Estuaries: The Vital Ecosystems Where Rivers Meet the Sea

Estuaries are partially enclosed coastal bodies of brackish water consisting of a mix of river freshwater and ocean saltwater. Rivers meet the sea in estuaries, where the lighter layer of freshwater tends to float on the surface, above the heavier layer of saltwater. This layering, together with the ebb and flow of tides, leads to a unique pattern of water circulation that results in high levels of nutrients, which stimulate primary productivity in plant and algal life. Estuaries are home to a wide variety of wildlife, including many threatened and endangered species. Estuaries are also important as sites of aquaculture, in which fish and shellfish are commercially raised, as well as locations for seaports. A number of estuaries have been severely affected by pollution from industrial and agricultural chemicals, as well as from sewage and other residential waste.

Types of Estuaries.Some estuaries are commonly referred to as bays, such as Chesapeake Bay in Maryland and Virginia—the largest of more than 100 estuaries in the United States. Chesapeake Bay and its tidal tributaries have a total surface area of 4,480 m² (11,600 km²). Other estuaries may be called coastal marshes, tidal flats, mud flats, fjords, lagoons, tidal streams, or other names, depending on the geographic region and geological characteristics. The formal names by which estuary types are classified include salt wedge estuaries, partially mixed (slightly stratified) estuaries, vertically homogeneous (vertically mixed) estuaries, and highly stratified estuaries (fjords). A small number of estuaries are classified as freshwater estuaries.

In salt wedge estuaries, the saltwater forms a wedge shape, which is thickest toward the ocean and tapers to a thin layer toward the land. The salty wedge extends farther up the river when the water is flowing slowly, and it retreats down the river during fastflowing floods. The surface water mixture of a salt wedge estuary draws in increasing amounts of salt from the lower layer the closer the water gets to the ocean. As the lower waters get mixed with the upper waters, the organic nutrients from decomposed plant and animal remains at the bottom of the estuary circulate throughout the estuary waters. Other nutrients sink from the upper to the lower layers and are carried upstream in the river, where they settle on the estuary bottom, providing rich sediment for the growth of saltwater-adapted plants, such as sea grasses.

The circulating nutrients make salt wedge estuaries very biologically productive. The nutrient-rich surface layers of these estuaries are home to a wide variety of phytoplankton and zooplankton. In tropical regions, mangrove forests develop in such estuaries. In temperate regions, salt marshes develop in these conditions.

In partially mixed estuaries, there is a very strong and active tide that repeatedly rises and falls, causing turbulent conditions and partial mixing of the upper freshwater with the lower saltwater. The surface water flow and bottom water flow are affected by the Coriolis effect, or the effect of Earth’s rotation. In the Northern Hemisphere, this effect causes the surface flow of the estuary to be stronger on the right side (facing toward the sea) and the bottom flow to be stronger on the left side. In the Southern Hemisphere, the surface flow is stronger on the left side, and the bottom flow is stronger on the right side.

In vertically homogeneous estuaries, the flow of the river water is slow and weak, whereas the flow of the tide is strong. These conditions prevent the development of water stratification, so that salinity levels remain mostly constant from the upper to lower layers and from one location to another. However, salinity levels are lowest where the river enters the estuary, and they are highest near the ocean.

In highly stratified estuaries, or fjords, the upper and lower layers are usually partially mixed, with the bottom being a basin of undiluted seawater kept in place by a submerged ridge, or sill, made of ancient glacial deposits. The salty water rises over this ridge to enter the estuary at medium depths when there is a strong landward flow of water at the bottom. However, the deep waters remain mostly undisturbed except when large infusions of seawater occur during storms. Fjords typically have steep sides and rocky shores, along which seaweeds grow.

Animal Life and Human Use.Although salt wedge estuaries are especially biologically productive, estuaries in general tend to be home to many diverse plants and animals. Shellfish, including mussels, oysters, crabs, and shrimp, are common invertebrates in these habitats. Fish are abundant, with some species living their entire lives in the estuaries and other species living there only during the juvenile stage. Amphibians, reptiles, birds, and mammals also make estuaries their homes, including using the biologically rich environments as nursery or nesting sites. Many migratory waterfowl species use estuaries as stopover sites during their journeys.

Some of the main animals raised in estuaries in the aquaculture industry include mussels, shrimp, salmon, and herring. The use of estuaries for ports and harbors dates back to ancient times, when many major civilizations arose in the river and coastal locations of the Middle East, North Africa, Italy, India, and China. Twenty-two of the thirty-two largest cities in the world were built along estuaries, including London, Hamburg, New York City, and Montreal.

Environmental Problems.Their proximity to heavily populated cities has made estuaries vulnerable to pollution from industrial, agricultural, and residential sources. The chemical pollution becomes concentrated by the same water circulatory patterns that concentrate nutrients in estuaries. The pollution filtered by the estuarine environment becomes trapped in the estuaries, thereby having the benefit of blocking its entry into the ocean. Estuaries also serve to stabilize shorelines, absorb water, and protect coastal cities during floods and hurricanes. Despite their ecological and societal value, many estuaries have been drained, dredged, filled, or dammed to create human developments. In the United States, the National Oceanic and Atmospheric Administration’s National Estuarine Research Reserve System and the Environmental Protection Agency’s National Estuary Program are charged with monitoring the ecological health of estuaries and helping local communities manage these resources. A. J. Smuskiewicz

FURTHER READING:Chesapeake Bay Program. 2012. “Facts & Figures.” http://www.chesapeakebay.net/discover/bay101 /facts. Accessed October 26, 2016.

National Geographic Society. 2016. “Estuary.” http://nationalgeographic.org/encyclopedia/estuary/. Accessed October 26, 2016.

National Oceanic and Atmospheric Administration. 2010. “Estuaries.” http://oceanservice.noaa.gov /education/tutorial_estuaries/welcome.html. Accessed October 26, 2016.

U.S. Environmental Protection Agency. 2016a. “National Estuary Program: Overview of the National Estuary Program.” https://www.epa.gov/nep/overview-national-estuary-program. Accessed October 26, 2016.

U.S. Environmental Protection Agency. 2016b. “What Are Estuaries?” https://www.epa.gov/nep. Accessed October 26, 2016.

U.S. Environmental Protection Agency. 2016c. “Estuarine Science.” http://omp.gso.uri.edu/ ompweb/doee/science/descript/whats.htm. Accessed October 26, 2016.