Substitution and phase shift within the Posidonia oceanica seagrass meadows of NW Mediterranean Sea

We investigated the conservation status and ecosystem substitution in six Posidonia oceanica (L.) Delile meadows in 2002 along 300 km of the Ligurian coast (NW Mediterranean Sea). We studied the meadows by scuba transects and we compared the meadows state of health through in situ shoot density counts and computing the Conservation Index (CI), the latter measuring the proportional amount of dead matte relative to live P. oceanica. Both measures evidenced a generalised state of regression for most of the six meadows. We also reported the occurrence, within the meadows, of the other common Mediterranean seagrass Cymodocea nodosa, the Mediterranean green alga Caulerpa prolifera, and the invasive green algae Caulerpa taxifolia and Caulerpa racemosa. We measured the replacement of P. oceanica by these species using the Substitution Index (SI), related to the proportional cover of each substitutes. These were ranked in order of their colonisation potential with respect to P. oceanica. We identified and measured a community ‘‘phase shift’’ occurring within the Mediterranean seagrass meadows using a combined index, the Phase-Shift Index (PSI). The analysis of the three environmental indices (CI, SI and PSI) at regional scale allowed to introduce three ‘‘regional’’ ordinal scales to classify the conservation status, the degree of substitution and the level of phase shift in P. oceanica meadows.