Nutrient dynamics of seagrass ecosystems: 15N 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. 15N-labeled phytodetritus was injected into the sediment and the appearance of 15N in the roots, the rhizomes and the leaves of the plants were measured after 1, 2, 4 and 8 d. The transfer of 15N 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 15N 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.