MEPS prepress abstract  -  doi: 10.3354/meps07405

ABSTRACT: 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 sitting on an intermediate branch of the main axis (i.e. among-shoots scale) and collected at several times within a one month period. We tested the following hypotheses: (i) developmental features in P. australis such as thicker and more open vascular system, higher primary production but lower leaf life-span will cause higher translocation in this species compared to P. sinuosa (ii) translocation of ¹⁵N and ¹³C will be largely influenced by source–sink organ relationships resulting in higher partitioning of C to rhizomes whereas N will be preferentially moved away to leaves; and (iii) ¹⁵N and ¹³C transport towards the apical region will be 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 neighboring plant parts for C requirements. Compared to Western Australian species, available information of P. oceanica from the literature indicates patterns of among-shoots N distribution that are similar to those of C which suggest that species are adapted to distinctive ambients.