Among the 64 species of marine phanerogams, Posidonia oceanica, a species endemic to the Mediterranean, appears to be the most effective in carbon fixation and storage. The average total carbon fixation of this plant (between its blades, sheaths and rhizomes) per year varies between 33.5 and 426.6 g C.m and the average carbon sequestration (carbon sink), corresponding to the tissues of the sheaths and rhizomes, varies between 7.7 and 84.4 g C.m-2, with a clear tendency to decrease with depth due to decreasing lawn density.
Based on a hundred measurements taken throughout the Mediterranean Sea and at depths ranging from 0.5 to 32 mt, it has been estimated that the total carbon fixation rate by Posidonia oceanica is 1302tCha- 1.yr- 1. This annual carbon fixation represents only 0.61% on average of CO2 emissions for all Mediterranean countries, but in the large Mediterranean islands this fixation averages 3.1% and can reach almost 14.4% for Corsica. Moreover, the main advantage of Posidonia oceanica seagrass lies in its ability to store carbon for centuries or millennia to the point that it can be compared to several terrestrial ecosystems considered efficient in carbon storage, such as peatlands.