Coastal lagoons are shallow bodies of water, close to the sea, generally separated from it by a sandy bar that has produced the closure of an ancient marine gulf. They are mostly the result of the accumulation of sands and gravels of continental origin that are dragged to the coast by a river and whose accumulation is also due to the action of persistent currents. A continuous input of materials is necessary for the accumulation to occur, as well as the beach current favoring deposition in shallow areas, gulfs and inlets where there is also no major river current [1]. The balance between the contributions from its basin and its vicinity, the strength of the contributions from the interior and the accumulation by the sea are the three factors necessary for a lagoon’s formation (Figure 1). Their presence is abundant on the coasts of the Mediterranean Sea, in the Baltic Sea and in some areas of the Atlantic (France, Portugal, Morocco, Brazil, Uruguay).

The Mediterranean Sea has its origin in the Tethys Sea, but in a totally different way from the one we know today, with most of the land surface we know today being flooded. About seven million years ago, the movement of the tectonic plates closed the Rifian and Betic corridors (previous to the actual Gibraltar strait), so that the transfer of water was severely affected and what is known as the Miocene saline crisis occurred, with the closure of the Mediterranean Sea [2]. The sea was left as an inland sea isolated from the Atlantic and fed only by the fluvial waters of its rivers. This resulted in a drop in sea level of hundreds of meters due to evaporation and direct communication between Africa and Europe through the southern Iberian and Italian peninsula. About five million years ago, the communication with the Atlantic Ocean was opened again through the current Gibraltar corridor, and once the same water level was recovered in the two seas, the formation of the coastline began, following the approximately present line.

Therefore, the geological origin of the current coastal lagoons is recent, a few thousand years old for the most part, and they are the result of coastal dynamics. Moreover, as with many lakes, they are ephemeral formations as they are created in a few thousand years and also disappear either by siltation of the sediments that reach them or by disappearance of the sandy bar by marine erosion [3]. All the Mediterranean coastal lagoons are from the most recent Holocene period, and other sub-current lagoons are known, such as the Elx lagoon in Spain, located about 15 km from the coast, which is a remnant of the ancient Mediterranean coast prior to the Miocene saline crisis [4].

The Mediterranean coastal zones exemplify the conflict between the human exploitation of water resources and the ecological needs of aquatic ecosystems [5]. The close relationship of lagoons with terrestrial ecosystem boundaries makes these environments very vulnerable to hydrological modifications (freshwater diversions or drainage discharges), water pollution and habitat loss ^[6][7][8][9][6,7,8,9], which deeply change the structure of the lagoons’ ecological dynamics [10].

Coastal lagoons are a habitat declared as a priority by the European Union in the Habitats Directive, i.e., they are threatened with extinction [11]. This directive includes as coastal lagoons, in addition to those described as a typical lagoon formation, coastal evaporation salt marshes, which are also frequent on the shores of the Mediterranean. However, a hydrological distinction must be made between a coastal lagoon open to the sea and an estuary, considered as a tidal floodplain. This means that, for the purposes of the Directive, the classification of similar formations as coastal lagoons or estuaries will depend on the strength of the fluvial inputs that flow into the body of water and the influence of the tide on water renewal. This detail means that lagoons are very common in the Mediterranean due to weak tides and rivers with few contributions, while in the Atlantic the tides are higher and the rivers tend to have more contributions [12].

Therefore, coastal lagoons are bodies of water with scarce renewal due to low freshwater inputs and reduced communication with the sea. Their salinity is usually lower than the main sea when freshwater inputs are high and their volume is low; but this can change abruptly due to the influence of marine storms that overtake the dune bar and flood a lagoon with seawater. However, they can also be hypersaline when there is no freshwater input and the entry of seawater increases salinity by evaporation. In this sense, it can be said that they will have a positive estuarine circulation in the first case (fresh water flows out) or a negative estuary (marine water enters); but let us not forget that they are not estuaries in any case (Figure 2) [13].