Nitrogen (N-15) retention in small Thalassia hemprichii seagrass plots in an offshore meadow in South Sulawesi, Indonesia

Nitrogen retention was investigated during 240 d in 1 x 1 m field plots of the tropical seagrass Thalassia hemprichii. Shoots were enriched with N-15 by brief exposure of the leaves to an elevated concentration of N-15 ammoni um in the water column. Hereafter, the N-15 absorbed in the seagrass system declined rapidly. The decline was initially dominated by the loss of N-15 in detached leaf fragments. Of the lost leaf fragments, 19% were recovered within the boundaries of the experimental plots, and 25% were deposited out side these boundaries but inside the seagrass meadow. Of the remaining 56%, the fate could not be resolved, but export from the meadow is probably lim ited to similar to 10%. During the course of time, the N-15 half-life incre ased from 1 to similar to2 months because of N-15 accumulation in compartme nts from which it was not easily exported (roots, rhizomes, and sedimentary detritus). The limited nitrogen retention in the seagrass plots is ascribe d to the combined effects of a major allocation of nitrogen to leaf product ion, 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 mead ow, however, nitrogen conservation via the detrital pathway could be of con siderable significance. We found indications for a rather efficient reabsor ption 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 .