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Zostera marina L. carbon dynamics have been intensively studied, yet in situ translocation rate estimates remain elusive, particularly for north temperate seagrasses. To better understand carbohydrate synthesis, allocation and use in Z. marina, we conducted summer and winter in situ C-13 labeling experiments and measured the delta C-13 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 C-13-dissolved inorganic carbon label, indicating that non-structural carbohydrate was translocated belowground. Rhizome delta C-13 increased for up to 2 wk after the C-13 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 C-13 loss rates (4 d average) were on the order of 11% C-13 d(-1), while the excess C-13 accumulation rate in the rhizome combined with roots was about 1.5% C-13 d(-1) (4 d average). Whole plant excess C-13 loss rates as a result of respiration, detrital production and exudation ranged between about 8.8 and 10% C-13 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.