ABSTRACT: The seasonal nutrient mass balance of the dominant seagrass of the Mediterranean, Posidonia oceanica (L.) Delile, was evaluated in NE Spain in order to test the hypothesis that the effect of seasonal nutrient imbalance can be reduced bythe reutilization of internal nutrient pools. To this end we investigated the seasonal and age-dependent variability of nitrogen and phosphorus concentration of the leaves, inferring from these data values of seasonal nitrogen and phosphorusincorporation, uptake, losses and retranslocation. Incorporation of nitrogen and phosphorus in leaves peaked in June and was lowest in September, thus following the seasonal growth pattern of the plant. Retranslocation of nitrogen and phosphorus was highfrom May to September and close to zero during the rest of the year. Losses of nitrogen and phosphorus were highest at the end of summer, associated with the major biomass losses. Nitrogen uptake by leaves reached maximum values in winter and was lowestduring August-September, while phosphorus uptake was highest in spring and lowest in August-September. On an annual basis nitrogen and phosphorus uptake accounted for 60 and 41% of the total nutrient incorporation, respectively, while retranslocation ofnutrients from old tissues accounted for the remaining 40 and 59%. Although roots and rhizomes function as sources of nutrients at the beginning of the summer, their contribution to the seasonal nutrient budget seemed to be minor.