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A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as a reef, a barrier island or islands, a barrier peninsula, or an isthmus. Lagoons are commonly divided into coastal lagoons (or barrier lagoons) and atoll lagoons. They have also been identified as occurring on mixed-sand and gravel coastlines. There is an overlap between bodies of water classified as coastal lagoons and bodies of water classified as Estuary. Lagoons are common coastal features around many parts of the world.
When used within the context of a distinctive portion of coral reef ecosystems, the term "lagoon" is synonymous with the term "back reef" or "backreef", which is more commonly used by coral reef scientists to refer to the same area.
Many lagoons do not include "lagoon" in their common names. Currituck Sound, Albemarle Sound and Pamlico Sound Sounds in North Carolina, Great South Bay between Long Island and the barrier beaches of Fire Island in New York, Isle of Wight Bay, which separates Ocean City, Maryland from the rest of Worcester County, Maryland, Banana River in Florida, US, Lake Illawarra in New South Wales, Australia, Montrose Basin in Scotland, and Broad Water in Wales have all been classified as lagoons, despite their names. In England, The Fleet at Chesil Beach has also been described as a lagoon.
In some languages the word for a lagoon is simply a type of lake: In Chinese a lake is hu (italic=no), and a lagoon is xihu (italic=no). In the France Mediterranean several lagoons are called étang ("lake"). Contrariwise, several other languages have specific words for such bodies of water. In Spanish, coastal lagoons generically are laguna costera, but those on the Mediterranean coast are specifically called albufera. In Russian and Ukrainian, those on the Black Sea are liman (italic=no), while the generic word is laguna (italic=no). Similarly, in the Baltic Sea, Danish has the specific , and German the specifics Bodden and Haff, as well as generic terms derived from laguna. In Poland these lagoons are called zalew ("bay"), and in Lithuania marios ("lagoon, reservoir"). In Jutland several lagoons are known as fjord. In New Zealand the Māori word hapua refers to a coastal lagoon formed at the mouth of a braided river where there are mixed sand and gravel beaches, while waituna, an ephemeral coastal waterbody, is neither a true lagoon, lake, nor estuary.
Some languages differentiate between coastal and atoll lagoons. In French, refers specifically to an atoll lagoon, while coastal lagoons are described as , the generic word for a still lake or pond. In Vietnamese, Đầm san hô refers to an atoll lagoon, whilst Đầm phá is coastal.
In Latin America, the term laguna in Spanish, which lagoon translates to, may be used for a small fresh water lake in a similar way a Stream is considered a small river. However, sometimes it is popularly used to describe a full-sized lake, such as Laguna Catemaco in Mexico, which is actually the third-largest lake by area in the country. The brackish water lagoon may be thus explicitly identified as a "coastal lagoon" ( laguna costera). In Portuguese, a similar usage is found: lagoa may be a body of shallow seawater, or a small freshwater lake not linked to the sea.
Coastal lagoons can be classified as leaky, restricted, or choked.
Coastal lagoons are usually connected to the open ocean by between barrier islands. The number and size of the inlets, precipitation, evaporation, and inflow of fresh water all affect the nature of the lagoon. Lagoons with little or no interchange with the open ocean, little or no inflow of fresh water, and high evaporation rates, such as Lake St. Lucia, in South Africa, may become highly saline. Lagoons with no connection to the open ocean and significant inflow of fresh water, such as the Lake Worth Lagoon in Florida in the middle of the 19th century, may be entirely fresh. On the other hand, lagoons with many wide inlets, such as the Wadden Sea, have strong tidal currents and mixing. Coastal lagoons tend to accumulate sediments from inflowing rivers, from runoff from the shores of the lagoon, and from sediment carried into the lagoon through inlets by the tide. Large quantities of sediment may be occasionally be deposited in a lagoon when storm waves overwash barrier islands. and Salt marsh plants can facilitate the accumulation of sediment in a lagoon. Benthos may stabilize or destabilize sediments.
Largest coastal lagoons
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