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

Abstract

The seasonal variability of specific growth rate and the carbon stable isotope ratio ( δ 13C) of leaf blades ( δ 13C leaf) of a temperate seagrass, Zostera marina (within 10 days old) were measured simultaneously, together with the δ 13C of dissolved inorganic carbon ( δ 13C DIC) at three sites in the semi-closed Akkeshi estuary system, northeastern Japan, in June, September, and November 2004. The δ 13C leaf ranged from −16.2 to −6.3‰ and decreased from summer to winter. The simultaneous measurement of the δ 13C leaf, growth rate, and morphological parameters (mean leaf length and width, mean number of leaves per shoot, and sheath length) of the seagrass and δ 13C DIC in the surrounding water allowed us to compare directly the δ 13C leaf and specific growth rate of seagrass. The difference in the δ 13C of seagrass leaves relative to the source DIC (Δ δ 13C leaf − 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 Δ δ 13C leaf − DIC and specific growth rate can be used to diagnose the growth of seagrasses. Δδ 13C leaf − DIC changed by −1.7 ± 0.2‰ when the leaf specific growth rate decreased by 1% d −1.