The estran is the part of the Littoral located between the known levels of highest and of lowest the seas. One also uses to indicate it the term “zone of Marnage” or the Anglicism “zone intertidal” (of English tidal meaning “relating to the tide”); in administrative and legal terms, one employs also the expression " zone of swinging of the marées".
The estran thus (at least partly) is covered at the time of the full seas and is discovered at the time of the low tides. The duration of exondation (important for the installation of the pledged organizations to this biotope) of the various parts of the estran depends on their sites compared to the mean level on the sea and the number (two on the Atlantic coasts of France but one in certain areas of the sphere) of the tides per day.
Precise details
The generic terms of estran characterize very different geomorphological facies which result in the installation of vegetable and animal populations having only few common points between them. If we consider the coasts bordering of the seas presenting of the important tides, it is easy to observe three principal systems:- the rock coasts
- the Beaches which they are of sand or rollers corresponding to zones of accumulation (sometimes of erosion) of sediments
- the zones estuariennes where there are more or less important terrigenous contributions settling in the form of muds. In the tropical and equatorial areas where the rivers have flows out of the commun run, with considerable terrigenous contributions, these zones give rise to the Mangroves.
Vertical structuring
This structuring in “stages” is the consequence of the more or less long periods of exondation of the estran according to the positioning of the studied place. Several factors intervene:- the Moisture: this one can come from sea water or the rain
- the Température: a sudden estran of great temperature variations when it is exondé (freezing in winter, strong temperatures in summer); on the other hand the temperature variations are weak when it is immersed
- the Lumière: luminous flows are rather quickly stopped by the layers of water.
The pledged organizations with the estrans play a part in their structuring; so we will have very different facies according to the substrate.
- a hard substrate (rock coasts) will allow the installation of algae which could be used as food with certain animals. The irregularities of this same substrate as well as the algae present their will provide also shelters of choice.
- a soft substrate will allow only very with difficulty the fixing of algae or plants; the animals will be rarer and generally in endogée position.
Lastly, the exposure of the estran to the action of the sea will not be alleviating. An open coast receiving the swell the broad one without this one not meeting an obstacle will not have the same structure as a sheltered coast.
On a rock coast four large stages can be defined:
- the stage supra littoral or zone of the spray, zone located at the top of the mean level of the open seas of spring tide, which is splashed easily by the spray, therefore washed rather violently by salt water, and, which is immersed only seldom at the time of the spring tides of spring tide (equinoctial tides) or at the time of certain storms. This stage makes the transition with the terrestrial fringe.
- the stage médiolittoral or normal zone of swinging of the tides where there is alternation of immersion and emergence; this stage is delimited upwards by the mean level of the open seas of spring tide and downwards by the mean level of the low tide of spring tide. It is characterized by the presence of the brown algas of the family of the fucales.
- the stage infralittoral emerged at the time of the tides of spring tide. It was defined formerly as the coastal area of low level where there was enough light so that the algae can develop; it went down according to the coast and from the sea to -10,-30 m, with an emerged part and another immersed. It is the stage characterized by the presence of laminar (brown algas) and that of the red algas.
- the stage sublittoral , current scientific denomination, corresponding to the part of the stage infralittoral which is never emerged. It is not an unknown zone of the fisherman to foot because the pledged flora and fauna on this floor can be observed (phenomenon of the increase of species) in the basins often present on the rock coasts.
On a sedimentary estran the vertical structuring of the beaches is much less clear. However, if the beach (which is sandy or of rollers) goes until the terrestrial fringe it is possible to distinguish the limit between the médiolittoral and the supralittoral because this last stage generally consists of dune systems. If these beaches end in a cliff or a rock unit, one finds oneself in the situation of the rock estrans, and supralittoral stages it corresponds to the zones covered with lichens. The limit between the médiolittoral and the infralittoral is much more delicate to observe. In general the sediments of the infralittoral much finer and are soaked much with water. However, there exist often blocks in place and those, by the algae which recover them, make it possible to be located.
In the estuaries all depends on the quantity of terrigenous contributions poured by the rivers and on flows of the latter. The supralittoral corresponds to the Schorre ; they are benches of more or less sandy compacted vases which are immersed the only with strongest tides. These benches are covered with vascular plants and the man often recovered them for the breeding of the animals known as of salted meadows; it should not be forgotten that these meadows are covered with sea water to each great equinoctial tide. The salt meadow is more or less cut out by channels of which the basal part is formed of more or less fluid vases according to sands which are mixed there; it is the Slikke which occupies all the médiolittoral on each side of the river when the river which ends in the sea has a low flow, the infralittoral corresponding then to a less muddy medium. If the rivère has a strong flow, the slikke banks of the river occupies medio- and infra-littoral. As the slikke is an unstable medium (with each tide the surface layer of the unit is more or less altered) the observer observes only one naked mud.
Ecology
The estran alternatively being covered by the sea and being exposed to the air, it is favourable with a specific ecosystem, adapted at the same time to the maritime and air conditions, able to resist the waves and the tide. Typical fauna includes sea anemones, Coquillage S (mould S, bernic, etc), starfishes, Crabe S, etc the flora includes/understands species of algae which are distributed on the estran according to their lifestyle and of the nature of the substrate. In tropical medium, the Marais with mangroves can occupy the zone of the estran.
Cartography
The level corresponding to the low limit of the estran (Leaves basic sea) is generally used as altitude of reference for the marine charts (or Zero hydrographic), unlike the terrestrial charts which use the Sea level as reference. On the other hand, all the cartographic organizations do not consider the same low part: in France for example, the altitude taken into account is the level of the low tides (coefficient of tide of 120), while certain British charts refer on the level of the average low tides of spring tides (coefficient of tide of 95).
Legal considerations
Just like the dry part of the littoral, the property and the use of the estran can give place to legal and political controversies:- In New Zealand, several groups maori S asserted their rights on the estran (and the ocean floor) on historical bases. The Foreshore and Seabed Act , which was voted in 2004, specifically establishes the state-owned property on these zones. The controversy always currently continues (and).
- With the the United States, for the private beaches, certain States as the Massachusetts use the bottom of the estran to separate the private property from the state-owned property. Other State as the California take into account the top of the estran.
- In France, the estran belongs to the field of the State (maritime Public domain, DPM), on the same basis since 1963 as the bottom and the basement of the territorial Sea. It can be conceded (temporary authorization of occupation, concession of use) for private uses (Conchyliculture, damping, cables…), but for limited times; these concessions do not involve a transfer of property of the estran.
See too
Zh-min-nan: Hái-phiâⁿ
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