In situ 13C tracer experiments elucidate carbon translocation rates and allocation patterns in eelgrass Zostera marina

Abstract

Zostera marina L. carbon dynamics have been intensively studied, yet in situ translo- cation rate estimates remain elusive, particularly for north temperate seagrasses. To better under- stand carbohydrate synthesis, allocation and use in Z. marina, we conducted summer and winter in situ 13 C labeling experiments and measured the δ 13 C of bulk tissue and individual carbohydrate compounds. Leaf tissue exhibited immediate isotope enrichment within hours of the tracer pulse. As the isotope ratio of the leaf tissue decreased over a period of days, enrichment became more evident in the belowground tissues (rhizome and roots) not directly exposed to the 13 C-dissolved inorganic carbon label, indicating that non-structural carbohydrate was translocated below- ground. Rhizome δ 13 C increased for up to 2 wk after the 13 C label pulse. The isotopic enrichment of glucose, fructose and sucrose were similar and significantly greater than the enrichment of myo-inositol. Maximum enrichment occurred in the glucose pool, with leaf tissue at 258 ± 61‰ and rhizome tissue at 55.1 ± 28.8‰ during the July 2004 labeling period. Leaf excess 13 C loss rates (4 d average) were on the order of 11% 13 C d −1 , while the excess 13 C accumulation rate in the rhi- zome combined with roots was about 1.5% 13 C d −1 (4 d average). Whole plant excess 13 C loss rates as a result of respiration, detrital production and exudation ranged between about 8.8 and 10% 13 C d −1 (4 d average). The results provide important estimates of carbon uptake and translocation that can be used to better understand whole plant carbon budgets as well as the transfer of sea- grass-derived carbon to other trophic communities.