Nitrogen retention was investigated during 240 d in 1 ϫ 1 m field plots of the tropical seagrass Thalassia hemprichii. Shoots were enriched with 15 N by brief exposure of the leaves to an elevated concentration of 15 N ammonium in the water column. Hereafter, the 15 N absorbed in the seagrass system declined rapidly. The decline was initially dominated by the loss of 15 N in detached leaf fragments. Of the lost leaf fragments, 19% were recovered within the boundaries of the experimental plots, and 25% were deposited outside these boundaries but inside the seagrass meadow. Of the remaining 56%, the fate could not be resolved, but export from the meadow is probably limited to ϳ10%. During the course of time, the 15 N half-life increased from 1 to ϳ2 months because of 15 N accumulation in compartments from which it was not easily exported (roots, rhizomes, and sedimentary detritus). The limited nitrogen retention in the seagrass plots is ascribed to the combined effects of a major allocation of nitrogen to leaf production, restricted nitrogen resorption from senescent leaves (28% of the gross N demand), and a dynamic environment facilitating detachment and export of leaf fragments from the experimental plots. At the scale of the whole meadow, however, nitrogen conservation via the detrital pathway could be of considerable significance. We found indications for a rather efficient reabsorption by the plant of nitrogen regenerated from seagrass leaf litter, with a meaningful role for the leaves, and postulate that increasing patch size may coincide with increasing nitrogen conservation in the system as a whole.Seagrasses are the only descendants of terrestrial angiosperms that have been able to invade the marine environment. The plants form extensive submarine meadows that can be found in oligotrophic and mesotrophic shallow marine waters all over the world (Den Hartog 1970). A comparison among different plant communities shows that the primary production of seagrass meadows ranks among the highest established, being in the range of tropical forest and swamps and marshes (Duarte and Chiscano 1999). The generally high productivity of seagrasses, which is logically paralleled by a high nutrient demand, often in nutrient-poor environments, has attracted attention since the expansion of seagrass research in the early 1970s. Nutrient-limited growth appears to be a quite common phenomenon in seagrass ecology, despite the capacity of seagrasses to exploit the nutrient reservoirs of both the sediment and the water column (Iizumi and