Lagoon

A lagoon is a shallow body of water separated from a larger body of water by a natural barrier such as reefs, barrier islands, barrier peninsulas or isthmuses. Found on many of the world’s coastlines, lagoons occur in a variety of geographic and ecological settings. They are typically categorised as coastal lagoons or atoll lagoons, though mixed-sediment coastlines may also host lagoonal environments. Because these waterbodies often share characteristics with estuaries, their classification has been the subject of ongoing scientific discussion.
Lagoons may vary significantly in salinity, shape and connection to the open sea, and their names in different languages reflect diverse cultural interpretations of these environments.

Definition and Terminology

Lagoons are generally defined as shallow, often elongated, waterbodies separated from a larger water mass by a shoal, reef or similar geomorphological feature. Some authorities consider only saline or brackish environments as lagoons, whereas others include freshwater equivalents. The distinction between lagoons and estuaries is particularly variable.
One classification restricts lagoons to waterbodies with little freshwater inflow and limited tidal exchange, while bays receiving substantial freshwater are termed estuaries. Another approach emphasises morphology: lagoons tend to run parallel to coasts, whereas estuaries occupy drowned river valleys oriented perpendicular to the shoreline.
Within coral reef systems, the term lagoon is often used interchangeably with back reef, referring to the sheltered waterbody landward of a reef crest. Many prominent lagoons do not include the term in their names—examples include Currituck Sound and Albemarle Sound in the United States, Montrose Basin in Scotland and Broad Water in Wales.
Linguistic differences further shape the concept. Some languages treat lagoons as a type of lake, while others create distinct names for coastal versus atoll lagoons. Examples include:

• French, where lagon denotes an atoll lagoon while étang may refer to coastal lagoons.
• Russian and Ukrainian terminology, with specific words for Black Sea lagoons.
• Māori, where traditional terms differentiate permanent coastal lagoons from ephemeral waterbodies.

Similar distinctions appear in Vietnamese, Spanish and Portuguese, highlighting the diverse conceptual frameworks applied across regions.

Etymology

The English word lagoon derives from the Italian laguna, referring originally to the Lagoon of Venice. It appeared in English texts by the early seventeenth century, later also rendered as lagune. European exploration accounts of the seventeenth and eighteenth centuries frequently used the term to describe shallow saltwater lakes adjacent to coasts or enclosed by islands.

Atoll Lagoons

Atoll lagoons develop through the long-term interaction between coral reefs and subsiding volcanic islands. As coral growth keeps pace with subsidence, a ring-shaped reef emerges while the central island gradually sinks beneath sea level. The result is a lagoon encircled by the reef, often with passes linking the deep inner basin to the open ocean.
These lagoons differ from those found behind fringing reefs because they may contain deeper sections and are structurally controlled by the evolution of the reef–island complex. Many Pacific and Indian Ocean atolls exhibit such lagoon systems.

Coastal Lagoons

Coastal lagoons form along gently sloping shorelines where barrier islands, spits or reefs arise offshore, creating an enclosed or semi-enclosed waterbody. Their formation requires a low tidal range; tidal amplitudes greater than one metre generally prevent stable lagoon formation. Because of their location on low-gradient coasts, coastal lagoons are typically shallow.
Changes in sea level have substantial effects on coastal lagoon stability. Lower sea levels may expose the lagoon bed, while rising sea levels may breach barrier systems or deepen reefs beyond their ability to protect lagoon waters. Coastal lagoons are geologically young and often short-lived, responding quickly to shifts in sediment supply, sea level or storm intensity.
Globally, they occur along approximately 15 per cent of shorelines, and in the United States they are particularly widespread along the Atlantic and Gulf coasts.
Coastal lagoons are commonly classified into:

• Leaky lagoons, with significant exchange through multiple inlets.
• Restricted lagoons, with limited but consistent water exchange.
• Choked lagoons, with narrow inlets leading to strong stratification and longer residence times.

Hydrological conditions vary depending on the balance between freshwater input, evaporation and tidal exchange. Lagoons with minimal freshwater and high evaporation may become hypersaline, whereas those with abundant inflow and no ocean connection may be entirely fresh. By contrast, systems with wide inlets experience strong mixing and tidal currents.
Sediment accumulation is a defining characteristic of coastal lagoons. Inputs arrive from rivers, coastal runoff and tidal transport. Storm overwash events may deposit large sediment volumes across barriers. Biological agents, including mangroves, saltmarsh vegetation and benthic organisms, may stabilise or destabilise these sediments, influencing lagoon morphology over time.

Global Distribution and Examples

Lagoons are found in a wide range of environments, from the brackish systems of the Baltic and Black Sea coasts to tropical atoll lagoons across the Pacific. Examples of coastal or mixed-type lagoons include:

• Great South Bay in New York, protected by Fire Island.
• Isle of Wight Bay in Maryland.
• Lake Illawarra in New South Wales.
• The Fleet Lagoon along Chesil Beach in England.
• Lagoon systems along the Western Pomerania coast in Germany.

In addition, many Mediterranean coastal lagoons—such as those along the French littoral—illustrate the close interplay between sedimentation, human activity and ecological dynamics.

Characteristics and Environmental Significance

Lagoons provide vital ecological services. They often support extensive wetlands, nursery habitats for fish, and rich biodiversity. Their shallow, sheltered waters make them sensitive to environmental pressures including pollution, eutrophication and changes in hydrological balance. Sedimentation patterns shape their long-term evolution, while coastal development can alter inlet structure and water exchange.