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.
lagoon, area of relatively shallow, quiet water situated in a coastal environment and having access to the sea but separated from the open marine conditions by a barrier. The barrier may be either a sandy or shingly wave-built feature (such as a sandbar or a barrier island), or it may be a coral reef. Thus, there are two main types of lagoons: (1) elongated or irregular stretches of water that lie between coastal barrier islands and the shoreline and (2) circular or irregular stretches of water surrounded by coral atoll reefs or protected by barrier coral reefs from direct wave action. Lagoon depths are maintained at a moderate level by sedimentation, and this compensates for the subsidence that commonly attends reef formation. Because the reef is an organic structure, the lagoonal sediments contain much calcareous material. The sheltered waters support highly productive ecosystems made up of a distinctive flora and fauna. Gippsland LakesPeter Firus, Flagstaffotos Barrier island, or coastal, lagoons are characterized by quiet water conditions, fine-grained sedimentation, and, in many cases, brackish marshes. Water movements are related to discharge of river flow through the lagoon and to the regular influx and egress of tidal waters through the inlets that normally separate the barrier islands. Coastal lagoons are generally characteristic of coasts of low or moderate energy, occurring especially on the east coasts of continents where the swells are less violent and in high latitudes where offshore ice provides some protection. They also are associated with low coasts and rarely occur where high cliffs form the coast. They can form only where there is abundant sediment for construction of the protective barrier islands. Too much sediment from the mainland, however, can lead to delta formation rather than lagoons, although lagoons frequently occur along the outer delta margin and between delta distributaries (see also river: Deltas). Coastal lagoons are widely distributed throughout the world and have been estimated to constitute about 13 percent of the total world coastline. Lagoons are more common on coasts with moderate to low tidal ranges; for example, they occur widely on low coasts of the southern Baltic, the southeast North Sea, the Black Sea, the Caspian Sea, and the Mediterranean Sea, as well as on low coasts of the southeastern United States and the Gulf of Mexico. Lagoon coasts also occur along southern Brazil, the east coast of Madagascar, northeastern Russia, Japan, and isolated parts of Africa, India, Australia, and New Zealand. Tuamotu Archipelago: Fakarava islandFrédéric Jacquot Coral lagoons are restricted to tropical open seas that provide the conditions necessary for coral growth. They are best exemplified by the roughly circular quiet waters that are surrounded by warm-water coral atoll reefs. Coral lagoons occur widely in the western Pacific, in parts of the Indian Ocean, and in isolated places in the Caribbean, mainly within 25° latitude of the Equator. Coral lagoons are of great importance to many island communities in the Pacific, particularly where they provide the only quiet water for use as harbours, although the passage through the reef into the lagoon is often perilous.
Water and its Varying Forms Even though water exists in three states, there is only one correct answer to the questions in this quiz. Dive in and test your knowledge of water...and see whether you sink or swim. Coral-reef lagoons also occur on marginal reefs such as the Great Barrier Reef of Australia, but the most spectacular examples are the atolls of the Pacific Ocean, some of which are more than 50 km (30 miles) across. Some atolls consist only of a lagoon, often with a fairly uniform depth, surrounded by a low-lying coral reef; some include one or more high, rocky volcanic islands, and others are complex, with small reefs surrounded by lagoons within a larger reef. All are thought to have been built by the upward growth of coral during a relative rise in sea level due to subsidence and eustatic (global) change. Coral lagoon dimensions range from small atolls to those so wide that the coral reefs on the far side cannot be seen across the lagoon. Atoll widths range from about 2.5 to nearly 100 km (1.5 to nearly 62 miles), but the mean value is about 20 km (about 12 miles). Depths rarely exceed 60 metres (about 200 feet) and many are shallower, usually less than 20 metres (about 65 feet) deep. The lagoon of Mayotte island in the Comoro archipelago in the Indian Ocean attains a maximum depth of about 92 metres (about 300 feet), but it is generally shallower. That lagoon is about 16 km (about 10 miles) in width at its widest point and lies behind a barrier reef that encircles the island, forming an atoll about 55 km (about 34 miles) in diameter. Get a Britannica Premium subscription and gain access to exclusive content. Subscribe Now
The smell associated with rainfall is called petrichor; the moisture causes bacteria in dry earth to release a molecule that human noses are very sensitive to. See All Good FactsBarrier island lagoons are usually elongated, though irregular ones may occur where river estuaries flood behind barriers. This occurs on the east coast of the United States, where lagoons extend intermittently for nearly 1,500 km (about 900 miles) along the coast. The Gippsland Lakes in Victoria, Australia, exemplify a complex lagoon system formed behind a 149-km (93-mile) beach. Elongated lagoons up to 64 km (about 40 miles) in length lie behind the beach barrier, and larger lagoons, such as Lake Wellington, lie behind the southwestern end. Postglacial subsidence has flooded the lowland in this area. The lagoons are shallow: Lake Wellington is less than 3.5 metres (11.5 feet) in depth, and much of Lake King is less than 6 metres (about 20 feet) deep. Scour holes as deep as 16.5 metres (about 54 feet) do occur, however. The elongated lagoon behind the barrier is only 1 to 1.5 metres (3 to 5 feet) deep, typical of barrier island lagoons. |
Lagoons that form behind barrier islands are examples of
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