Abstract
The spatial scale and the magnitude of carbon and nitrogen translocation was examined in 5 tropical (Cymodocea serrulata, Halophila stipulacea, Halodule uninervis, Thalassodendron ciliatum, Thalassia hemprichii) and 3 temperate (Cymodocea nodosa, Posidonia oceanica, Zostera noltii) seagrass species using Carbon-13 (C-13) and Nitrogen-15 (N-15) as tracers in experiments conducted in situ. Seagrass leaf and rhizome production during the study period varied from <0.001 to 0.015 gDW shoot-1 d-1 and 0.002 to 0.017 gDW rhizomeapex-1 d-1, respectively. Based on measured leaf and rhizome growth rates, the demand of resources for leaf production varied from 0.19 to 4.99 mgC shoot-1 d-1, and from 0.01 to 0.24 mgN shoot-1 d-1, while the demand for rhizome production varied from 0.62 to 5.57 mgC rhizome apex-1 d-1 and from 0.02 to 0.12 mgN rhizomeapex-1 d-1. Seagrass leaves incorporated the isotopes at rates ranging from 0.04 to 0.63 µg C-13 gDW-1 h-1, and <0.01 to 0.35 µg N-15 g DW-1 h-1. After 4 d, all incubated shoots had shared part of the incorporated C-13 and N-15 with ramets placed at maximum distances ranging from 2.7 (H. stipulacea) to 81 cm (C. nodosa), indicating that seagrass clonal integration may be maintained between 1.6 d (H. stipulacea) and 5.4 yr (P. oceanica). Resource translocation within seagrass clones was stimulated towards horizontal rhizome apices. Seagrass ramets, in 4 d, shared with their neighbours between 0.37 and 390 µg C-13 and between 0.02 and 178 µg N-15. During the study period, resource translocation would supply <5% and up to 40% of the leaf carbon and nitrogen
required by a neighbouring developing ramet, respectively, and <5% and up to 36% of the carbon and nitrogen required for rhizome growth; provided that the incorporated resources over 1 d were mobilised at similar rates over 4 d. These results conclusively demonstrate physiological integration between seagrass ramets, and that resource translocation may be an important mechanism for young seagrass ramets to acquire resources and for seagrass clones to expand and persist. | 14 pages, 6 figures, 5 tables. | This study was funded by the grant no.
IC18-CT96-0065 of the Commission of the European Communities within the framework of the INCO Programme. N.M. was supported by a post-doctoral grant from the Spanish Ministry of Education, M.A.M. by a post-doctoral grant from the European Commission (MAS2-ct-94-5022), and J.T. and E.G. by return contracts of the Spanish Ministry of Education. This is the contribution number
2852 of the NIOO Centre for Marine and Coastal Ecology Yerseke, The Netherlands.