New insights for an old topic: seagrasses as bioindicators of coastal trace element pollution

The marine magnoliophyte Posidonia oceanica (L.) Delile has been widely used since the mid-70th to biomonitor the coastal pollution of the Mediterranean in Cr, Ni, Cu, Zn, Cd, Pb and/or Fe. In contrast, other trace elements (TEs) like As, V, Ag, Be, Al, Mn, Co, Se, Mo, Sn, Sb and Bi, many of them categorized as TEs of environmental emerging concern, have been subject to nearly no ecotoxicological survey with that species. It has been shown that the French Mediterranean littoral was submitted to local, diffuse and/or chronic contaminations both by TEs broadly or little biomonitored with P. oceanica; high TE levels could further be linked to specific anthropic activities such as agriculture (Mo), mining (Cr, Sb, Zn), industries (As), storage and refinement of oil products (V, Pb) or presence of major ports and urban centres (Sn, Bi, Ag). It seems therefore necessary to expand the short list of the seven metals commonly monitored to other TEs, what is today easily achievable as current analytical methods allow the simultaneous determination of all a series of TEs within the same sample. Furthermore, only a multielement analysis in appropriate bioindicator species allow to correctly intercompare the pollution status of numerous sampling sites. To do this, we have calculated proper environmental indices, the trace element pollution index TEPI and the trace element spatial variation index TESVI. The TEPI is an index of the global contamination of a site, giving the same weight to each TE after mean normalization of their environmental concentrations. The TESVI estimates the global spatial variability of environmental concentrations of each TE levels, taking into account both punctual contaminations in impacted sites and the overall coastal spatial heterogeneity between all monitored sites.These two indices were successfully applied both at large (French Mediterranean littoral) and small (a Bay) spatial scales. Furthermore, they can be used in the framework of an intercomparative study compiling data from any previous monitoring surveys. We also highlighted that the ecophysiology and surrounding levels of TEs influenced in an equivalent manner the bioaccumulation process of TEs in P. oceanica. Consequently, this natural cyclic evolution of TE concentrations should be systematically quantified in regional reference sites. Finally, the rapid and proportionnal accumulation of TEs in P. oceanica traps huge amounts of contaminants and can stock them for longer periods of time in their bellow grounds tissues. P. oceanica meadows therefore play an efficient role of natural filter of TE coastal pollutions.