The coastline

The coastline, a space with many definitions

The coastline is the place where the land and sea meet and it is not easy to define. In Ecology it is described as a “ecotone”, that is a transitional zone between the terrestrial and marine ecosystems, while also highlighting the diversity of the fauna and flora that is found there, and the fragility of these biological riches. In Geography, the coastline corresponds to the zone of the sea’s influence on the land, but also that of the land on the sea... In Law, the coastline does not have an enshrined definition but it is a space that is interesting in legal terms, and all rights apply to it.

The concept of the coastline can then take on many definitions, depending on the views and approaches that we bring to it. In practice, its physical boundaries remain very tricky to judge and probably cannot be grasped, to the degree to which the coastline is a space of multiple dimensions.

The best way to get to know the coastline (and to find a way to define it), is to take the time to observe it. A fragile space, that is constantly evolving, the coastline charms, and reveals all its poetry to the most attentive observers. Before you head off into these striking terrestrial and underwater landscapes, you can discover some useful information here to help you to organize your sortie.

How biologists divide the coastline

Biologists define several successive stages (or zones) along the coastline, due to the varied living conditions that determine the distribution of living organisms along its length. Whatever the morphology and nature of the coast, it can be divided theoretically into 5 coastline stages, even though it is often difficult to discern them in the field.

Moving from the land towards the sea, we encounter: 

  • The halophile stage, which is composed entirely of mainland, but which is influenced by the sprays composed of currents of air charged with droplets of salt water, which therefore carry “salts” and “humidity”. This stage is occupied by flowering plants that are adapted to these conditions. The presence of the wind, which is generally important on the coastline, can shape the vegetation’s hold, as it leans according to the direction of the wind,
  • The adlittoral stage corresponds to the last stage of the mainland, or to the most extreme conditions (with the greatest variations in physicochemical parameters), which only allows a few species, such as lichens or salicorn [salicornia] to gain a foothold.
  • The supralittoral stage is the first marine stage. It is never submerged, even at high tide, but is always subject to high levels of dampening by the sea spray. The importance of this stage is highly dependent on the strength of the waves: the more frequent and stronger the waves are, the more this stage extends. You can find lichens here, but also cyanobacteria that are grazed on by small marine mollusks called “winkles [littorina]”, plus crustaceans, etc.
  • The mediolittoral stage is submerged at each high tide and exposed during low tides. This stage is submerged depending on the waves and tides. It is inhabited by the more adaptive organisms, which can tolerate long periods of immersion and dryness. We can refer to resistant species, which have a strong capacity to adapt, but also to competitive species. In general, the boundaries of this stage depend on physicochemical factors, which at the lower limit are constrained by competition for space between species,
  • Then, there is the infralittoral stage, which remains totally submerged, even at low tide. The lower limit of this stage corresponds to the light’s penetration limit, and to species’ ability to realize photosynthesis (between 30 and 40 meters’ depth),
  • Beyond that is the circalittoral stage.


Within the framework of BioLit, we are essentially interested in the supralittoral, mediolittoral and infralittoral stages.