Lagoons line about 15 percent of the world's coasts, often under other names
North Carolina's Pamlico Sound, Scotland's Montrose Basin and England's Fleet behind Chesil Beach are all lagoons, whatever they are called. These shallow waters, cut off from the sea by a reef or sandbar, are geologically young and restless, and a small shift in sea level can drain one or drown it.
A lagoon is a shallow stretch of water walled off from a bigger one by a narrow strip of land: a reef, a barrier island, a peninsula or an isthmus. The two main kinds are coastal lagoons and the lagoons inside atolls. Definitions blur at the edges. Some experts allow fresh water, others insist on salt, and the line between lagoon and estuary shifts by author. Richard A. Davis Jr. reserves lagoon for waters with little river inflow and little tide, while Timothy M. Kusky notes that lagoons usually run parallel to the shore and estuaries, which are often drowned river valleys, run across it.
The word comes from Venetian łaguna, the waters around Venice, itself from the Latin for lake. English had it by 1612. William Dampier described a salt lagoon on the Mexican coast in 1697, and James Cook in 1769 noted an oval island with a lagoon at its centre. Other languages carve the idea differently: French keeps lagon for atolls and étang for coastal ones, Spanish Mediterranean lagoons are albuferas, Black Sea ones are limans, and German Baltic ones are Bodden or Haff.
Atoll lagoons form as coral keeps growing upward while the island it rings slowly sinks, until only the reef stands above the waves; they can exceed 20 metres deep. Coastal lagoons need gently sloping shores, rising relative sea level and tides under about 4 metres, so they never appear on steep, rocky coasts. They fringe more than 75 percent of the United States' Eastern and Gulf Coasts.
Their chemistry depends on the balance of inlets, rain, evaporation and river water. South Africa's Lake St. Lucia, with little exchange and strong evaporation, can turn very salty, while Florida's Lake Worth Lagoon was once entirely fresh. The Wadden Sea, with many wide inlets, churns with tidal currents. Sediment arrives from rivers, tides and storm waves washing over barriers, and mangroves and marsh plants help trap it.
Source: Lagoon
