Estuaries

Introduction

An estuary is a zone where a river flows into the sea, mixing freshwater with saltwater. The river brings nutrient-rich sediment into an estuary that can support a high level of biological productivity. Estuaries are usually relatively warm and calm, protected from the force of the ocean, but the constant ebb and flow of the tides, together with the presence of both freshwater and saltwater can make it a challenging habitat. Species living in estuaries, like salt-resistant grasses, are those that can adapt to the margins of the sea.

Many fish and shellfish spawn and develop in estuaries, protected from the open ocean. Estuaries have traditionally provided a rich hunting ground for species such as crabs, clams, and oysters. However, many estuaries are now polluted from sources such as agricultural runoff arising from human overuse of pesticides and fertilizers, and industrial waste. Efforts are being made to restore some of the world’s great estuaries, including Chesapeake Bay, but their recovery is a slow process.

Historical Background and Scientific Foundations

An estuary is the place where a river, stream, or creek inlet meets the sea or ocean, creating a complex physical environment and a set of ecosystems that is productive, although not necessarily diverse. There may be many water inlets, giving the estuary a complex shape, or just one. An estuary is partially enclosed by some combination of coastland, marshes, wetlands, and barrier islands, and is thus protected from the direct force of ocean wind and waves.

Freshwater from the inlets bring nutrients to the estuary in its sediments and dilutes the saltwater. In some temperate estuaries, the less dense freshwater floats on top of the more dense saltwater. Estuarine ecology is shaped by the tides. The saltwater layer picks up nutrients from the muddy bottom as it moves with the tides and then mixes these with the freshwater layer. Estuarine water is defined as brackish, meaning having a salt content in between that of freshwater and seawater. Typically, its salt content is not constant, but will vary with location in the estuary and over time. Estuaries are usually shallow and so the euphoric zone, which is the layer receiving enough sunlight for photosynthesis, tends to extend to the bottom, which gives the whole zone high biological productivity.

Both terrestrial and aquatic plants and animals are found in estuaries. Terrestrial species include storks, pelicans, coniferous and deciduous trees, wildflowers, and butterflies. Aquatic species include sea grasses, crabs and lobsters, turtles, marine birds, and marine mammals like sea lions. Filter feeders like the horseshoe crab pull larvae and plankton out of the water moving past them. Oysters adhere to the ocean floor and are a source of food not just for humans but also for fish, sponges, and snails. These species are all closely linked to one another in a complex food web.

There are four types of estuary, classed according to geological formation. The coastal plain estuary is created when rising sea levels fill an existing river valley, as in the Chesapeake Bay in Maryland or the harbor in Charleston, South Carolina. Tectonic estuaries are formed by the folding of land surfaces and are found along major fault lines. Such estuaries include San Francisco Bay. The bar-built estuary is one in which a shallow bay or lagoon is protected from the ocean by a barrier island or a sand bar. One example is the Eastern seaboard or the Gulf Coast of North America. Fjords are U-shaped valleys that have been formed by glacial action. They are found in Norway, Alaska, and Canada.

The tides and the flow of freshwater into saltwater create three main types of habitat within an estuary. The salt marsh is farthest from the sea and is a shallow wetland that is occasionally or regularly flooded with saltwater. Notable salt-marsh locations in the United States include the eastern coast from Maine to Florida and continuing on to Louisiana and Texas along the Gulf of Mexico. Some of these marshes are brackish or freshwater rather than saltwater marshes. They are all influenced by ocean tides that create two zones. The lower or inter-tidal marsh is generally covered and exposed every day by the tide, while the upper or high marsh is only covered sporadically. This affects the plant ecology in the two zones. Tall cordgrass tends to grow in the lower marsh, while a shorter version grows in the upper marsh, along with various other salt-resistant grasses. Salt marshes also provide food and shelter for clams, crabs, juvenile fish, and migratory wildfowl.

Mudflats and sandflats are found closer to the sea. They comprise large zones of mud, which is a fine-grained sediment, or sand, which is coarser; both do not support vegetation. Mudflats are exposed to the air at low tide and submerged at high tide. They are, therefore, a challenging habitat because of the constant switch between land and aquatic conditions, from fresh to saltwater and changes in temperatures. Therefore, the biodiversity of mudflats is not great, but there may be large populations of the few species that can survive here. They are rich in invertebrates such as worms, clams, and other shellfish and often support commercial oysterbeds. Mudflats are also good foraging grounds at low tides for shorebirds and, at high tide, for birds and fish.

The mudflats of the San Francisco Bay area are extensive and there are smaller ones throughout California and some Pacific areas. Because they are shal-low, and near the shore, they are at risk from dredging, filling for development, sewage pollution, and agricultural runoff. The final main habitat in an estuary is the pathways full of water known as channels, which connect the salt marshes and mud flats to the sea or ocean. These serve as nurseries for fish and shellfish, many of which have commercial value, because of the protection and food supplies they offer. Other habitats found in estuaries include rock pools, mangrove swamps, and coral reefs.

Every estuary is unique. An estuary may be known as a bay, harbor, lagoon, or sound. However, not all areas given these names are estuaries. The defining feature of an estuary is the mixing of fresh and salt water. Some well-known examples of estuaries in the United States include Mobile Bay, Puget Sound, Boston Harbor, the New York/New Jersey Harbor, and Tampa Bay.

Chesapeake Bay, which extends for 200 mi (322 km) from Havre de Grace in Maryland to Virginia Beach, Virginia, illustrates many of the typical features of an estuary. It is 34 mi (55 km) wide at its widest point, which is the mouth of the Potomac River, and its average depth is about 21 ft (6.5 m). It is home to more than

WORDS TO KNOW

CHANNEL: A water-filled path which connects mudflats and salt marshes to the ocean in an estuary.

FLYWAY: A flying route regularly taken by migratory birds.

FOOD WEB: An interconnected set of all the food chains in the same ecosystem.

MUDFLAT: An area of low-lying muddy land that is covered at high tide and exposed at low tide.

SALT MARSH: Wetland which is sometimes flooded with seawater.

SEDIMENT: Solid unconsolidated rock and mineral fragments that come from the weathering of rocks and are transported by water, air, or ice and form layers on Earth’s surface. Sediments can also result from chemical precipitation or secretion by organisms.

WATERSHED: The expanse of terrain from which water flows into a wetland, water body, or stream.

3,600 species of animal and plant including 348 species of fish and 173 species of shellfish. There are 2,700 different plant species and 29 species of waterfowl in Chesapeake Bay, which is an important resting ground for birds along the Atlantic flyway. A million waterfowl winter in the bay every year.

Impacts and Issues

Estuaries are among Earth’s most biologically productive ecosystems. They produce more organic matter every year than a forest, grassland, or agricultural land of comparable size. More than two thirds of commercial fish and shellfish spend part of their lives in estuaries, and about 80 to 90% of all recreational fishing is done here, Many migratory birds rest and refuel in estuaries during their long journeys, thanks to the abundant food supplies and protection from the ocean. Estuaries also act as filters for sediments and pollutants running off the land toward the sea or ocean, creating clearer, cleaner water, which benefits both people and wildlife. Estuarine soils and plants have a spongelike capacity that helps prevent flooding and storm surges. Salt-marsh grasses and other estuarine plants also help to prevent erosion of the sea shore.

Many estuaries are places of outstanding natural beauty or scientific interest, where people like to go to bird watch, fish, surf, swim, or sail. They also have important commercial value in terms of the fishing and related industry. Estuarine fishery trade is worth billions of dollars in the United States; many estuaries include ports or harbors of importance in shipping and transportation. Together commercial and recreational fishing, boating, tourism, and other coastal industries in and around estuaries provide more than 30 million jobs in the United States. For instance, two of the major North Atlantic ports, Baltimore and Hampton Roads, are situated on Chesapeake Bay.

Most estuaries in the United States and elsewhere, however, are now under threat because of human activities. This is mainly because coastal communities have grown in recent years. The population of the Chesapeake Bay area is 16.6 million and it is growing by 170,000 people a year. Therefore, estuarine channels have been dredged, waters polluted, and marshes filled in to create housing, roads, and industry. This has led to polluted water and beaches, development of harmful algal blooms, and loss of habitat for formerly abundant species.

The damage to estuaries worldwide has been going on for many years. The most comprehensive study ever of 12 important temperate estuaries and coastlines in

Europe, North America, and Australia, reported in 2006, found a high degree of degradation and depletion at all sites. The international team of researchers used paleontological, archaeological, historical, and ecological records to discover what the estuaries were like in the past so as to plot the changes that have occurred. The exploitation and decline of estuaries began as far back as the Roman Empire, but have accelerated dramatically in the last 150 to 300 years. Over 90% of formerly important species have been depleted to at least some degree and 65% of seagrass and wetland habitat is now destroyed, while water quality has been degraded. Most of the damage can be attributed to rapid population growth and the accompanying increase in demand. Commercialization of resource use and the growing demand for luxury foods like salmon, lobster, and oysters, have accelerated decline.

This study found that estuarine locations with the longest history of human impact and the highest human populations were among the most degraded, and these include the Adriatic, Wadden, and Baltic Seas. The outer Bay of Fundy, Southern Gulf of St. Lawrence, and Massachusetts Bay were the least degraded estuaries. Stocks of most mammals, birds, and reptiles were already depleted by 1900 and had declined still further by 1950 because of over exploitation for food or increasing demand for luxury items like furs.

For food, salmon and sturgeon stocks were depleted first, then tuna, shark, halibut, herrings, and sardines, demonstrating a shift from high- to lower-value species. The study also showed that oysters were the first invertebrate to be depleted because of their high value and destructive exploitation methods. The reef-forming and filter-feeding capacity of oysters has meant that their depletion had a deleterious effect on the estuarine ecosystem by reducing its ability to provide high water quality and a complex habitat. The advent of seafloor trawling hit previously unaffected filter feeders like corals and sponges. Mussels and crabs went into decline because, although they have long been used for food, they only recently become the target of commercial fish-ries.

The report also found that loss of coastal vegetation in estuarine zones has been a factor in the loss of nursery habitats for fish and also in protecting coastlines from erosion. However, although many species have been depleted, only 7% have actually become extinct. Restoration efforts are working, but slowly. There has been a partial recovery, although estuaries today remain far from their original ecosystem structure and function. Reducing exploitation, safeguarding water quality, and protection of habitats are the key to restoring estuaries.

Restoration of estuaries is going on all around the world. The United States National Estuary Program was established in 1987 following amendments to the Clean Water Act. Its mission, however, extends far beyond

merely ensuring water quality and is more about protecting and restoring the nation’s estuaries through maintaining the integrity of the whole system in terms of its physical, chemical, and biological properties. The program encourages local communities to take the lead in managing their own estuaries. Currently, 28 estuaries are participating in the program. They include San Francisco Bay, Puget Sound, Indian River Lagoon, and the New Hampshire estuaries.

A separate program is underway to protect the largest estuary in the United States, Chesapeake Bay, which was the first to be targeted for protection and restoration in 1983. This is proving to be a long-term project, as the damage of the last 100 years or so cannot be reversed in only a couple of decades. In a recent assessment, the overall health of the Bay area was still judged to be degraded. Just 12% of the Bay and its tidal tributaries met dissolved oxygen standards. This means that pollution with nutrients is still in the water, causing overgrowth of algae and so encouraging the growth of microorganisms that use up oxygen. Lower oxygen levels will decrease the population of fish and other species. The area of grasses increased to 65,000 acres, but this is only about a third of the ultimate restoration goal. Native oyster and blue crab populations continue to be lower than their targets. However, striped bass populations remained high, although some fish are not healthy.

The main factors that are impeding the recovery of Chesapeake Bay include population growth, agricultural runoff, and climate variability, which are the same as those affecting estuaries around the world. The program has achieved only part of its goals of controlling nitrogen, phosphorus, and sediment coming from agricultural lands and wastewater. But progress has been made. A total of 2,266 mi (3,647 km) of freshwater stream habitat has been re opened to migratory fish. Maryland, Virginia, Pennsylvania, and the District of Columbia have permanently preserved 6.88 million acres of land, which brings them close to their 2010 goal of preserving 20% of their combined land within the Chesapeake watershed.

See Also Bays and Estuaries; Coastal Ecosystems; Oceans and Coastlines

BIBLIOGRAPHY

Books

Kaufmann R., and C. Cleveland. Environmental Science. New York: McGraw-Hill International Edition, 2008.

Web Sites

Chesapeake Bay Program. “About the Bay.” March 2008. http://www.chesapeakebay.net/aboutbay.aspx?menuitem=13953 (accessed March 31, 2008).

Estuaries.gov. “About Estuaries.” August 8, 2006. http://www.estuaries.gov/about.html (accessed March 31, 2008).

Lenfest Ocean Program. June 23, 2006. http://www.lenfestocean.org/publications/Lotze-etal_06_Science.pdf (accessed May 22, 2008).

U.S. Environmental Protection Agency. “National Estuaries Program.” January 16, 2008. http://www.estuaries.gov/about.html (accessed March 31, 2008).

Susan Aldridge