A lagoon is a shallow body of water located near a coast, separated from the ocean by a natural barrier. This feature creates a sheltered environment where the dynamic forces of the sea are muted. It forms a unique aquatic habitat and a transitional zone between the terrestrial and marine worlds.
Essential Characteristics of Lagoons
Lagoons are defined by their physical structure, which results in predictable environmental conditions. They are shallow, with typical depths seldom exceeding a few meters, often less than 5 meters. This limited depth means the water temperature fluctuates widely, closely mirroring air temperature—becoming warmer than the ocean in summer and colder in winter.
The defining physical trait is the barrier separating the lagoon from the sea, such as a sand spit, a barrier island, or a coral reef. This restricted exchange causes the water’s salinity to be highly variable, ranging from brackish (nearly freshwater) to hypersaline (saltier than the ocean). Salinity depends on the balance between freshwater inflow, evaporation rates, and tidal exchange through the inlet.
The Geological Processes of Lagoon Formation
Lagoons are formed through two distinct geological processes, creating either coastal or atoll structures. Coastal lagoons, the more common type, develop along gently sloping coastlines with an ample supply of sediment. Wave action and long-shore currents deposit this sediment, gradually building up barrier islands or sand spits parallel to the shore. The resulting body of water trapped between the barrier and the mainland becomes the coastal lagoon.
Atoll lagoons form in tropical oceans through the subsidence of a volcanic island over millions of years. As the central volcanic mass sinks, the fringing coral reef continues to grow upwards, maintaining its position near the sea surface. Eventually, a ring-shaped reef remains, surrounding a central body of water. This enclosed water body is the atoll lagoon, with the reef acting as the protective barrier.
Distinguishing Lagoons from Other Coastal Features
Lagoons are often confused with other partially enclosed coastal water bodies, but their characteristics distinguish them from estuaries and bays. The primary distinction from an estuary lies in freshwater input and tidal flow. Estuaries are defined by the continuous mixing of large volumes of river freshwater with ocean saltwater, creating a distinct salinity gradient and stronger currents. Lagoons, conversely, receive little large river inflow, leading to sluggish water circulation and a highly variable salinity profile.
Lagoons are separated from the sea by a substantial physical barrier, which severely restricts tidal exchange. Bays and sounds, while also partially enclosed, are generally larger, deeper inlets that are more open to the ocean and experience greater tidal influence.
The Unique Biology of Lagoon Environments
The highly variable conditions within lagoons support specialized biological communities adapted to fluctuating salinity and temperature. Organisms must tolerate a wide range of salt concentrations, which limits the diversity of species compared to the open ocean. Specialized flora, such as submerged seagrasses, saltmarsh grasses, and mangrove forests, thrive in these brackish conditions.
These plant communities form the base of a productive food web and serve an important ecological function as nursery habitats. Juvenile fish, shrimp, and shellfish spend their early developmental stages within the calm, nutrient-rich waters of the lagoon before migrating to the sea. The environment is also home to invertebrates, including mollusks and crustaceans, which provide a food source for larger animals like the European Eel and numerous species of wading and shorebirds.