Utilisation de Posidonia oceanica (L.) Delile comme bio-indicateur de la contamination métallique

The coastal zone of the continental shelf contains a high ecological richness. However, this zone is very fragile towards xenobiotics ; the pollution of this environment can be damaging to the structure of the biocenoses and to the productivity of the ecosystems. Metals, contrarily to numerous contaminants, are normal constituents of the environment at a trace level, but are all toxic above a certain threshold. At the end of the 1970s, several researchers proposed the use of organisms to evaluate the quality of the marine environment ; it is in this context that the bio-indicator concept arises. The seagrass Posidonia oceanica (L.) Delile, endemic of the Mediterranean Sea, has been the subject of several works proposing to use this species to determine the metal concentrations of the environment. However, most studies carried out explorative research in order to qualify this species as bio-indicator. The results of this work validate the use of Posidonia oceanica as a reliable tool for the evaluation of metal contamination of coastal ecosystems. Indeed, this work allowed : (i) to better understand the trace metals accumulation processes by Posidonia oceanica (influence of age and growth of foliar tissues in the metal accumulation) ; (ii) to confirm the high sensitivity of this species as a tracer of metal contamination (identification of metal inputs associated to the previous asbestos mine of Canari and evaluation of the extent of the impact of this previous exploitation) ; (iii) to confirm its reliability as bio-indicator of metal contamination (same trends observed for this species and the bio-indicator Mytilus galloprovincialis Lamark, 1819). Therefore, Posidonia oceanica appears as an interesting tool in the frame of setting up biomonitoring networks at the scale of the Mediterranean Sea. A technique, which consists in collecting only the blades of the adult leaves is proposed insofar as it will allow to carry out trace metals analyses without inducing the mortality of the plant (protected species in numerous Mediterranean countries) and conserving its integrity. Preliminary scales to interpret the level of metal contamination in the environment have equally been proposed for cadmium, mercury, nickel and lead (priority substances ; Decision No 2455/2001/EC ; EC, 2001). Finally, this work shows equally the possible use of Posidonia oceanica as a « tracer » of the past mercurial contamination thanks to the lepidochronology technique.