13C and 15N translocation within and among shoots in two Posidonia species from Western Australia

Translocation of 13C and 15N was investigated at the spatial scales of within-shoot (i.e. the seagrass clonal unit including leaves and associated vertical rhizome) and among-shoots in a mixed meadow of Posidonia sinuosa and P. australis. Incubation with 13C and 15N was conducted in either the oldest leaf of a shoot (i.e. within-shoot scale) or in the first shoot on the 4th or 5th branch of the main axis (i.e. among-shoots scale) and collected several times within a 1 mo period. We tested the following hypotheses: (1) developmental features in P. australis such as thicker and more open vascular system, higher primary production but lower leaf lifespan cause higher translocation in this species than in P. sinuosa, (2) translocation of 15N and 13C are largely influenced by source–sink organ relationships resulting in higher partitioning of C to rhizomes, whereas N is preferentially moved away to leaves, and (3) 15N and 13C transport towards the apical region is more dominant in P. australis than in P. sinuosa. As predicted, higher isotope content was found at both spatial scales in P. australis but differences were related to enhanced incorporation during incubation in this species. When both spatial scales were compared, both species showed higher 15N translocation to young leaves within the same shoot, whereas in the among-shoots experiment most of the material remained within the leaves of the incubated shoot. In contrast, translocation of 13C occurred mainly to rhizomes and tended to be higher at the among-shoots scale, particularly in P. sinuosa. No directionality was detected for either P. australis or P. sinuosa, possibly as a result of the low rates of N translocation at the among-shoots scale and the morphology of the vascular system allowing the integration of neighbouring plant parts for C requirements. Unlike for Western Australian species, the available literature on P. oceanica indicates patterns of among-shoots N distribution that are similar to those of C, which suggests that species are adapted to distinctive ambients.