Estuary

I

Introduction

Estuary, the tidal area of the lower part of a river. Sea inlets, extending as far as the tides are strongly felt, also have estuarine characteristics of varying quality. There are usually three distinct but overlapping zones in an estuary: an open connection with the sea where marine water dominates; a middle area subject to strong salt and fresh water mixing; and a tidal river zone characterized by fresh water. Variation in the seasonal discharge of rivers causes the limits of these zones to shift, and this increases the overall ecological complexity of estuaries.

II

Geological Formation

Geologically speaking, present-day estuaries are young and ephemeral coastal features. Today's estuaries began to take their current form during the beginning of the present interglacial period, when sea level rose about 120 m (400 ft) between 15,000 years ago and 5,000 years ago, when it reached its current level. When sea level was lower (glaciation periods), estuaries were greatly restricted compared to their present form and were located on what is now the continental slope. The relatively high sea levels and extensive estuaries found today have only been characteristic of about 10 to 20 per cent of the last million years. Unless sea level rises the tendency of estuaries is to fill with sediments and become greatly reduced in size. The sediments are derived from river-borne terrestrial materials from the eroding continents and from sand transported upstream by the tides from the continental shelf.

Estuaries are generally divided into four main groups. Coastal plain estuaries were formed during the last great rise in sea level when, in temperate latitudes, melting glaciers caused river valleys to become increasingly flooded, or 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. This type of estuary is reminiscent of 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, and although they have a well-defined drainage network they are not usually fed by rivers, and are thus dominated by salt water. This type of estuary is common, for example, in parts of the Florida coastline of the United States.

Lagoons, in contrast to the salt-marsh type of estuary, have a less well-defined drainage-channel network, larger open areas, and are usually shallow (often less than two metres). A raised ridge, or sand barrier, is characteristic of the edge of lagoons, such as that formed by the Lido, separating the lagoon of Venice from the Adriatic Sea. This feature was formed during the Pleistocene interglacial stage some 80,000 years ago when sea shorelines were six metres 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.

Fiords, a major type of estuary in both the northern and southern temperate latitudes above about 45° latitude, were formed when continental glaciers scoured out river valleys. They usually have a U-shaped cross-sectional form and water depths in the interior part that can exceed 500 metres.

Tectonic estuaries owe their origins to major geological events such as faulting, volcanic eruption and landslides. San Francisco Bay is perhaps the best known estuary formed by tectonic activity.

III

Water and Vegetation Patterns

Estuaries are the areas where most freshwater run-off in the world encounters the oceans. Because freshwater is lighter than salt water there are major density differences and hence a salinity gradient. Unless mixed by the tides or winds, freshwater remains at the surface. On the other hand the tides force seawater inland as a countercurrent and produce a salt-water wedge below the freshwater surface. This means that the gravitational circulation within the estuary is much greater than if only river water were flowing to sea. Abrupt rises in tidal water (tidal bores) occur in estuaries, such as in the Bay of Fundy and at the mouth of the Amazon, when strong tides are forced upwards by shallows before actually reaching shore areas.

Estuaries are very rarely in a static condition. Tides are the principal energy source causing estuarine mixing, but wind, wave motions, and river run-off can also be important locally. Salt water and freshwater mix to form what is generally called brackish water. Due mostly to oscillations in river flow, the three main estuarine zones—salt water, brackish, and freshwater—can shift seasonally and vary greatly between different areas. Also, an area of the estuary can change from stratified to well-mixed during the spring-neap tide cycles. The most highly stratified estuaries are ones that receive a large amount of freshwater but have a relatively low tidal range. The Volga delta is a good example. Partially mixed estuaries have moderate freshwater inflow and tidal range, such as Chesapeake Bay. The brackish zone might have a salinity of 2 to 10 parts per million (ppm), compared with salt water of about 35 ppm. Where there is a large tidal range but little freshwater inflow, such as in the Bristol Channel, mixing will be heavy. Wind is usually a more important mixer than tides in coastal lagoons where there are large open waters, small tidal range, and low freshwater inputs.

Freshwater shows great chemical variability whereas salt water is fairly uniform. River chemistry, then, produces considerable differences in estuarine nutrient cycles. In general the most important compounds for estuarine life supplied by freshwater are silicon, iron, nitrogen, and phosphorus. Seawater provides sulphates and bicarbonate. Estuaries contain some of the most productive habitats on Earth because of the accumulation and availability of nutrients, along with adequate light conditions, that fuel phytoplankton production. Phytoplankton are tiny, single-celled algae that drift in the water. These algae are highly adapted to the nutrient-rich but often rigorous conditions of estuarine waters. A group of algae called diatoms are especially important.

On a worldwide basis estuaries have very diverse vegetation types, which range from small, sparsely distributed grasses to large rainforest trees with a closed canopy. Salt marshlands are intertidal communities of rooted plants that are inundated by the tides. They are dominant in the temperate and boreal regions, and more than 600 plant species have been recorded from these habitats throughout the world. Puccinellia is especially common in Arctic salt marshes; Spartina is abundant along the more temperate coasts. The Mississippi delta is 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, dominate over salt marshes in the tropics, such as those of northern Australia, in Papua (formerly Irian Jaya) in Indonesia, and the Sunderbans in Bangladesh.

Though seldom recognized because so little is yet known about them, tidal rainforests are probably the richest estuarine habitats on Earth. Especially important are the huge tidal rainforests of the Amazon River, which are much more abundant there than mangroves.

Finally, 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 these are usually dominated by the so-called seagrasses, such as eelgrass (Zostera spp.) and turtlegrass (Thalassia testudinum). Seagrasses, however, do not belong to the grass family.

IV

Animal Life

Animal life found in estuaries varies greatly. Tropical estuaries are generally more diverse than temperate latitude counterparts. High productivity and habitat complexity can lead to considerable wildlife diversity in estuaries, and in some cases to huge concentrations of animals as well. All of the main vertebrate groups are represented by species in estuaries. Of mammals, rodents, cetaceans, carnivores and ungulates are usually present. Waterfowl, such as ducks, geese, herons, and egrets, are often found in great numbers. Huge flocks of songbirds, such as red-wing blackbirds in eastern North America, will also visit estuaries in search of seasonally abundant food. Other species use estuaries as stopping-off sites during annual migrations.

Aquatic animals must have physiological or behavioural adaptations to changes in salinity. Many oceanic species, such as some of the sea catfishes, are able to move inwards during the low-water season of rivers when salinity increases in estuaries. Conversely, freshwater fish move further out to sea with the annual floods. Due to stressful conditions in the mixed zones of estuaries, diversity is often reduced despite extremely high productivity of a few fish species.

Small to microscopic animals are less noticeable than vertebrates, but much more diverse and essential to food webs and community structure. Zooplankton feeds on phytoplankton; in turn zooplankton is eaten by fishes and other animals. The rich organic layer on the bottom, which is composed of material deposited from river inflow and the “rain” of dead organisms produced in situ, such as plankton, is the home to a myriad animals, which include barnacles, bryozoans, sponges, mussels, hydroids, and others. In general the diversity of the bottom, or benthic, fauna decreases from salt water to freshwater. Fish, crabs, and shrimp are often highly mobile in their search for food, and thus able to take advantage of the high invertebrate production found in estuaries. This extremely high productivity is what makes the estuary such as attractive place as nursery habitat for many crustacean and fish species, and of course fishing grounds for humans.