Carbon and nitrogen translocation between seagrass ramets

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 13Carbon (13C) and 15Nitrogen (15N) as tracers in experiments conducted in situ. Seagrass leaf and rhizome production during the study period varied from <0.001 to 0.015 g DW shoot-1 d-1 and 0.002 to 0.017 g DW rhizome apex-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 mg C shoot-1 d-1, and from 0.01 to 0.24 mg N shoot-1 d-1, while the demand for rhizome production varied from 0.62 to 5.57 mg C rhizome apex-1 d-1 and from 0.02 to 0.12 mg N rhizome apex-1 d-1. Seagrass leaves incorporated the isotopes at rates ranging from 0.04 to 0.63 μg 13C g DW-1 h-1, and <0.01 to 0.35 μg 15N g DW-1 h-1. After 4 d, all incubated shoots had shared part of the incorporated 13C and 15N 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 13C and between 0.02 and 178 μg 15N. 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.