Lagoons are relatively shallow bodies of coastal water that are separated from the open sea by a barrier such as a sandbar, barrier island, reef, beach ridge or other depositional feature, while often remaining connected to the sea through one or more openings. The term is broad and is used in physical geography, coastal geomorphology, ecology and ordinary geographical description. Lagoons differ substantially in origin, shape, salinity, circulation and degree of isolation.
Some are marine lagoons that form behind barrier beaches or islands, while others develop behind coral reefs or other carbonate structures. Their water may be almost as saline as the adjacent ocean, significantly fresher because of river and rainfall inputs, or hypersaline where evaporation greatly exceeds freshwater replacement. Lagoon depth is usually limited compared with adjacent offshore waters, although particular systems can be locally deeper.
The defining feature is generally the partial separation of coastal water from the open sea rather than a particular chemical composition. Lagoons occur on many continental margins and islands and are especially common along low gradient coasts where sediment transport, wave action and sea level change permit barriers to develop. Their boundaries can shift substantially over time as sediment accumulates, channels migrate, barriers breach and sea level changes.
Lagoon formation is closely connected with coastal sediment transport and relative sea level. Along suitable shorelines, waves approaching obliquely can move sediment laterally through longshore transport. Sand may accumulate into spits, barrier beaches or barrier islands that partially enclose water behind them. Where sufficient sediment is available and the coastline is relatively low lying, continued deposition can create a lagoon behind the barrier.
Tidal inlets provide exchange between lagoon water and the ocean, but these openings may migrate, widen, narrow or close depending on waves, tides and sediment supply. Storms can breach barriers and create new inlets, while subsequent sediment transport may progressively close them. Sea level rise can cause barriers to migrate landward if sediment supply and coastal morphology permit, while insufficient sediment can lead to erosion and loss of the barrier.
Reef associated lagoons have a different developmental history because the barrier may be created by coral or carbonate accumulation rather than sandy sediment. In atoll systems, lagoon formation is closely connected with reef growth and island development. Lagoon morphology therefore records interactions among waves, tides, sediment transport, biological production, sea level, storms and tectonic or volcanic history.
The physical and chemical properties of a lagoon are strongly controlled by its connection with the sea. A lagoon with several wide tidal entrances may experience substantial tidal exchange and maintain salinity close to marine conditions. A lagoon with a narrow or restricted inlet can develop strong salinity gradients, temperature differences and residence times. Freshwater from rivers, groundwater, precipitation and surface runoff can reduce salinity, while evaporation can increase it.
In arid climates, restricted lagoons may become hypersaline and develop distinctive mineral precipitation or specialised biological communities. Water circulation can be driven by tides, wind, density differences, wave induced setup and freshwater discharge. Shallow depth means that wind can mix the water column efficiently under many conditions, although strong density stratification can develop where freshwater forms a persistent surface layer.
Restricted circulation can promote accumulation of nutrients and organic matter and can contribute to oxygen depletion when biological decomposition consumes dissolved oxygen faster than physical exchange replenishes it. Lagoon water quality is therefore closely related to flushing time, nutrient inputs, sediment dynamics and human land use in the surrounding catchment.
Lagoons are ecologically important because they combine marine, freshwater and terrestrial influences and create diverse habitats within a relatively small area. Common lagoon environments include mudflats, sandy shoals, salt marshes, mangrove forests, seagrass meadows, oyster beds, tidal channels and shallow subtidal habitats. These environments provide feeding, nursery and refuge areas for fish, crustaceans, molluscs, birds and other organisms. Seagrass and marsh vegetation can trap sediment, reduce wave energy and store organic carbon.
Mangrove dominated lagoons in tropical regions can be especially productive and can provide important coastal protection and habitat. Lagoon ecosystems are influenced by salinity, temperature, dissolved oxygen, nutrient concentration, turbidity, tidal range, sediment grain size and connectivity with adjacent marine waters. Excess nutrient input from agriculture, sewage or urban runoff can stimulate algal growth and alter oxygen conditions. Physical modification of inlets, dredging, embankment construction and land reclamation can change circulation and habitat availability. Conservation commonly requires maintaining ecological connectivity while controlling pollutant and sediment inputs from surrounding land.
Lagoons have diverse human uses and have supported settlement, fishing, navigation, aquaculture, salt production and recreation for centuries. Shallow sheltered waters can provide favourable conditions for small vessel navigation and traditional fisheries. Lagoon fisheries may target finfish, shellfish and crustaceans, while aquaculture can involve oysters, mussels, fish or other organisms depending on local conditions. Salt production can occur where evaporation concentrates seawater in shallow restricted basins. Tourism may depend on beaches, calm water, wildlife, boating and scenic landscapes.
Lagoons can also provide natural protection from wave energy, although their barrier systems remain vulnerable to severe storms and long term erosion. Urban development around lagoons can increase pollution and reduce wetlands through land reclamation. Engineering structures intended to stabilise shorelines can interrupt sediment transport and sometimes shift erosion to neighbouring areas. Sustainable management therefore needs to consider the lagoon and its barrier as a connected sedimentary system rather than treating individual beaches or channels as independent features.
Several geographical terms overlap with lagoon terminology but describe different physical relationships. An estuary is generally a coastal water body where river discharge mixes with seawater, whereas a lagoon is defined primarily by partial separation from the open sea. Some lagoons receive substantial river inflow and can therefore exhibit estuarine processes, while some estuaries have barrier features that make their classification context dependent.
A lake is an inland standing water body and is not necessarily connected to the sea, although some coastal lagoons may become effectively isolated and acquire lake like characteristics. A bay is a broad indentation of the coastline and does not require a barrier. A tidal flat is an intertidal sedimentary surface rather than a water body, although tidal flats commonly occur inside lagoons. A coastal pond can resemble a lagoon at small scale but generally lacks the same geomorphological significance. These distinctions are useful because lagoon classification depends on origin, connectivity and geomorphology rather than simply on whether the water looks enclosed.
Lagoons are highly dynamic environments and can change substantially over periods ranging from individual storms to centuries. Barrier migration, inlet relocation, sediment deposition, erosion, vegetation expansion and sea level change can all modify their shape. A storm surge may temporarily raise lagoon water levels, erode barrier dunes and open new channels. Following the event, sediment may accumulate and gradually reduce the opening.
Longshore sediment transport can cause an inlet to migrate in the direction of net littoral drift, while tidal currents may maintain an inlet where tidal prism and channel capacity are sufficient. Human intervention can interrupt these natural processes by constructing groynes, seawalls, breakwaters, training walls or dredged channels. Climate change introduces additional pressures through sea level rise, changing storm patterns, warming water and altered rainfall. The future condition of a lagoon depends on the balance between barrier landward migration, sediment supply, accommodation space and the ability of ecosystems to adjust.
Lagoons also provide important scientific information about coastal evolution and environmental change. Sediment cores can preserve pollen, microfossils, shells, organic matter and geochemical signals that record former salinity, vegetation, sea level and hydrological conditions. Remote sensing can map changes in water area, vegetation, shoreline position and sediment distribution over large regions. Numerical models can simulate tidal circulation, wave transformation, sediment transport and future responses to sea level rise.
Because lagoons lie at the interface between terrestrial and marine systems, they can be useful indicators of changes in both environments. Their sedimentary records can reveal periods when barriers were breached, lagoons became isolated, or marine connectivity increased. Their ecological records can also show responses to eutrophication, pollution, habitat modification and temperature change. Understanding lagoons therefore requires an integrated approach combining geomorphology, hydrology, oceanography, ecology, sedimentology and human geography.
