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

Abstract

Nitrogen retention was investigated during 240 d in 1 × 1 m field plots of the tropical seagrass Thalassia hemprichii. Shoots were enriched with 15N by brief exposure of the leaves to an elevated concentration of 15N ammonium in the water column. Hereafter, the 15N absorbed in the seagrass system declined rapidly. The decline was initially dominated by the loss of 15N 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 ∼10%. During the course of time, the 15N half-life increased from 1 to ∼2 months because of 15N 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.