Coastal Lagoons are relatively shallow bodies of water that are separated from the open sea by a barrier such as a sand bar, barrier island, spit, reef structure or other coastal feature while remaining connected to marine waters through one or more inlets or restricted exchanges. Their salinity, water level, circulation and sediment conditions can vary greatly according to the strength and permanence of their connection with the sea, freshwater inflow, evaporation, precipitation, tidal exchange and wind driven circulation. Coastal Lagoons occur in many climatic settings and can support diverse habitats including wetlands, mudflats, salt marshes, mangroves, seagrass beds and shallow subtidal environments. They are dynamic transitional systems situated between terrestrial, freshwater and marine environments.
The morphology of a Coastal Lagoon is strongly controlled by its barrier and inlet configuration. Barrier islands and spits can form through sediment transport driven by waves and longshore currents, creating sheltered water bodies behind them. Inlets permit exchange between lagoon and ocean, but their size and position can change through storms, sediment deposition, tidal flows and human engineering. Some lagoons have permanent inlets, while others open intermittently when storms or high water levels breach a barrier. Lagoon basins may become progressively infilled by marine sediment, river sediment, organic material and biological carbonate production. Their long term development therefore involves a balance between sediment supply, erosion, accommodation space, sea level and water exchange.
Coastal Lagoons have distinctive hydrological and chemical characteristics. Freshwater from rivers, groundwater and rainfall can reduce salinity, while evaporation can increase salinity in poorly flushed systems. Tidal exchange can transport seawater into and out of the lagoon and produce strong gradients near inlets. Restricted circulation may cause differences in temperature, dissolved oxygen and nutrient concentration between the lagoon and adjacent ocean. Nutrient enrichment can increase biological productivity but excessive nutrient input may contribute to eutrophication, algal blooms and oxygen depletion. The degree of flushing is therefore a major factor controlling lagoon water quality and ecological condition.
Coastal Lagoons support many habitats and can provide important nursery grounds for fish and invertebrates. Shallow waters can support seagrasses and submerged aquatic vegetation, while intertidal margins can support salt marshes, mangroves and mudflats depending on climate and tidal conditions. Birds may use lagoon wetlands for feeding, nesting or migration. Sediment rich environments can support organisms adapted to variable salinity and periodic exposure. Because lagoons contain strong environmental gradients, biodiversity may reflect the interaction of marine and terrestrial influences. Habitat distribution can change when inlet positions shift, salinity changes, vegetation expands or sediment accumulation alters water depth.
Coastal Lagoons are important sedimentary environments. Waves deliver sediment toward barrier systems, while longshore transport can move material along the coast. Tidal currents can carry sediment through inlets and redistribute it within the lagoon. Rivers can deliver fine sediment and organic matter from inland catchments. Biological processes can also generate carbonate sediments in tropical settings. When sediment accumulation exceeds the ability of water movement to remove material, lagoon areas may become shallower and gradually convert to wetlands or terrestrial environments. Conversely, erosion or rising sea level can deepen or expand lagoon waters. The observed lagoon form therefore represents a continually changing balance between sediment supply and removal.
Human activities can substantially alter Coastal Lagoons. Dredging may deepen or maintain navigation channels, while artificial barriers and inlet stabilisation can change circulation and sediment transport. Coastal development can increase nutrient and pollutant inputs, while removal of wetlands can reduce natural filtration and habitat. Agricultural runoff may introduce nutrients and sediment, and groundwater abstraction can affect freshwater inputs. At the same time, lagoons can support fisheries, aquaculture, tourism, recreation, navigation and port activities. Management therefore requires attention to both ecological functions and human demands, particularly because engineering interventions in one location can alter sediment and water movement elsewhere in the lagoon or along the coast.
Coastal Lagoons differ from estuaries, although the two environments can overlap and share characteristics. An estuary is generally defined by the mixing of freshwater and seawater in a semi enclosed coastal environment, whereas a Coastal Lagoon is defined primarily by its separation from the open ocean by a barrier and its restricted marine connection. A lagoon can be strongly influenced by freshwater and may function partly like an estuary, while a lagoon with little freshwater input can remain predominantly marine. Lagoons also differ from ordinary bays because the barrier separating a lagoon from the open sea is a defining morphological characteristic. Terminology varies among disciplines and regions, so descriptions should consider both physical morphology and hydrological function.
Coastal Lagoons are valuable indicators of coastal change because their boundaries and water conditions respond to sea level, storms, sediment supply, river discharge and human modification. Remote sensing can monitor changes in lagoon area, vegetation, turbidity and inlet position, while bathymetric surveys can measure changes in water depth and sediment accumulation. Field monitoring can measure salinity, temperature, nutrients, oxygen and biological communities. Long term interpretation requires distinguishing natural variability from persistent environmental change. Coastal Lagoons therefore provide important examples of coupled coastal systems in which marine processes, terrestrial inputs, biological activity, sediment dynamics and human intervention continuously interact.
