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The uptake of nitrate and ammonium by the roots and leaves of Posidonia oceanica were determined between February 1997 and June 1999 by in situ experiments using the isotope 15 of nitrogen ( 15 N) as a tracer in a nutrient-poor coastal zone of the NW Mediterranean Sea (Revellata Bay, Corsica). Nitrate and ammonium leaf uptakes are recorded at 0.05 and 0.1 µM respectively. The high variability observed cannot be explained solely by the variation of the substrate concentrations in the water column. For leaves, mean specific uptake rates were 47 ± 45 and 43 ± 64 µg N g N -1 h -1 . Nitrate and ammonium leaf uptake fluxes (g N m -2 yr -1 ) seem to have the same importance on an annual basis. Nitrate uptake occurs mainly in winter and early spring, when nitrate concentrations in the water column are highest. The uptake of N, and mainly of ammonium, is significant throughout the year with maxima at the beginning of spring, but it is insufficient to meet the annual N require- ment of the plant. Posidonia root biomass was very high and corresponded to high specific N uptake rates by the roots. Ammonium was incorporated by the roots 6 times faster than nitrate. In the sedi- ment, this uptake capacity is limited by the nutrient diffusion rate, and the root uptake is therefore insufficient to meet the N requirements of the plant. In fact, P. oceanica of Revellata Bay have a com- plex N budget involving uptake and recycling processes and allowing the plants to meet their N requirements in one of the most nutrient-poor areas of the NW Mediterranean Sea. We calculated that leaf and root would contribute to 40 and 60% of the annual N uptake, respectively, and 60% of the annual N requirement of the plant.

Annual nitrogen budget of the seagrass Posidonia oceanica as determined by in situ uptake experiments