Carbon stable isotope discrimination: a possible growth index for the kelp Undaria pinnatifida

Undaria pinnatifida sporophytes were grown in a marine bay under differing culture conditions in order to obtain individuals with different growth rates. Carbon stable isotope compositions of seawater and cultured algae were measured to calculate carbon stable isotope discrimination due to carbon assimilation (Δ) by the thalli. High and significant correlations were found between growth rates (absolute in length, absolute in weight, or relative in weight) and Δ, which suggests that growth rate was the most important factor determining Δ. The correlation between Δ and relative growth rate in weight was especially good, following the formula: Δ = 28.6 – 1.4RGR (r = –0.97), in which RGR is the relative growth rate in weight (% d^–1). This very good correlation, if confirmed for other instances, would allow Δ to be used as a growth index for the species. Results also suggested that HCO₃^– uptake may be the main mechanism of inorganic carbon assimilation for the studied species.