Proceedings of the Second Mediterranean Symposium on Marine

fraction <strong>of</strong> <strong>the</strong> decomposition. The existence <strong>of</strong> passive fixation mechanisms on <strong>the</strong> scales
in function <strong>of</strong> <strong>the</strong> time has also been showed for Iron (Pergent-Martini et al., 1993), but
this mechanism seems circumscribed to this element. Finally, a decrease <strong>of</strong> <strong>the</strong> mercury
contents in <strong>the</strong> environment can be possible but does not seem to be confirmed with
regard to what is observed for <strong>the</strong> rhizomes. Indeed, <strong>the</strong>se latter show an increase <strong>of</strong>
mercury concentrations, in all sites, since 1998 (Table 1). Pergent (1987) evokes <strong>the</strong> fact
that <strong>the</strong> growth <strong>of</strong> <strong>the</strong> rhizome, situated at <strong>the</strong> apex, is not achieved and that it can
continue during two-three years, causing consequently an increase <strong>of</strong> its biomass. Pergent-
Martini (1994) shows it exists a negative correlation between <strong>the</strong> quantity <strong>of</strong> mercury
memorized by <strong>the</strong> rhizomes and <strong>the</strong> size <strong>of</strong> <strong>the</strong>se latter. The quantities <strong>of</strong> mercury,
recorded in <strong>the</strong> rhizomes for <strong>the</strong> three last years, may be so overestimated. A correction
factor, based on <strong>the</strong> «<strong>the</strong>oretical» relative contribution <strong>of</strong> scales and rhizomes to <strong>the</strong>
belowground biomass, can be calculated taking into consideration <strong>the</strong> relative contribution
observed in 1997 and 1998, in each sites (Fig. 3). So <strong>the</strong> <strong>the</strong>oretical mercury
concentration obtained shows an important decrease between 2000 and 2001. Indeed,
<strong>the</strong> correction does not integrate <strong>the</strong> fact that if <strong>the</strong> rhizome had continued to grow, it
would have probably continued to accumulate mercury.
Fig 3: Corrected temporal evolution <strong>of</strong> mercury concentrations in <strong>the</strong> rhizomes <strong>of</strong> Posidonia oceanica, in
Bastia, Lumio and Macinaggio.
Although it seems necessary improving <strong>the</strong> correctional factor to apply to <strong>the</strong> more
recent years, <strong>the</strong> results obtained confirm all <strong>the</strong> interest <strong>of</strong> <strong>the</strong> use <strong>of</strong> Posidonia
oceanica to evaluate <strong>the</strong> mercurial contamination <strong>of</strong> <strong>the</strong> coastal environment. Indeed,
because <strong>of</strong> its big sensitivity, it allows establishing a gradient <strong>of</strong> quality between sites little
contaminated and allows moreover to inform on <strong>the</strong> past contamination levels and to
apprehend <strong>the</strong> future kinetics.
REFERENCES
Andral B., Stanisiere J.Y. (1999) - Réseau Intégrateurs Biologiques Rinbio: évaluation de la qualité des
eaux basées sur l’utilisation de stations artificielles de moules en Méditerranée, résultats de la campagne
1998. Rapport détaillé Ifremer, Agence de l’eau RMC: 46 pp + Ann.
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