Abstract.Seasonal changes in nitrogen, free amino acids, and carbon were investigated in the three Mediterranean seagrassesPosidonia oceanica, Cymodocea nodosa, andZostera noltii.Leaves, rhizomes, roots, as well as dead plant material were analysed separately. Highest N‐concentrations were obtained in the winter months, regardless of species or plant part. In contrast to the other two species, the N‐content inPosidoniawas higher in the rhizomes than in the leaves.In the investigated species, marked differences in the free amino acid (FAA) composition were detected between species: Proline, lacking inPosidonia, was the main component inCymodocea.InPosidonia, FAA decreased from 320umol g‐1(dry wt) in leaf sheaths to 1.5 umol g‐1(dry wt) in the leaf tips. The function of proline as a possible storage and/or stress metabolite is discussed.High C/N values in deadP. oceanicaandC. nodosarhizomes as well as inP. oceanicawrack seem to be related to considerable resistance to decomposition. Low C/N ratios did not increase in detachedC. nodosaandZ. noltiileaves, which began to decompose shortly after detachment from the