The study of intermattes in Posidonia oceanica meadows

Among Mediterranean marine ecosystems, Posidonia oceanica (L.) Delile seagrass meadows form a habitat playing an important ecological and economical role. They form heterogeneous seascapes influenced by the impact of natural and anthropogenic phenomenon (of which the anchoring of leisure ships). These impacts lead to the formation of sand and bare mat patches called ‘intermattes’. By describing and using intermatte characteristics observed in Calvi Bay and more widely around Corsica, these PhD works precisely described intermatte and P. oceanica seascape dynamics in order to develop new methods of analysis. On the one hand, natural sandy intermattes formed by bottom currents show two types of border. Each has its own characteristics at the level of the sediments biogeochemistry. On the other hand, anthropogenic intermattes generated by anchoring (bare mat) present different sizes and shapes resulting from specific dynamics. They are not recolonized when the anchoring pressure remains high. A depletion of the substrate chemical quality occurs after the mechanical destruction. A new cartographic seascape index, the Patchiness Source Index, was created using the size and the nature of intermattes at large spatial scale. After a comparison of methods used for the study of intermattes, two new indices (Map Anchoring Index and Anchoring Index) for managers of coastal areas were developed to evaluate the impact of anchoring within P. oceanica meadows.STARECAPME