THE POSIDONIA OCEANICA MATTE: A UNIQUE COASTAL CARBON SINK FOR CLIMATE CHANGE MITIGATION AND IMPLICATIONS FOR MANAGEMENT

Seagrass meadows have long been recognized for their high ecological and economic value (ecosystem services). More recently, a potential role in climate regulation, due to their ability to fix and sequester carbon, has been the focus of intensive study. In the Mediterranean Sea, the matte, a specific structure built by the seagrass Posidonia oceanica, is of particular interest because it keeps buried for thousands of years massive amounts of carbon. Recent studies carried out along the Corsican coasts show a mean fixation of 1.62 Mg C ha(-1) yr(-1), with a sequestration rate of between 27 and 30 %, a mean matte thickness of 210 cm and 711 Mg C ha(-1) of organic carbon trapped in the matte. That is to say, a stock corresponding to 1,580 years of P. oceanica carbon sequestration, confirmed by radiocarbon analysis. An extrapolation to the Mediterranean basin (1.0 to 1.5 million hectares covered by P. oceanica meadow; mean matte thickness: 210 cm) shows that the total stock of organic carbon sequestered in the P. oceanica matte might be as much as 711 to 1,067 million Mg C. The conservation of the P. oceanica meadows thus constitutes an issue of major importance since any degradation of the matte, which has been built up over the past millennia, would very likely result in the release of considerable quantities of carbon. Rather than playing a major role in the attenuation of the impact of climate change (blue carbon sequestration), the P. oceanica meadow would then become a source of carbon that would be likely to amplify the greenhouse gas emissions. Management of P. oceanica meadows should take into account not only their role in carbon sequestration, but also the whole the full range of their ecosystem services, in relation with the functioning of the ecosystem.