Trench









Molloy Deep



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ocean The salt-water mass that occupies more than two-thirds of the surface of the Earth (70.8 per cent). The oceans contain 1370 × 106 km3 of water; the average depth is 3730 m.

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ocean Salt water mass that occupies more than two-thirds of the surface of the Earth (70.8%). The oceans contain 1370 × 106 km3 of water; the average depth is 3730 m.

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AILSA ALLABY and MICHAEL ALLABY. "ocean." A Dictionary of Earth Sciences. 1999. Retrieved May 31, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-ocean.html


oceanashen, fashion, passion, ration •abstraction, action, attraction, benefaction, compaction, contraction, counteraction, diffraction, enaction, exaction, extraction, faction, fraction, interaction, liquefaction, malefaction, petrifaction, proaction, protraction, putrefaction, redaction, retroaction, satisfaction, stupefaction, subtraction, traction, transaction, tumefaction, vitrifaction •expansion, mansion, scansion, stanchion •sanction •caption, contraption •harshen, Martian •cession, discretion, freshen, session •abjection, affection, circumspection, collection, complexion, confection, connection, convection, correction, defection, deflection, dejection, detection, direction, ejection, election, erection, genuflection, imperfection, infection, inflection, injection, inspection, insurrection, interconnection, interjection, intersection, introspection, lection, misdirection, objection, perfection, predilection, projection, protection, refection, reflection, rejection, resurrection, retrospection, section, selection, subjection, transection, vivisection •exemption, pre-emption, redemption •abstention, apprehension, ascension, attention, circumvention, comprehension, condescension, contention, contravention, convention, declension, detention, dimension, dissension, extension, gentian, hypertension, hypotension, intention, intervention, invention, mention, misapprehension, obtention, pension, prehension, prevention, recension, retention, subvention, supervention, suspension, tension •conception, contraception, deception, exception, inception, interception, misconception, perception, reception •Übermenschen • subsection •ablation, aeration, agnation, Alsatian, Amerasian, Asian, aviation, cetacean, citation, conation, creation, Croatian, crustacean, curation, Dalmatian, delation, dilation, donation, duration, elation, fixation, Galatian, gyration, Haitian, halation, Horatian, ideation, illation, lavation, legation, libation, location, lunation, mutation, natation, nation, negation, notation, nutation, oblation, oration, ovation, potation, relation, rogation, rotation, Sarmatian, sedation, Serbo-Croatian, station, taxation, Thracian, vacation, vexation, vocation, zonation •accretion, Capetian, completion, concretion, deletion, depletion, Diocletian, excretion, Grecian, Helvetian, repletion, Rhodesian, secretion, suppletion, Tahitian, venetian •academician, addition, aesthetician (US esthetician), ambition, audition, beautician, clinician, coition, cosmetician, diagnostician, dialectician, dietitian, Domitian, edition, electrician, emission, fission, fruition, Hermitian, ignition, linguistician, logician, magician, mathematician, Mauritian, mechanician, metaphysician, mission, monition, mortician, munition, musician, obstetrician, omission, optician, paediatrician (US pediatrician), patrician, petition, Phoenician, physician, politician, position, rhetorician, sedition, statistician, suspicion, tactician, technician, theoretician, Titian, tuition, volition •addiction, affliction, benediction, constriction, conviction, crucifixion, depiction, dereliction, diction, eviction, fiction, friction, infliction, interdiction, jurisdiction, malediction, restriction, transfixion, valediction •distinction, extinction, intinction •ascription, circumscription, conscription, decryption, description, Egyptian, encryption, inscription, misdescription, prescription, subscription, superscription, transcription •proscription •concoction, decoction •adoption, option •abortion, apportion, caution, contortion, distortion, extortion, portion, proportion, retortion, torsion •auction •absorption, sorption •commotion, devotion, emotion, groschen, Laotian, locomotion, lotion, motion, notion, Nova Scotian, ocean, potion, promotion •ablution, absolution, allocution, attribution, circumlocution, circumvolution, Confucian, constitution, contribution, convolution, counter-revolution, destitution, dilution, diminution, distribution, electrocution, elocution, evolution, execution, institution, interlocution, irresolution, Lilliputian, locution, perlocution, persecution, pollution, prosecution, prostitution, restitution, retribution, Rosicrucian, solution, substitution, volution •cushion • resumption • München •pincushion •Belorussian, Prussian, Russian •abduction, conduction, construction, deduction, destruction, eduction, effluxion, induction, instruction, introduction, misconstruction, obstruction, production, reduction, ruction, seduction, suction, underproduction •avulsion, compulsion, convulsion, emulsion, expulsion, impulsion, propulsion, repulsion, revulsion •assumption, consumption, gumption, presumption •luncheon, scuncheon, truncheon •compunction, conjunction, dysfunction, expunction, function, junction, malfunction, multifunction, unction •abruption, corruption, disruption, eruption, interruption •T-junction • liposuction •animadversion, aspersion, assertion, aversion, Cistercian, coercion, conversion, desertion, disconcertion, dispersion, diversion, emersion, excursion, exertion, extroversion, immersion, incursion, insertion, interspersion, introversion, Persian, perversion, submersion, subversion, tertian, version •excerption

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"ocean." Oxford Dictionary of Rhymes. 2007. Encyclopedia.com. 31 May. 2016 <http://www.encyclopedia.com>.

"ocean." Oxford Dictionary of Rhymes. 2007. Encyclopedia.com. (May 31, 2016). http://www.encyclopedia.com/doc/1O233-ocean.html

"ocean." Oxford Dictionary of Rhymes. 2007. Retrieved May 31, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O233-ocean.html

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