Nutrient dynamics of seagrass ecosystems: N-15 evidence for the importance of particulate organic matter and root systems

The availability of nitrogen in sediment phytodetritus to seagrass was investigated in 5 tropical species (Thalassia hemprichii, Halodule uninervis, H, pinifolia, Halophila ovalis/ovata and Syringodium isoetifolium) from Indonesia. N-15-labeled phytodetritus was injected into the sediment and the appearance of N-15 in the roots, the rhizomes and the leaves of the plants were measured after 1, 2, 4 and 8 d. The transfer of N-15 from sediment phytodetritus to the plant tissues was very rapid (within 1 d) and continued over the 8 d period, indicating an efficient retention of the phytodetritus and a net mineralization of nitrogen rather than an immobilisation by bacteria. Relative enrichment of roots versus leaves indicated that N-15 was taken up by the roots and then transferred to the leaves. The combined efficiency of seagrass canopy-induced trapping and retention of sestonic particles and root-uptake, results in the acquisition of nutrients released upon mineralization of particulate organic matter (POM), giving them a competitive advantage over other primary producers in oligotrophic environments.