LEPIDOCHRONOLOGICAL ANALYSIS IN THE MEDITERRANEAN SEAGRASS POSIDONIA-OCEANICA STATE-OF-THE-ART AND FUTURE-DEVELOPMENTS

After blade shedding in the seagrass Posidonia oceanica, leaf sheaths remain attached to the rhizome and can persist for several decades within a ''matte'' They show, according to their insertion rank, cyclical variations in parameters such as thickness and tissue structure. These cycles are always annual, and may be influenced by environmental parameters such as light, temperature or water movement. The study of cyclic changes in Posidonia oceanica sheaths is thus a technique that can be used to investigate past changes in the near-shore environment. By analogy with dendrochronology, this study is termed lepidochronology. A number of applications of lepidochronological analysis have already been successfully tested : (i) Estimation of the growth rate and annual production of rhizomes; (ii) Modelling of leaf renewal cycles (e.g. rhythm of leaf formation and of leaf fall, leaf longevity); (iii) Detection and dating of previous flowering from the remains of flower stalks inserted between the sheaths (iv) Evaluation of primary production. This new technique is much faster than the classic methods and has the further advantage that primary production for previous years can also be estimated. Twenty-two sites in the Mediterranean sea? between 0.7 and 32.0 m depth have been investigated using this technique. The possibility that chemical events (e.g. heavy metals) may be memorized in Posidonia oceanica sheaths and rhizomes is currently being investigated.