An estuary is a partly enclosed coastal area where freshwater from rivers or streams mixes with saltwater from the ocean. Estuaries act as transition zones between rivers and the sea, and they are called ecotones, which are areas where different ecosystems meet. These areas are shaped by both ocean processes, like tides and waves, and river processes, such as the flow of freshwater and the movement of sediment. The mixing of saltwater and freshwater creates high levels of nutrients in the water and in the soil, making estuaries some of the most productive natural habitats on Earth.

Most estuaries formed during the Holocene epoch, a time when rising sea levels flooded valleys carved by rivers or glaciers about 10,000–12,000 years ago. Scientists classify estuaries using four main criteria: how salt is distributed, their physical shape, how water moves, and how energy is used in the system. Estuaries may also be called bays, harbors, lagoons, inlets, or sounds, though some of these areas may not fully meet the definition of an estuary and could be entirely saltwater.

Many estuaries are harmed by factors such as soil erosion, cutting down trees, overgrazing, overfishing, and filling in wetlands. Too many nutrients from sources like runoff, sewage, and animal waste can cause eutrophication. Pollutants such as heavy metals, polychlorinated biphenyls, radioactive materials, and hydrocarbons from sewage, along with activities like building dikes or dams for flood control or water use, also damage estuaries.

Definition

The word "estuary" comes from the Latin word aestuarium, which means a tidal inlet of the sea. This term is based on aestus, the Latin word for tide. Many definitions have been suggested to describe an estuary. The most commonly used definition states: "a partially enclosed coastal area of water that connects to the open sea and where seawater is mixed with freshwater from land drainage." However, this definition does not include some coastal areas, such as lagoons and brackish seas.

A broader definition describes an estuary as "a partially enclosed body of water connected to the sea up to the point where tides reach or saltwater stops moving inland. It receives freshwater from rivers and streams, though this flow may not happen all year. The connection to the sea might be blocked for part of the year, and tides might not strongly affect the area." This definition includes areas like fjords, lagoons, river mouths, and tidal creeks.

An estuary is a changing ecosystem that connects to the open sea. Seawater enters the estuary with the rising and falling of tides. The movement of water in estuaries can create complex effects that influence the ecosystem and water flow. Freshwater from rivers and streams mixes with seawater, but the pattern of this mixing depends on the amount of freshwater, the height of the tides, and how much water evaporates in the estuary.

Estuarine ecosystem

Earth has many different types of living things, each needing certain conditions to survive. No living thing exists alone; plants, animals, and tiny organisms in an area depend on each other in many ways. These connections between living things and their environment create an ecosystem.

Estuaries are special areas near the coast where rivers meet the ocean. Freshwater from rivers mixes with saltwater from the ocean, creating changing conditions that affect how habitats look, how nutrients are used, and how much life can grow in the area.

These areas support many different living things that depend on each other. Animals and plants from rivers, oceans, and land live together in estuaries, forming food chains and helping move nutrients through the environment. These connections make estuaries important places that help protect many species, keep natural processes working, and connect land, rivers, and oceans into one system.

Estuaries help both nature and people in many ways. They are places where young fish and shellfish grow before moving to the ocean, and they support plants like mangroves, seagrasses, and salt marshes that help keep water clean and support fish. These areas also help fish move between the ocean and rivers, and they provide resources like wood, honey, and fuel for nearby people.

Estuaries also give homes to birds and animals that are rare or in danger, which helps protect the variety of life on Earth. Water from mountains and forests carries dirt, nutrients, and pollution into estuaries, where wetlands like swamps and salt marshes naturally clean the water, making it clearer and safer for both animals and people. Plants like salt marsh grasses help stop the shoreline from being damaged by storms and rising sea levels, protecting both nature and human buildings.

Provisioning Services
– Help fish and shellfish grow, reproduce, and find food.
– Provide fish that are important for food and trade.
– Offer materials like wood, fuel, wax, and honey from mangroves.
– Help people grow food through activities like farming fish and shellfish.

Regulating Services
– Clean water by trapping dirt, nutrients, and pollution in wetlands.
– Make water cleaner for fish and people to use.
– Keep harmful chemicals like heavy metals from spreading.
– Help clean water by using plants like mangroves, seagrasses, and salt marshes.
– Stop the shoreline from being worn away by waves and storms.
– Protect land and buildings from floods and strong waves.

Supporting Services
– Give homes to birds, fish, and other animals.
– Help young ocean animals grow before they move to the open sea.
– Keep the environment healthy by moving nutrients and helping tiny organisms work.
– Help fish move between the ocean and rivers.
– Give birds places to rest and find food during long trips.
– Help protect many types of animals, including those that are rare or endangered.

Cultural and Economic Services
– Help people enjoy nature through activities like visiting, swimming, and seeing beautiful places.
– Support jobs and money in coastal areas.
– Help build ports, docks, and roads that are important for shipping.
– Help people live better lives by providing food, clean water, and other benefits.

Classification based on geomorphology

Drowned river valleys are also called coastal plain estuaries. When sea levels rise compared to the land, seawater slowly moves into river valleys, and the shape of the estuary stays similar to a river valley. This is the most common type of estuary in temperate climates. Examples include the Severn Estuary in the United Kingdom and the Ems Dollard near the Dutch-German border.

These estuaries usually have a large width-to-depth ratio, appearing wedge-shaped (in side view) near the land and becoming wider and deeper as they move toward the sea. Water depths rarely go above 30 meters (100 feet). In the United States, examples include the Hudson River, Chesapeake Bay, and Delaware Bay along the Mid-Atlantic coast, and Galveston Bay and Tampa Bay along the Gulf Coast.

Bar-built estuaries form where sediment deposits have kept up with rising sea levels, creating shallow estuaries separated from the sea by sand spits or barrier islands. These estuaries are common in tropical and subtropical regions. They are partially enclosed by barrier beaches (such as barrier islands and sand spits), with only narrow openings connecting them to the ocean. Bar-built estuaries often develop on gently sloping plains near the edges of continents and along coastal areas with stable tectonic activity. They are widespread along the Atlantic and Gulf coasts of the U.S. in regions with active sediment deposition and where tidal ranges are less than 4 meters (13 feet). Barrier beaches form in several ways:

• Offshore bars are built by waves, with sand from the seafloor forming long, parallel bars near the shore.
• Rivers deposit sediment that is reshaped by waves, currents, and wind into beaches, overwash flats, and dunes.
• Sea level rise causes mainland beach ridges (formed by erosion of coastal sediments about 5,000 years ago) to be flooded, creating shallow lagoons.
• Barrier spits grow from the erosion of headlands by longshore currents, extending in the direction of the littoral drift.

Fjord-type estuaries form where glaciers during the Pleistocene era deepened and widened river valleys, creating U-shaped cross-sections. At their mouths, rocks, bars, or sills made of glacial deposits often block the estuary, affecting water movement.

These estuaries are shaped by glaciers and have steep sides, rock bottoms, and underwater sills formed by glacial activity. They are shallowest at their mouths, where glacial moraines or rock bars create sills that limit water flow. In the upper parts, depths can exceed 300 meters (1,000 feet). The width-to-depth ratio is usually small. In estuaries with shallow sills, tidal movements only reach the depth of the sill, leaving deeper water stagnant for long periods. If the sill is deep, water exchange between the estuary and ocean happens more steadily. Fjord-type estuaries are found along coasts in Alaska, the Puget Sound region of Washington, British Columbia, eastern Canada, Greenland, Iceland, New Zealand, Chile, and Norway.

Tectonic estuaries form when land sinks due to faulting, volcanoes, or landslides, or when rising sea levels during the Holocene Epoch flood areas. Only a few tectonic estuaries exist, such as San Francisco Bay, which formed when movement along the San Andreas Fault caused the lower parts of the Sacramento and San Joaquin rivers to flood.

Classification based on water circulation

In this type of estuary, the flow of water from the river is much greater than the flow of seawater from the ocean, and tides have little effect. Freshwater sits on top of seawater, forming a layer that gets thinner as it moves toward the sea. The heavier seawater moves toward the land along the bottom of the estuary, creating a wedge-shaped layer that becomes thinner near the shore. When the speed of the two layers differs, forces between them create waves that mix seawater upward with freshwater. Examples include the Mississippi River and the Mandovi estuary in Goa during the monsoon season.

When tides become stronger, the flow of seawater into the estuary becomes greater than the river's outflow. This causes turbulence from the movement of water, mixing the entire water column so that salt levels change mainly along the length of the estuary, not up and down. This creates a condition where the water is somewhat layered. Examples are the Chesapeake Bay and Narragansett Bay.

When tidal forces are stronger than the river's flow, the water in the estuary mixes completely, and the difference in salt levels between the top and bottom of the water column disappears. The boundary between freshwater and seawater vanishes because of strong mixing and swirling water movements. Examples include the lower parts of Delaware Bay and the Raritan River in New Jersey, where the water has the same salt level throughout.

Inverse estuaries form in dry areas where water evaporates more than the amount of freshwater flowing in. A zone with the highest salt level develops, and both river water and seawater flow toward this zone near the surface. This water then moves downward and spreads along the bottom in both directions. Examples include Spencer Gulf in South Australia, the Saloum River and Casamance River in Senegal.

The type of estuary depends greatly on how much freshwater flows into it and can change from a completely ocean-like area to any of the other estuary types.

Physiochemical variation

The most important features of estuary water include the amount of dissolved oxygen, the saltiness (salinity), and the amount of sediment. Salinity changes greatly across different areas, ranging from almost no salt near where rivers meet the estuary to 3.4% salt at the estuary's mouth. At any one location, salinity can change a lot over time and with seasons, making it a difficult environment for living things. Sediment often settles in intertidal mudflats, which are hard for organisms to live in because there are no surfaces for algae to attach to, so plant-based habitats do not form. Sediment can also block feeding and breathing parts of animals, and species living in mudflats have special traits to handle this challenge. Lastly, changes in dissolved oxygen levels can harm life. Nutrient-rich sediment from human activities can increase the growth of plants and algae, which may later decay and reduce oxygen in the water, leading to areas with very low or no oxygen.

Implications of eutrophication on estuaries

Nitrogen is a major cause of eutrophication in estuaries in temperate regions. During eutrophication, natural chemical processes reduce the amount of silica available in the water. These processes also increase the supply of nitrogen and phosphorus, creating conditions where harmful algal blooms can continue for long periods. Because the nitrogen cycle is now unbalanced, estuaries may become limited by phosphorus instead of nitrogen. Estuaries can be greatly affected by an unbalanced phosphorus cycle, as phosphorus interacts with the availability of nitrogen and silica.

When there is an abundance of nutrients in an ecosystem, plants and algae grow rapidly and eventually die. As these organisms decompose, they release large amounts of carbon dioxide. While releasing carbon dioxide into the water and atmosphere, these organisms also take in most of the available oxygen, creating low oxygen conditions and an unbalanced oxygen cycle. The excess carbon in the form of carbon dioxide can lower pH levels and cause ocean acidification, which is more harmful to vulnerable coastal areas like estuaries.

Eutrophication has been shown to negatively affect many plant communities in estuarine ecosystems. Salt marshes, a type of ecosystem found in some estuaries, have been harmed by eutrophication. Cordgrass is the main plant in salt marshes. Excess nutrients allow plants to grow more above ground, but less energy is used for root growth because nutrients are plentiful. This leads to less root growth below ground, which weakens the stability of the marsh and increases erosion. A similar situation occurs in mangrove swamps, another ecosystem in estuaries. Increased nitrogen causes more growth in the above-ground parts of mangroves but less growth in their roots. Weaker root systems make mangroves less able to survive during dry seasons, which can lead to their death. This shift in growth caused by eutrophication can reduce the success of plants in these ecosystems.

In all biomes, eutrophication often leads to plant death, but the effects do not stop there. Plant death changes the structure of the food web, which can cause the death of animals in the affected area. Estuaries are rich in biodiversity and support a large portion of commercial fish catches, making the effects of eutrophication even more severe in these areas. Some estuarine animals are more affected by eutrophication than others. For example, whitefish in the European Alps have experienced local extinctions because eutrophication reduced oxygen levels so much that their eggs could not survive. However, some animals, such as carnivorous fish, may benefit from nutrient-rich environments. This can be seen in populations of bass or pikes.

Eutrophication can harm many marine habitats and lead to economic consequences. The commercial fishing industry depends on estuaries for about 68 percent of its catch by value due to the high biodiversity in these ecosystems. During algal blooms, fishermen have observed a sudden increase in fish numbers. A sudden rise in primary productivity can temporarily boost fish populations, but continued loss of oxygen in the water eventually causes fish populations to decline. These effects often begin in estuaries and can impact surrounding waters. This can reduce fishing industry sales in one area and across the country. In 2016, fishing—both commercial and recreational—contributed billions of dollars to the United States' gross domestic product (GDP). A decline in fishing production can affect the 1.7 million people employed in the fishing industry in the United States each year.

Implications for marine life

Estuaries are changing environments where temperature, salt levels, water cloudiness, depth, and water movement change daily because of tides. This change makes estuaries very productive places for life, but it also makes it hard for many species to live there all year. Because of this, fish populations in estuaries change a lot with the seasons. In winter, fish communities are mostly made up of strong, ocean-dwelling species. In summer, many ocean and anadromous fish move in and out of estuaries to take advantage of the high productivity. Estuaries are important habitats for many species that depend on them for parts of their life cycles. Pacific Herring (Clupea pallasii) lay their eggs in estuaries and bays, surfperch give birth in estuaries, juvenile flatfish and rockfish move to estuaries to grow, and anadromous salmonids and lampreys use estuaries as paths for migration. Also, migratory birds, like the black-tailed godwit, rely on estuaries.

Two major challenges for life in estuaries are changes in salt levels and the amount of sediment. Many fish and invertebrates have ways to handle or adjust to these changes in salt levels, and are called osmoconformers and osmoregulators. Many animals dig into the sediment to avoid predators and live in a more stable environment. However, large numbers of bacteria are found in the sediment, and they use a lot of oxygen. This lowers the oxygen levels in the sediment, sometimes creating areas with very little oxygen, which can be made worse by slow water movement.

Phytoplankton are important producers of food in estuaries. They move with the water and are carried in and out by the tides. Their growth depends on how cloudy the water is. The main types of phytoplankton in estuaries are diatoms and dinoflagellates, which are common in the sediment.

A main food source for many organisms in estuaries, including bacteria, is detritus from the settling of sediment.

Human impact

Out of the thirty-two largest cities in the world in the early 1990s, twenty-two were located on estuaries.

Estuaries are ecosystems that face dangers from human actions, such as pollution, overfishing, sewage, coastal development, and land clearing. These areas collect materials like pollutants and sediments from far upstream. Runoff from land, along with waste from industrial, agricultural, and household sources, flows into rivers and then into estuaries. Some harmful substances, like plastics, pesticides, furans, dioxins, phenols, and heavy metals, do not break down quickly in the ocean.

These toxins can build up in the bodies of aquatic animals through a process called bioaccumulation. They also gather in benthic environments, such as estuaries and bay muds, which act as a geological record of human activities over the past century. The chemical makeup of biofilm in these areas can show where human impacts have occurred. Over time, these changes may alter the basic structure of the ecosystem, causing both temporary and permanent shifts in its non-living and living parts.

Estuaries are naturally rich in nutrients because land runoff carries nutrients into them. Human activities have added more chemicals, such as fertilizers used in farming and waste from animals and people, into the water. Too many oxygen-depleting chemicals can lead to low oxygen levels, creating dead zones where marine life cannot survive. This harms water quality and reduces fish and other animal populations. Overfishing is also a problem. For example, the Chesapeake Bay once had a large oyster population that was nearly destroyed by overfishing. Oysters help clean water by filtering pollutants and removing excess nutrients. In the past, they could filter the entire estuary’s water every three to four days. Now, this process takes almost a year, leading to problems like excess sediment, nutrients, and algae in local waters.

Some major rivers that flow through deserts once had large estuaries, but these areas have shrunk due to dams and water diversions. An example is the Colorado River Delta in Mexico, which was once covered with marshes and forests but is now mostly a salt flat.