Posidonia oceanica is an iconic and highly productive Mediterranean seagrass. As most studies have focused on the fate of its production, temporal and plant part-specific variations of isotopic composition and biochemical content were overlooked. Combined seasonal and plant-part stable isotope composition and biochemical concentrations were measured at the lower depth limit of a P. oceanica meadow (~ 25 meter depth), and explained on the basis of previous knowledge of the specific metabolic functioning of each part. The predominance of compounds with complex chemical structure was reflected by the high concentrations of insoluble carbohydrates, high C/N ratios and high ä13C values. Plant parts clustered in 3 groups with similar isotopic or biochemical features and metabolism: rhizomes and juvenile leaves, intermediate and adult leaves, senescent and drifting leaves. This result agrees with the vegetative phenology of the plant. The biochemical composition and the isotopic composition of the plant parts were consistent with previous knowledge regarding the photosynthetic activity and its seasonal variation. Correlations were found between N-linked descriptors (ä15N and protein content), and between ä13C and insoluble carbohydrate concentration. Epibiont values differed considerably from those of the leaf, as this community is taxonomically diverse and seasonally variable. Biochemical and isotopic composition measured confirmed that the current complex metabolism of P. oceanica results from adaptations to the specific features of life in a marine oligotrophic environment.