Lagoons are aquatic environments found at the dynamic interface between land and sea, offering a contrast to the turbulent open ocean. Their characteristics and origins reveal a complex interplay of geological forces and marine processes.
Defining a Lagoon
A lagoon is a shallow body of water situated in a coastal area, typically separated from a larger body of water like an ocean or sea. This separation is achieved by a natural barrier, which can be a sandbar, a barrier island, or a coral reef. The presence of this barrier often results in water that is calmer than the adjacent open sea, with restricted water exchange. While some lagoons might have direct connections to the ocean through inlets, others experience water exchange primarily through seepage or occasional over-topping by high tides.
The water within a lagoon can exhibit a wide range of salinities, from nearly freshwater to hypersaline conditions. Freshwater input from rivers or rainfall can lower salinity, while high evaporation rates can lead to hypersaline conditions where salt concentration exceeds that of typical seawater. Many lagoons contain brackish water, a mixture of fresh and saltwater, with salinity levels fluctuating seasonally.
How Lagoons Form
The creation of lagoons involves natural geological processes that shape coastal landscapes. One common mechanism involves the formation of barrier islands or sand spits that gradually enclose a section of the sea. This process occurs along gently sloping coastlines where there is an abundant supply of sediment, and the sea level is either stable or gradually rising. Waves and currents transport and deposit sand and gravel, building up these elongated landforms that act as natural walls, creating a sheltered basin.
Another way lagoons form is through the growth of coral reefs, particularly in tropical ocean environments. Atoll lagoons, for instance, develop as coral reefs grow upwards around a subsiding volcanic island. Over thousands of years, the central island may sink beneath the water, leaving a ring-shaped coral reef that continues to grow, encircling a central body of water. This process can take as long as 300,000 years.
Common Types of Lagoons
Lagoons are broadly categorized into two primary types: coastal and atoll.
Coastal Lagoons
Coastal lagoons, also known as barrier lagoons, are the most prevalent type and are found along coastlines worldwide. These form when a barrier, such as a sandy spit or barrier island, develops parallel to the shoreline, separating coastal waters from the open ocean. Coastal lagoons are often shallower than atoll lagoons and typically have inlets connecting them to the sea, allowing for water exchange.
Atoll Lagoons
Atoll lagoons are distinctive features of tropical oceans. They are characterized by a circular or irregular stretch of water surrounded by a ring of coral reef. These lagoons originate from the subsidence of volcanic islands, with corals building up around the island’s perimeter. As the island sinks, corals continue to grow, eventually forming a circular barrier reef that encloses a central lagoon, which can sometimes be quite deep.