Lagoons are shallow, often elongated bodies of water separated from a larger body of water by a shallow or exposed shoal, coral reef, or similar feature. Some authorities (such as Nybakken) include fresh water bodies in the definition of “lagoon”, while others explicitly restrict “lagoon” to bodies of water with some degree of salinity. The distinction between “lagoon” and “estuary” also varies between authorities. Davis restricts “lagoon” to bodies of water with little or no fresh water inflow, and little or no tidal flow, and calls any bay that receives a regular flow of fresh water an “estuary”. Davis does state that the terms “lagoon” and “estuary” are “often loosely applied, even in scientific literature.” Kusky characterizes lagoons as normally being elongated parallel to the coast, while estuaries are usually drowned river valleys, elongated perpendicular to the coast. 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. Coastal lagoons are classified as inland bodies of water.
Atoll lagoons form as coral reefs grow upwards while the islands the reefs surround subside, until eventually only the reefs remain above sea level. Unlike the lagoons that form shoreward of fringing reefs, atoll lagoons often contain some deep (>20m) portions.
Coastal lagoons form along gently sloping coasts where barrier islands or reefs can develop off-shore, and the sea-level is rising relative to the land along the shore (either because of an intrinsic rise in sea-level, or subsidence of the land along the coast). Coastal lagoons do not form along steep or rocky coasts, or if the range of tides is more than 4 metres (13 ft). Due to the gentle slope of the coast, coastal lagoons are shallow. They are sensitive to changes in sea level. A relative drop in sea level may leave a lagoon largely dry, while a rise in sea level may let the sea breach or destroy barrier islands, and leave reefs too deep under water to protect the lagoon. Nybakken describes coastal lagoons and barrier islands as a “coupled system”. Coastal lagoons are young and dynamic, and may be short-lived in geological terms. Coastal lagoons are common, occurring along nearly 15 percent of the world’s shorelines. In the United States, lagoons are found along more than 75 percent of the eastern and Gulf coasts.
Coastal lagoons are usually connected to the open ocean by inlets 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.Mangroves and marsh plants can facilitate the accumulation of sediment in a lagoon. Benthic organisms may stabilize or destabilize sediments.
In Mallorca there is a very important Coastal Lagoon. S’Albufera (Albufera of Mallorca in Mallorca) is the largest wetland and most important of the Balearic Islands, is a former lagoon separated from the sea by a belt of dunes, which for many centuries, especially in the last two-and human influence has been filled up with sediments become an extensive floodplain. The Natural Park of Albufera de Mallorca affords protection to some 1708 hectares of marsh and dunes.