The seasonal variability of specific growth rate and the carbon stable isotope ratio (delta C-13) of leaf blades (delta C-13(leaf)) of a temperate seagrass, Zostera marina (within 10 days old) were measured simultaneously, together with the delta C-13 of dissolved inorganic carbon (delta C-13(DIC)) at three sites in the semi-closed Akkeshi estuary system, northeastern Japan, in June, September, and November 2004. The delta C-13(leaf) ranged from -16.2 to -63 parts per thousand, and decreased from summer to winter. The simultaneous measurement of the delta C-13(leaf), growth rate, and morphological parameters (mean leaf length and width, mean number of leaves per shoot, and sheath length) of the seagrass and delta C-13(DIC) in the surrounding water allowed us to compare directly the delta C-13(leaf) and specific growth rate of seagrass. The difference in the delta C-13 of seagrass leaves relative to the source DIC (Delta delta C-13(leaf) - DIC) was the least negative (-11 to -7 parts per thousand) in June at all three sites and became more negative (-17 to -8 parts per thousand) as the specific growth rate decreased. This positive correlation between Delta delta C-13(leaf) (-) (DIC) and specific growth rate can be used to diagnose the growth of seagrasses. Delta delta C-13(leaf) (-) (DIC) changed by -1.7 +/- 0.2 parts per thousand. when the leaf specific growth rate decreased by 1% d(-1). (C) 2008 Elsevier B.V. All rights reserved.