The existence of a fully fledged ecological domain has been confirmed; this term being understood in the widest and most multidisciplinay sense which has its own dynamics: the paralic domain. Its principal originality lies in its obvious geographical dispersion at the present time and in the spatial and temporal variability of its abiotic parameters. Paradoxically but logically, these two fundamental characteristics lead to both a deep biological unity and biological stability.
The geographical dispersion of the paralic domain is only apparent, since every coast, even that most open to the sea, has its more or less extensive paralic fringe, if only because there is always an intertidal fringe (Appendix 3). Moreover, there are many transfer agents between the different basins, the sea itself, the living beings it contains, sea birds, etc.
The variability of the abiotic parameters results in incomparably greater hydrochemical gradients than those found in the other domains of the hydrobiosphere, even if one considers endoreic basins, which are often highly homogeneous at a given moment of the seasonal cycle.
The biological unity of the paralic domain appears in a qualitative and quantitative zoning common to the entire sub-domain close to the marine domain (the Near paralic). This zonal organization, which affects the species (mainly thalassoid) depends on the confinement (relative to the sea), an inconspicuous but obligatory parameter in any paralic system. Beyond this, in the Far paralic, the communities very rapidly become richer in freshwater species, or in species characteristic of the evaporitic milieux: confinement becomes too great and appears therefore to lost its leading biological role. However, perhaps later studies may prove the contrary.
In order to be able to compare the spatial and temporal variations of the different biotic and abiotic parameters which, in each basin of the Near paralic or within the sub-domain as or whole, describe the confinement field (and gradient), is a scale proposed based on the communities of benthic invertebra which best assimilate the minor fluctuation of the milieu. The other links of the trophic chain, along with the quantitative variations of the different biological parameters (number of species, biomass, density, production, productivity), can satisfactorily and logically be incorporated in this scale.
The exact way in which confinement affects the distribution of the paralic living beings still remains unclear, but various reasons allow the supposition that confinement results in an impoverishment of the milieu in “vital” elements of essentially marine origin, trace elements, small organic components (vitamins, alkaloids, etc.), around which the communities are organized.
These different considerations lead to a qualification of the paralic domain as a biological cross-roads between the marine and continental domains (evaporitic and freshwater), to abandon the marginal status it was hitherto conceded, and on the contrary to grant it a key position in the history of the biosphere. The paralic domain is moreover a preferential place for the accumulation of biogenic sediments (deltaic series, evaporites) and organic matter on the surface of the Earth, and, in this respect, plays a fundamental part in the history of the lithosphere.
To conclude, the various original characteristic features of the paralic domain - both biological and geological - open up considerable economic possibilities, incomparably superior to those of the Oceans, both in the present and in the fossil realms. In this respect, the study of confinement and its various mineral and biological expressions appears as an effective, reliable, and inexpensive instrument for the development and the survival of the paralic domain.
