This dataset gathers biochemical and isotopic ratios measured seasonally in different plant-part of Posidonia oceanica shoots collected in the bay of Marseilles. Values are triplicated measurement of the same samples, but for ashes, that were determined once only. Plant parts were separated based on the classification of Giraud (1979): juvenile (less than 5 cm long, with intact leaf tip), intermediate (more than 5 cm and without basal sheath) and adult (more than 5 cm, with a basal sheath). Adult leaves were subsequently separated between the basal green part without the sheath (photosynthetically active, hereafter adult leaf) and the apical brown section (senescent leaf). In addition, dead P. oceanica leaves drifting away were collected at random close to the meadow, to investigate the ultimate evolution of isotopic and biochemical parameters of the P. oceanica leaves.