Le confinement, paramètre essentiel de la dynamique biologique du domaine paralique

Abstract

Recent studies on present lagoonal faunas show that, contrary to accepted opinions, salinity is not the main ecological parameter in paralic environments. Location of species and quantitative biological gradients (biomass, density, ...) in paralic populations essentially depend on an original parameter which could be expressed, in each point, as the lapse of time necessary to renew the marine elements at that point. It is called confinement (with regard to the open sea), on the analogy of the usual meaning of this word. In any given point of a paralic basin, the geochemistry (salinity, chemical composition) depends not only on confinement but also on the water balance of this basin : positive or negative it would induce lower or higher salinities than sea-water. Hence, the biological zonation of paralic middles do not fit the geocnemical and sedimentological zonations. The existence of strictly paralic species, the great stability of lagoonal populations due to the wide changeableness of abiotic parameters and a common biological dynamism contrast with the geochemical diversity of the paralic middles. Hence, they should be no longer considered as dependencies of the marine or continental realms neither as intermediate middles : they form together a genuine paralic realm apart from both traditionnal realms of hydrobiosphere. This is reinforced by the sedimentological features of paralic areas : high sedimentation rates, importance of biogenic phases. Such a point of view beeing accepted, the classifications of paralic middles according to salinities must be abandonned, the ecological importance of osmoregulation must be reconsidered, the biological indicators of paleosalinities must be suspected. On the whole, the role played by the paralic realm in the biosphere and lithosphere history seems more important than usually assessed. At least, the confinement could be a guide for the economical exploitation of paralic middles.