Vulnerabilité des ecosystèmes Méditerranéens à la pollution

Abstract

Abstract First the main peculiarities of the Mediterranean Sea from the hydrographic point of view are noted: water budget; cyclonic gyre of Atlantic surface water, more marked in the western basin; high summer temperature in surface waters; isothermal layer below 250–300 m depth; strong northerly winds in the northern areas, etc. The input of pollutants is particularly marked in the vicinity of the larger urban and industrial areas of the northern coasts. Because rivers are being more and more used to provide increasing amounts of hydroelectricity, the spring flood, which formerly pushed away mud and clay towards the lower continental slope and the abyssal plain has decreased or even disappeared; therefore, pollutants adsorbed on mineral particles remain in the estuaries and on the shelf, and the noxious effect of these pollutants more or less inhibits the enrichment which usually arises from the nutrients brought into the sea by the rivers. Moreover, owing to the lack of tides — except in the northern part of the Adriatic Sea and the eastern coast of Tunisia — the dilution and diffusion of sewage and industrial liquid wastes in the seawater are weaker than along shores where there is a large tidal range; hence, the pollution levels are often very high in the shallow-water areas. From the ecological point of view, the Mediterranean Sea ecosystems exhibit a rather high diversity index; moreover, especially on the northern coasts of both the western and the eastern basins, the ecosystems, particularly benthic assemblages, include a mixture of temperate (eurythermal or sometimes slightly cold-loving) and subtropical (warmth-loving) species. Such a heterogeneous composition of the ecosystems results in rather large seasonal changes of many benthic assemblages in the shallow-water areas, where — especially on the northern coasts — the annual temperature range is over 10°C or even more. Many thermophilic species in winter and psychrophilic species in summer are close to their thermal tolerance threshold and therefore become more vulnerable to an additional stress by pollutants. Once damaged, and after the pollution has been abated, the restitution of the Mediterranean benthic ecosystems seems to be slow and incomplete. It is well known that the Mediterranean Sea as a whole has a low level of primary production: offshore areas are highly oligotrophic and productivity is important almost only in some coastal areas, especially near the estuaries. With regard to the plankton, one often observes nearshore a clear juxtaposition of areas occupied by highly eutrophicated and polluted ecosystems, and areas occupied by oligotrophic ecosystems. Therefore, the high level of pollution in the nearshore areas may result, in the future, in increasing impoverishment of Mediterranean marine life. Owing to the fact that no surface water flows out through the Strait of Gibraltar, the amount of hydrocarbons, which tend to concentrate in the uppermost layer, is particularly high in the Mediterranean surface water. Therefore one has to fear that an increase of other light, lipophilic pollutants, such as organochlorines and some surfactants, which are less degradable than hydrocarbons, might result in a progressive adulteration of the pelagic ecosystems, even in offshore areas. Finally, it must be emphasized that Mediterranean ecosystems are highly vulnerable to thermal discharges from coastal power plants, especially in summer.