• lagoon
    The tropical lagoons of Bora Bora are protected from harsh ocean waves by atolls and barrier islands.

    Photograph by George F. Mobley

    Hapua
    Hapua ecosystems are lagoons located near river mouths. As rivers carve deep channels parallel to the coastline, they create a unique type of coastal lagoon. Hapua are primarily freshwater ecosystems, but interact strongly with ocean tides. Hapua are identified almost entirely with river systems in New Zealand.

    Blue Lagoon
    The world's most famous lagoon, the Blue Lagoon in Iceland, is not a lagoon at all. It is a manmade feature where water from a local geothermal power plant is pumped over a lava bed rich in silica and sulfur. These elements react with the warm water to create a bright blue lake used as a spa.

    A lagoon is a shallow body of water protected from a larger body of water (usually the ocean) by sandbars, barrier islands, or coral reefs. Lagoons are often called estuaries, sounds, bays, or even lakes.

    Coastal Lagoons

    Lagoons sheltered by sandbars or barrier islands are called coastal lagoons. Coastal lagoons form along coastal plains—flat or gently sloping landscapes. They form in areas with small tidal ranges. Coastal lagoons are created as a shallow basin near the shore gradually erodes, and the ocean seeps in between the sandbars or barrier islands.

    The size and depth of coastal lagoons often depend on sea level. When the sea level is low, coastal lagoons are swampy wetlands. When the sea level is high, they can look like coastal lakes or bays.

    The Outer Banks are barrier islands along the coast of the U.S. states of North Carolina and Virginia. The Outer Banks create a series of lagoons known as sounds: Currituck Sound, Albemarle Sound, and Pamlico Sound. These areas are sheltered from storm surges and other waves that often pound the shore during the Atlantic Ocean's hurricane season.

    The Outer Banks are actually enormous sandbars. They are not anchored to the earth, and suffer from coastal erosion during storms. The protection they offer the shores and lagoons is vital to the environment and economy of the region. Engineers continually monitor and maintain the Outer Banks by dredging sand from the seafloor to fortify the islands.

    The lagoons of the Outer Banks have mostly brackish water, a mix of saltwater from the Atlantic Ocean and freshwater from many river mouths in the area. The area is rich in biodiversity: waterfowl and fish from flounder to bass thrive in the region.

    The tourism industry also thrives in the coastal lagoons of the Outer Banks. Besides fishing, visitors to the sounds enjoy boating and recreational activities such as water skiing and parasailing.

    Lagoons with more protection from the open ocean have a more freshwater habitat. Lake Nokoue, Benin, is a lagoon whose narrow mouth to the Atlantic Ocean is almost entirely protected by sandbars. Its salinity varies with the seasons. During the rainy season, when rivers flood the lake with their outflow, Lake Nokoue is almost entirely freshwater. During the dry season, when river slow to a trickle and seawater seeps in, Lake Nokoue has a more brackish ecosystem. Fish indigenous to Lake Nokoue, such as tilapia, have adapted to survive in both brackish and freshwater.

    Coastal lagoons, which offer protection from harsh ocean waves, are often used as harbors. Lake Piso, for example, is the largest lake in the African country of Liberia. It is a lagoon protected from the Atlantic Ocean by big barrier islands. Lake Piso was used as a harbor for U.S. seaplanes during World War II.

    Lake Nokoue offered a different type of protection during the 16th and 17th centuries. Slave-trading tribes were forbidden from entering the waters of the lagoon, so local communities constructed an entire town, Ganvie, directly in the water. Homes and businesses were built on sturdy stilts, and transportation was limited to boats and bridges. Inhabitants were protected from capture and enslavement.

    Venice

    The city of Venice, Italy, is built on barrier islands and a coastal lagoon of the Adriatic Sea. In fact, Venice's nickname is "Queen of the Adriatic."

    The Venetian Lagoon is the largest wetland in the Mediterranean. It consists mostly of saltwater marshes and mudflats. Two large rivers (the Sile and the Brenta) empty into the lagoon. Its thin barrier islands have three narrow openings to the Adriatic.

    Venice, however, has been one of the largest cities in Italy since the rise of Ancient Rome. Human activity has radically altered the ecosystem of the Venetian Lagoon.

    Today, Venice sits on 118 islands. Not all of these islands are natural features of the landscape. For more than 500 years, engineers and city leaders have dredged the lagoon to create a series of islands and canals. Wetland areas have also been drained to create land for housing and industry.

    The growth of Venice has also drained the aquifer beneath the lagoon and surrounding coast. As the aquifer shrank, the land above it subsided—Venice sank. Venice's lower elevation made it increasingly vulnerable to strong seasonal tides from the Adriatic.

    Artesian wells were banned in the 1960s, and engineers have developed a sophisticated tide barrier project to reduce subsidence and protect the city from flooding.

    The Venetian Lagoon has recovered. Subsidence has slowed, although the famous aqua alta (high water) tide still floods the city in as much as 1.5 meters (5 feet) of water every winter.

    Atoll Lagoons

    Atoll lagoons are similar to coastal lagoons. Instead of being sheltered by sandbars or barrier islands, however, atoll lagoons are protected by coral reefs. Atoll lagoons are very common in the tropical waters of the South Pacific and Indian Oceans.

    Atoll lagoons form as coral reefs form around volcanic islands. Over millions of years, the island subsides into the ocean. The ring of coral reefs, however, remain. The reefs become the atoll, protecting an enclosed lagoon where the volcano used to be.

    Atoll lagoons are marine ecosystems. The organisms found in atoll lagoons are usually the same ones found outside it. Because of the ringing atoll, many lagoons have few indigenous species at all. Organisms, such as fish and jellies, surf in as waves from the ocean crash over the atoll and dump them in the lagoon. Many species of jellies thrive in this protected environment, but larger predators have few food resources.

    The water of atoll lagoons are often a striking light blue due to their shallow depth and their interaction with limestone. Coral reefs and coral sand are made of limestone, the remains of billions of tiny coral exoskeletons. As limestone leaches into the lagoon, it turns the water bright blue.

    The billion-dollar tourism industry of the South Pacific relies on pristine beaches and bright blue lagoons. These atoll lagoons are also the site of some of the most intense debates about climate change and sea level rise.

    Lagoons and atolls are low-lying ecosystems vulnerable to even the slightest change in sea level. Sea level rise could drown the lagoons, and even their ringing atolls. Island nations such as Maldives could lose not only their primary industry (tourism), but the land itself. Maldivian leaders have worked to combat sea level rise and coastal erosion by pursuing international agreements to limit human contributions to global warming, erecting buildings on stilts, and even considering evacuating the entire population.

  • Term Part of Speech Definition Encyclopedic Entry
    alter Verb

    to change.

    aquifer Noun

    an underground layer of rock or earth which holds groundwater.

    Encyclopedic Entry: aquifer
    artesian well Noun

    type of confined aquifer that flows to the Earth's surface without the need for pumping.

    atoll Noun

    a coral reef or string of coral islands that surrounds a lagoon.

    Encyclopedic Entry: atoll
    atoll lagoon Noun

    shallow, circular body of water between the ocean and a ring-shaped atoll.

    barrier island Noun

    long, narrow strip of sandy land built up by waves and tides that protects the mainland shore from erosion.

    basin Noun

    a dip or depression in the surface of the land or ocean floor.

    Encyclopedic Entry: basin
    biodiversity Noun

    all the different kinds of living organisms within a given area.

    Encyclopedic Entry: biodiversity
    brackish water Noun

    salty water, usually a mixture of seawater and freshwater.

    canal Noun

    artificial waterway.

    climate change Noun

    gradual changes in all the interconnected weather elements on our planet.

    Encyclopedic Entry: climate change
    coastal erosion Noun

    wearing away of earth or sand on the beach by natural or man-made methods.

    coastal lagoon Noun

    shallow body of water between the coast and a series of sandbars or barrier islands.

    coastal plain Noun

    low, flat land lying next to the ocean.

    Encyclopedic Entry: coastal plain
    coral reef Noun

    rocky ocean features made up of millions of coral skeletons.

    dredge Verb

    to remove sand, silt, or other material from the bottom of a body of water.

    ecosystem Noun

    community and interactions of living and nonliving things in an area.

    Encyclopedic Entry: ecosystem
    engineer Noun

    person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

    erode Verb

    to wear away.

    exoskeleton Noun

    the hard external shell or covering of some animals.

    fortify Verb

    to strengthen.

    global warming Noun

    increase in the average temperature of the Earth's air and oceans.

    Encyclopedic Entry: global warming
    harbor Noun

    part of a body of water deep enough for ships to dock.

    Encyclopedic Entry: harbor
    hurricane season Noun

    time of year when the risk of hurricanes is greatest. Atlantic hurricane season runs from June 1 to November 30.

    lagoon Noun

    shallow body of water that may have an opening to a larger body of water, but is also protected from it by a sandbar or coral reef.

    Encyclopedic Entry: lagoon
    leach Verb

    to separate materials by running water or another liquid through them.

    limestone Noun

    type of sedimentary rock mostly made of calcium carbonate from shells and skeletons of marine organisms.

    marsh Noun

    wetland area usually covered by a shallow layer of seawater or freshwater.

    Encyclopedic Entry: marsh
    mouth Noun

    place where a river empties its water. Usually rivers enter another body of water at their mouths.

    Encyclopedic Entry: mouth
    mudflat Noun

    coastal wetland formed as rivers or tides deposit sediment.

    Outer Banks Noun

    barrier islands off the coast of the U.S. state of North Carolina.

    pristine Adjective

    pure or unpolluted.

    salinity Noun

    saltiness.

    sandbar Noun

    underwater or low-lying mound of sand formed by tides, waves, or currents.

    sea level rise Noun

    increase in the average reach of the ocean. The current sea level rise is 1.8 millimeters (.07 inch) per year.

    seep Verb

    to slowly flow through a border.

    sound Noun

    body of water, larger than a bay, partially surrounded by land.

    storm surge Noun

    abnormal rise in sea level accompanying a hurricane or other intense storm. Also called a storm tide.

    Encyclopedic Entry: storm surge
    subside Verb

    to return to a lower level.

    tidal range Noun

    the difference in height between an area's high tide and low tide.

    tourism Noun

    the industry (including food, hotels, and entertainment) of traveling for pleasure.

    volcanic island Noun

    land formed by a volcano rising from the ocean floor.

    wetland Noun

    area of land covered by shallow water or saturated by water.

    Encyclopedic Entry: wetland
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