por A estuary It is a geomorphological accident that forms an ecosystem located at the mouth of a great river to the sea. It is identified by the existence of a single outlet to the sea with a strong influence of the tides that prevent sedimentation.
What defines the estuary is the dynamics established between the fresh water of the river and the salty water of the sea. The resulting ecosystem gathers characteristics of both riparian (river) and marine ecosystems.
Estuaries are found on all coasts of the world where a large river flows under strong tides. It is considered a biome of great importance, due to the diversity of ecosystems it generates.
In those estuaries located in tropical and subtropical zones, mangrove swamps and underwater meadows develop. The fauna is home to a wide variety of fish, aquatic turtles, crustaceans, mollusks and marine mammals, such as river dolphins.
The climate of the estuaries is variable, depending on the latitude where they are located, and can occur in a tropical, temperate or cold climate. However, due to its coastal condition, its climate is tempered by the influence of the ocean mass.
Productivity in the estuary is high due to the different aquatic environments it hosts and the nutrient load provided by the river. In them are some of the most important fishing areas.
Some examples of estuaries are the La Plata river between Argentina and Uruguay and the Guadalquivir estuary in Spain.
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estuary features
The estuary is a transition zone between a river of great flow and depth and the sea, where the tides determine the hydrological dynamics. The clear, open shape, with a single mouth channel that characterizes the estuary, is due to the tides.
In this area, the tides are strong and when they ascend they retain the water of the river, letting it out suddenly when they withdraw, thus preventing sedimentation. This differentiates it from the deltas where the river accumulates sediments that form the characteristic channels with multiple mouths.
The tides
The tide is the movement of rise and fall experienced by marine waters due to the gravitational attraction of the Sun and the Moon. This flow (rise) and ebb (fall) of marine waters occurs every 6 hours, that is, twice a day.
high tide and low tide
High tide is called the maximum point of height that the tide reaches in a given area, as well as the time that this phenomenon lasts. While the low tide is the lowest point of the sea level reached in the ebb of the tide.
Dynamics in the estuary
During the high tide, the rise of the marine waters towards the continent exerts pressure against the waters of the river that advance to flow. This force of the marine waters retains the waters of the river, which is why the channel at the mouth widens.
In the same way, it causes overflow, forming marshes (saltwater swamps). Then when the sea waters recede at low tide, the river waters forcefully advance towards the sea.
The flow that the river reaches, the force of the tide as well as the direction and speed of the winds also affect this dynamic. For example, if the river flow is low, the action of the tide dominates and the salinity of the water will be maximum.
salinity gradient
In the estuary there is a dynamic salinity gradient, with areas where there is a greater amount of fresh water and others where salt water dominates. This is the product of the interaction of fresh water from the river with seawater.
Zoning and stratification
The salinity gradient occurs both horizontally and vertically. Therefore, there is fresh water towards the entrance of the river and salt water in the sea, with an intermediate estuarine zone of medium salinity.
On the other hand, given the difference in density between fresh and salt water, differentiated layers are present. As fresh water is less dense, it tends to occupy the upper layer while salt water forms the lower layer of the estuary.
These layers move in opposite directions, where fresh water heads to the sea and salt water tends to penetrate inland.
Productivity
Due to the contribution of nutrients carried by the river, the estuaries are very productive, attracting a large number of marine, river and terrestrial species. In some estuaries the contribution of fresh water with nutrients is considerable, such as that of the La Plata river, where the Paraná and Uruguay rivers contribute 20,000 m3/sec
types of estuaries
Given the complexity of marine and fluvial factors that act, there are various types of estuaries.
By the amplitude of its tides
We speak of microtidal estuaries when the height difference between high tide and low tide is less than 2 m. While in the mesomareales the difference is from 2 to 4 m and in the macrotidals between 4 and 6 m.
Likewise, there are hypertidal estuaries where the difference between high tide and low tide is greater than 6m.
Due to its topography (landform)
Taking into account the relief of the coast in the estuary, the estuaries of coastal plains, fjords and estuaries with barriers are presented. The former are formed by flooding the valley where the mouth occurs.
These coastal plain estuaries are funnel-shaped towards the sea and no deeper than 30 m, except towards the mouth. If the coastal valley is flooded by the sea instead of the river, an estuary is formed.
Fjords occur in regions that were covered with ice in the Pleistocene. The weight of these masses of ice excavated pre-existing valleys making them narrow, deep and rectangular in shape, with almost vertical walls.
For their part, barrier estuaries are similar to those of the coastal plain, but sedimentation has formed a transversal barrier at the mouth.
tectonic estuaries
There are also estuaries caused by tectonic movements, for example uplifts. As for example the estuary that constitutes the Bay of San Francisco in the United States.
Location
Estuaries are located at the mouths of large rivers on the coasts of all continents.
Flora
Since the estuary is an ecosystem defined by geomorphological and hydrological characteristics, present at different latitudes, the flora varies greatly from one to another. In all cases, marsh and halophyte plants are frequent.
On the other hand, estuaries are home to various ecosystems associated with their own species, such as mangroves, seagrass meadows, tidal mudflats, and salt marshes. For example, grasslands of spartinacombined with different species of algae.
Mangroves and seagrass beds
In those estuaries located in tropical and subtropical zones, whose waters do not exceed 20 ºC, the mangrove ecosystem develops on its coasts. In the same way you can find submerged meadows of aquatic herbs such as Thalassia and seagrass.
Fauna
As with the flora, the fauna varies depending on where the estuary occurs, be it tropical, temperate or cold latitude. A prominent animal in many tropical and subtropical estuaries is the river dolphin, of which there are four genera (superfamily Platanistoidea).
Birds
An abundant group in estuary areas are birds, with various species of associated seabirds. Common among these are the alcatraz (morus bassanus) and the seagull (family Laridae).
In general, the most abundant aquatic species are those euryhalines, that is, those that support wide variations in salinity. This is natural if we consider the differences in this factor along the estuary.
On the other hand, there are abundant species of fish, turtles, crustaceans and molluscs, both river and marine.
breeding area
For some marine species, the estuary constitutes an area for the development of their larvae that go up the river and return to the sea as adults. Such is the case of the Atlantic tarpon (Brevoortia tyrannus), a fish of the herring group.
Climate
Estuaries can originate in any climate, whether in tropical, temperate or cold regions, depending on the latitude at which they are located. But, due to its coastal character, its climate is tempered by the influence of the oceanic masses.
In such a way that even in a cold zone, its climate will be less extreme than the characteristic climate of the interior.
Examples of estuaries in the world
– La Plata river estuary (Argentina and Uruguay)
This estuary arises at the joint mouth of the Paraná and Uruguay rivers in the Atlantic, between Argentina and Uruguay. It is one of the largest in the world, with a width of 40 km in its internal zone and 200 km in the external zone.
On its coasts there are marshes populated with species of spartina and other plants that support salinity (halophytes).
Climate
It has a temperate climate tempered by the incidence of the oceanic mass of the western Atlantic.
Fauna
It is characteristic of its waters the river dolphin called Franciscana or delfin del Plata (Pontoporia blainvillei) and the tonina is also located (Tursiops gephyreus). On the other hand, there are various species of aquatic turtles such as the seven-keeled turtle (Dermochelys coriacea) and the loggerhead turtle (caretta caretta).
Among the fish, 72 species are recognized, including the anchovy (engraulis anchovy) and the white croaker (Micropogonia furnieri).
– Guadalquivir estuary (Spain)
It forms at the mouth of the Guadalquivir river in the Province of Andalusia, Spain, near the city of Seville. In ancient times the estuary opened into a lagoon or gulf (Gulf of Tartessian or lacus ligustinus), which sedimented.
The so-called Guadalquivir marshes, part of the Doñana reserve, were established on these swampy sediments.
In this ecosystem, halophytic herbs dominate, such as wire grass (spartina spp.). On its margins there are also trees such as the holm oak (Quercus rotundifolia) and white poplar (populus alba).
Climate
The Guadalquivir estuary area is subject to a Mediterranean oceanic climate, with hot summers and mild winters.
Fauna
Fish such as eels abound (eel eel), the jarabugo (Anaecypris hispanica), the fang (malarial cobitis) and common trout (psalm trutta). While in the surrounding properties is the Iberian lynx (lynx pardinus), the roe deer (Capreolus capreolus) and the wild boar (sus scrofa).
In the same way, birds such as the Iberian imperial eagle (aquila adalberti), the white-headed duck (Oxyura leucocephala) and flamenco (Phoenicopterus roseus).
References
Bosch, E. (1987). The estuarine ecosystem of the Rio de La Plata (ARGENTINA AND Uruguay). Annals of the Institute of Marine Sciences and Limnology.
Calow P (Ed.) (1998). The encyclopedia of ecology and environmental management.
Cole, S. (1998). The Emergence of Treatment Wetlands. Environmental Science & Technology.
RAMSAR Convention (Seen on September 21, 2019). ramsar.org/es
Cowardin, LM, Carter, V., Golet, FC & LaRoe, ET (1979). Classification wetlands and deepwater habitats of the United States.
Malvárez AI and Bó RF (2004). Documents of the course-workshop «Ecological bases for the classification and inventory of wetlands…