Specific growth rate as a determinant of the carbon isotope composition of the temperate seagrass Zostera marina

The seasonal variability of specific growth rate and the carbon stable isotope ratio (δ13C) of leaf blades (δ13Cleaf) of a temperate seagrass, Zostera marina (within 10 days old) were measured simultaneously, together with the δ13C of dissolved inorganic carbon (δ13CDIC) at three sites in the semi-closed Akkeshi estuary system, northeastern Japan, in June, September, and November 2004. The δ13Cleaf ranged from −16.2 to −6.3‰ and decreased from summer to winter. The simultaneous measurement of the δ13Cleaf, growth rate, and morphological parameters (mean leaf length and width, mean number of leaves per shoot, and sheath length) of the seagrass and δ13CDIC in the surrounding water allowed us to compare directly the δ13Cleaf and specific growth rate of seagrass. The difference in the δ13C of seagrass leaves relative to the source DIC (Δδ13Cleaf − DIC) was the least negative (−11 to −7‰) in June at all three sites and became more negative (−17 to −8‰) as the specific growth rate decreased. This positive correlation between Δδ13Cleaf − DIC and specific growth rate can be used to diagnose the growth of seagrasses. Δδ13Cleaf − DIC changed by −1.7 ± 0.2‰ when the leaf specific growth rate decreased by 1% d−1.