Lagoon

A lagoon is a shallow body of water separated from a larger sea or ocean by barrier landforms such as sandbars, spits, barrier islands, or coral reefs. Lagoons are typically shallow, often less than 5 to 10 meters in depth, and can be restricted or intermittently connected to the open sea, which influences their salinity and water exchange.

Lagoons form behind coastal barriers and are connected to the open sea through one or more inlets or channels. The exchange of water is controlled by tides, winds, and the barrier geometry, and salinity can range from freshwater to marine, often becoming brackish when substantial freshwater input from rivers or rainfall dilutes seawater. Sediment tends to accumulate in lagoons, gradually filling them in over long times and sometimes converting parts of the lagoon into wetlands.

Lagoons support specialized ecosystems, including seagrass beds, mangroves or salt-tolerant marshes, and provide nursery habitats for many fish and invertebrates. They also support diverse bird communities and contribute to coastal protection by absorbing wave energy and trapping sediments.

Lagoons vary in origin and salinity. Coastal lagoons lie behind barrier islands; coral reef lagoons occur within atolls and along reef systems; freshwater or brackish lagoons form where rivers and rainfall strongly influence salinity. The Venice Lagoon in Italy is a well-known example, and many tropical coastlines host carbonate barrier lagoons.

Lagoons face threats from development, dredging, pollution, and nutrient runoff, which can alter water quality and sediment balance. They are vulnerable to sea level rise and climate change, which can erode barriers, change exchange with the open sea, and affect habitats. Conservation efforts focus on protecting barrier dynamics, restoring wetlands, and maintaining hydrological connectivity.