Estuary

I

Introduction

Estuary, semienclosed coastal area, where seawater mixes with fresh water from rivers; also, the tidal area of the lower part of a river. There are usually three overlapping zones in an estuary: an open connection with the sea where marine water preponderates, a middle area where strong salt water and fresh water mix, and a tidal river zone where fresh water preponderates. Tidal forces create variable estuarine characteristics in sea inlets. Variation in the seasonal discharge of rivers causes the limits of these zones to shift, and this increases the overall ecological complexity of estuaries. Estuaries are highly productive ecosystems, accounting for one-half of the living matter of the world's oceans.

II

Geological Formation

In terms of geology, present-day estuaries are young and ephemeral coastal features. Today's estuaries began to take their current form during the last interglacial period (a warm period during which polar caps melt), when sea level rose about 120 m (about 394 ft). However, the relatively high sea levels and extensive estuaries found today have been characteristic of only about 10 to 20 percent of the last million years. When sea level was lower, during glaciation periods, estuaries were much smaller than they are at present and were located on what is now the continental slope. Unless sea level rises, estuaries tend to fill with sediments and become much smaller. The sediments come from riverborne terrestrial materials from the eroding continents and from sand transported upstream by the tides from the continental shelf. See also Glacier; Ice Ages.

Tidal bores—sudden, extreme rises in tidal water—occur in some estuaries, such as the Bay of Fundy and the mouth of the Amazon River. When strong tides are forced upward through narrow channels, the water can rise swiftly and often to great heights.

III

Water and Vegetation Patterns

It is in estuaries that most of the world's freshwater runoff encounters the oceans. Because fresh water is lighter, or less dense, than salt water, unless the two are mixed by the tides or winds, the fresh water remains at the surface, resulting in a salinity gradient. Tides force seawater inland as a countercurrent and produce a saltwater wedge below the freshwater surface waters.

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Estuaries are generally divided into four main groups. Coastal-plain estuaries were formed during the last great rise in sea level, when melting glaciers in temperate latitudes flooded river valleys. In some tropical regions of low relief, such as the Amazon Basin, a combination of rising sea levels and increased rainfall led to greater flooding. Coastal-plain estuaries resemble a V-shaped river channel, usually less than 20 m (65 ft) deep, with an accompanying floodplain.

Salt-marsh estuaries are also part of the coastal plain. Although they have a well-defined drainage network, they are not usually fed by rivers and thus they contain predominantly salt water. This type of estuary is common from Cape Fear, North Carolina, to Cape Canaveral, Florida.

Lagoons, in contrast to salt-marsh estuaries, have a less well defined drainage network and larger open areas and are usually shallow—often less than 2 m (6.5 ft) deep. A raised ridge, or sand barrier, is characteristic of lagoons. This feature was formed during the interglacial stage of the Pleistocene Epoch, some 80,000 years ago, when sea shorelines were about 6 m (20 ft) above present average levels. During the last ice age, fluvial and atmospheric processes eroded the earlier coast. When sea levels rose anew, the areas behind the barrier were once again flooded. Lagoons are present on all continents.

Fjords, a major type of estuary in both the northern and southern temperate latitudes above about 45°, were formed when continental glaciers scoured out river valleys. Such estuaries usually have a U-shaped cross-sectional form. Water in the interior part of the fjord can exceed 500 m (1600 ft) in depth.

Tectonic estuaries result from major geological events such as faulting, volcanic eruption, and landslides. San Francisco Bay is the best-known estuary formed by tectonic activity.

Estuaries are rarely in a steady state. Tides are the principal energy source causing estuarine mixing, but wind, wave motions, and river runoff can also be important locally. Salt water and fresh water mix to form brackish water. Due mostly to oscillations in river flow, the three main estuarine zones—saltwater, brackish, and freshwater—can shift seasonally and vary greatly from one area to another. Also, an area of an estuary can change from stratified to well-mixed during the spring neap-tide cycles. The most highly stratified estuaries are the ones that receive a large amount of fresh water but that have a relatively low tidal range; the Mississippi Delta is one example. Partially mixed estuaries have moderate freshwater inflow and tidal range, such as Chesapeake Bay. The brackish zone of such estuaries may have a salinity of 2 to 10 parts per thousand (ppt), compared with the salinity of salt water, which is about 35 ppt. Where there is a large tidal range but little freshwater inflow, as in the North Inlet of South Carolina and in the Bay of Fundy, mixing is more complete. In coastal lagoons, where there are large open waters, small tidal range, and low freshwater inputs, wind is usually a more important mixing agent than tides.

The chemical components of fresh water can vary greatly, whereas salt water is fairly uniform. Consequently, variations in the chemistry of river water produce considerable differences in estuarine nutrient cycles. In general, the most important compounds for estuarine life that are supplied by fresh water are silicon, iron, nitrogen, and phosphorus. Seawater provides sulfate and bicarbonate. Estuaries provide some of the most productive habitats on earth because of the accumulation and availability of nutrients—along with adequate light conditions—that fuel the production of phytoplankton, the tiny, single-celled algae that drift in the water. Phytoplankton are highly adapted to the nutrient-rich but often rigorous conditions of estuarine waters. They are critical to the food web; a group of phytoplankton called diatoms are especially important.

The estuaries of the world have very diverse vegetation types, ranging from small, sparse grasses to large rain-forest trees with a closed canopy. Salt marshes are intertidal communities of rooted plants that are inundated by the tides. They are most common in the temperate and boreal regions, and more than 600 plant species have been recorded from these habitats. In the United States, the Mississippi Delta is especially famous for its huge salt marshes and the robust, highly productive plant communities associated with them.

Mangroves, which are adapted to saline, boggy, and periodically inundated soils, are common in the salt marshes of the Tropics, although they also extend as far north as the Gulf Coast of the United States. There are only about ten mangrove species in the Americas; the western region of the Indo-Pacific has more.

Although they are seldom recognized because so little is known about them, tidal rain forests are probably the richest estuarine habitats on earth. Especially important are the huge tidal rain forests of the Amazon River.

There are often dense communities of submerged vascular plants in the shallower and at least partially protected areas of estuaries. These plants usually root in soft sediments. In the more saline zones of the northern latitudes, the so-called seagrasses, such as eelgrass and turtlegrass, are usually the most common.

IV

Animal Life

The animal life in estuaries varies greatly. Tropical estuaries are generally more diverse than their temperate counterparts. High productivity and habitat complexity can lead to considerable wildlife diversity and, in some cases, to huge concentrations of animals as well. All of the main vertebrate groups are represented in estuaries: mammals, rodents, carnivores, and ungulates. Waterfowl such as ducks, geese, herons, and egrets are often found in large numbers. Huge flocks of songbirds, such as the red-winged blackbird in eastern North America, visit estuaries in search of food. Other species use estuaries as resting sites during annual migrations.

To survive in estuaries, aquatic animals must have physiological or behavioral adaptations to changes in salinity. Many oceanic species, such as some of the sea catfish, are able to move inward during the low-water season of rivers, when the salinity of estuaries increases. Freshwater fish move oceanward with the annual floods. Due to the stressful conditions that fish face in the mixed zones of estuaries, there is often little diversity despite the extremely high productivity of a few fish species.

The rich diversity of animal life in estuaries represents a complex food web. The foundation of the estuarine food web is plankton. These microscopic animals are more diverse and essential to food webs and community structure than are vertebrates. Zooplankton feed on phytoplankton and are, in turn, eaten by fish and other animals. Dead plankton make up an important element of the organic bottom layer of estuaries. Benthic (bottom-dwelling) fauna such as barnacles, bryozoans, sponges, mussels, and hydroids live on the rich estuary floor. Fish, crabs, and shrimp take advantage of the large numbers of invertebrates found in estuaries. This extremely high productivity makes the estuary an attractive nursery habitat for many crustacean and fish species—and such an attractive fishing ground for humans.

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