Estuary


An estuary is a partially enclosed coastal body of brackish water where freshwater from rivers or streams meets and mixes with saltwater from the open sea. Estuaries form transition zones between riverine and marine environments and are classified as ecotones, areas where different ecosystems overlap. They are influenced by both marine processes and fluvial processes. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.
Most existing estuaries originated during the Holocene epoch with the flooding of river-eroded or glacially scoured valleys when the sea level began to rise about 10,000–12,000 years ago. Estuaries can be classified using four fundamental criteria. These include salinity distribution, geomorphology, water circulation and vertical stratification, and system energetics.They can have many different names, such as bays, harbors, lagoons, inlets, or sounds, although some of these water bodies do not strictly meet the above definition of an estuary and could be fully saline.
Many estuaries suffer degeneration from a variety of factors including soil erosion, deforestation, overgrazing, overfishing and the filling of wetlands. Eutrophication may lead to excessive nutrients from nitrogen run off, sewage and animal wastes; pollutants including heavy metals, polychlorinated biphenyls, radionuclides and hydrocarbons from sewage inputs, and diking or damming for flood control or water diversion.

Definition

The word "estuary" is derived from the Latin word aestuarium meaning tidal inlet of the sea, which in itself is derived from the term aestus, meaning tide. There have been many definitions proposed to describe an estuary. The most widely accepted definition is: "a semi-enclosed coastal body of water, which has a free connection with the open sea, and within which seawater is measurably diluted with freshwater derived from land drainage". However, this definition excludes a number of coastal water bodies such as coastal lagoons and brackish seas.
A more comprehensive definition of an estuary is "a semi-enclosed body of water connected to the sea as far as the tidal limit or the salt intrusion limit and receiving freshwater runoff; however the freshwater inflow may not be perennial, the connection to the sea may be closed for part of the year and tidal influence may be negligible". This broad definition also includes fjords, lagoons, river mouths, and tidal creeks.
Ovarall, an estuary is a dynamic ecosystem having a connection to the open sea through which the sea water enters with the rhythm of the tides. The effects of tides on estuaries can show nonlinear effects on the movement of water which can have important impacts on the ecosystem and waterflow. The seawater entering the estuary is diluted by the fresh water flowing from rivers and streams. The pattern of dilution varies between different estuaries and depends on the volume of freshwater, the tidal range, and the extent of evaporation of the water in the estuary.

Estuarine ecosystem

Earth supports a wide diversity of life forms, each with specific environmental requirements for survival. No organism exists in isolation; plants, animals, and microorganisms within a given environment depend on one another through complex and interconnected relationships. These interactions between living organisms and their physical surroundings form an ecosystem.
Estuaries are dynamic coastal ecosystems shaped by the interaction of riverine and marine processes. The mixing of freshwater inflows with oceanic waters creates constantly changing physical and chemical conditions that influence habitat structure, nutrient availability, and biological productivity.
This transitional environment supports a diverse and highly interconnected community of organisms. Species from freshwater, marine, and terrestrial systems coexist and interact within estuaries, forming complex food webs and nutrient cycles. Together, these interactions make estuaries essential ecosystems that maintain biodiversity, regulate environmental processes, and link land, freshwater, and marine environments into a single integrated system.

Estuarine Ecosystem Services

Estuaries provide a wide range of ecosystem services that sustain both ecological productivity and human well-being. They function as nursery habitats, supporting the early life stages of many marine fish and shellfish, while also maintaining mangroves, seagrasses, salt marshes, and seaweeds that recycle nutrients and support productive fisheries. These ecosystems serve as critical migration corridors for anadromous and catadromous fish and provide essential natural resources, including timber, honey, and fuel, for nearby communities.
In addition, estuaries offer habitat for migratory birds and endangered species, enhancing biodiversity and supporting ecotourism opportunities. Water draining from upland areas carries sediments, nutrients, and pollutants into estuaries, where wetlands such as swamps and salt marshes naturally filter these materials improving water clarity and quality for both marine organisms and coastal communities. Vegetation such as salt marsh grasses also stabilizes shorelines and reduces erosion, providing a natural buffer against storm impacts and sea-level rise, thereby protecting coastal environments and human infrastructure.
Provisioning Services
  • Support fisheries by providing breeding, nursery, and feeding grounds for commercially important finfish and shellfish.
  • Contribute to national and export economies through fishery resources.
  • Provide raw materials from mangrove ecosystems such as timber, fuel, wax, and honey.
  • Support aquaculture activities that contribute to food production and local livelihoods.
Regulating Services
  • Filter sediments, nutrients, and pollutants from upland runoff through wetlands such as swamps and salt marshes.
  • Improve water quality, resulting in clearer and cleaner waters for marine life and human use.
  • Regulate and retain conservative pollutants, including heavy metals.
  • Support bioremediation through mangroves, seagrasses, salt marsh grasses, and seaweeds.
  • Stabilize shorelines and prevent coastal erosion.
  • Act as natural buffers against floods and storm surges, protecting upland habitats and coastal infrastructure.
Supporting Services
  • Function as critical natural habitats for birds, mammals, fish, and other wildlife.
  • Serve as the "nurseries of the sea, supporting early life stages of marine organisms.
  • Maintain high biological productivity and complex food webs.
  • Support nutrient recycling and biogeochemical cycling, with microbes playing a key role.
  • Serve as migratory corridors for anadromous and catadromous fish.
  • Provide resting and refueling sites for migratory birds.
  • Contribute to biodiversity conservation, including migratory and endangered species
Cultural and Economic Services
  • Support tourism, recreation, and aesthetic value in coastal regions.
  • Provide the foundation for coastal economies and local livelihoods.
  • Support ports, harbors, and transportation infrastructure essential for trade and shipping.
  • Deliver direct and indirect benefits that sustain human well-being and societal development.

    Classification based on geomorphology

Drowned river valleys

Drowned river valleys are also known as coastal plain estuaries. In places where the sea level is rising relative to the land, sea water progressively penetrates into river valleys and the topography of the estuary remains similar to that of a river valley. This is the most common type of estuary in temperate climates. Well-studied estuaries include the Severn Estuary in the United Kingdom and the Ems Dollard along the Dutch-German border.
The width-to-depth ratio of these estuaries is typically large, appearing wedge-shaped in the inner part and broadening and deepening seaward. Water depths rarely exceed. Examples of this type of estuary in the U.S. are the Hudson River, Chesapeake Bay, and Delaware Bay along the Mid-Atlantic coast, and Galveston Bay and Tampa Bay along the Gulf Coast.

Lagoon-type or bar-built

Bar-built estuaries are found in a place where the deposition of sediment has kept pace with rising sea levels so that the estuaries are shallow and separated from the sea by sand spits or barrier islands. They are relatively common in tropical and subtropical locations.
These estuaries are semi-isolated from ocean waters by barrier beaches. Formation of barrier beaches partially encloses the estuary, with only narrow inlets allowing contact with the ocean waters. Bar-built estuaries typically develop on gently sloping plains located along tectonically stable edges of continents and marginal sea coasts. They are extensive along the Atlantic and Gulf coasts of the U.S. in areas with active coastal deposition of sediments and where tidal ranges are less than. The barrier beaches that enclose bar-built estuaries have been developed in several ways:
  • building up of offshore bars by wave action, in which sand from the seafloor is deposited in elongated bars parallel to the shoreline,
  • reworking of sediment discharge from rivers by a wave, current, and wind action into beaches, overwash flats, and dunes,
  • engulfment of mainland beach ridges due to sea level rise and resulting in the breaching of the ridges and flooding of the coastal lowlands, forming shallow lagoons,
  • elongation of barrier spits from the erosion of headlands due to the action of longshore currents, with the spits growing in the direction of the littoral drift.

    Fjord-type

Fjords were formed where Pleistocene glaciers deepened and widened existing river valleys so that they become U-shaped in cross-sections. At their mouths there are typically rocks, bars or sills of glacial deposits, which have the effects of modifying the estuarine circulation.
Fjord-type estuaries are formed in deeply eroded valleys formed by glaciers. These U-shaped estuaries typically have steep sides, rock bottoms, and underwater sills contoured by glacial movement. The estuary is shallowest at its mouth, where terminal glacial moraines or rock bars form sills that restrict water flow. In the upper reaches of the estuary, the depth can exceed. The width-to-depth ratio is generally small. In estuaries with very shallow sills, tidal oscillations only affect the water down to the depth of the sill, and the waters deeper than that may remain stagnant for a very long time, so there is only an occasional exchange of the deep water of the estuary with the ocean. If the sill depth is deep, water circulation is less restricted, and there is a slow but steady exchange of water between the estuary and the ocean. Fjord-type estuaries can be found along the coasts of Alaska, the Puget Sound region of western Washington state, British Columbia, eastern Canada, Greenland, Iceland, New Zealand, Chile, and Norway.