Use of cavity ring‐down spectrometry to quantify 13C‐primary productivity in oligotrophic waters

Cavity ring‐down spectroscopy (CRDS) is a highly sensitive laser technique that allows the analysis of isotopic signals and absolute concentration of individual molecular species in small‐volume samples. Here, we describe a protocol to quantify photosynthetic 13C‐uptake rates of marine phytoplankton by using the CRDS technique (13C‐CRDS‐PP). We validated our method by comparing the 13C‐PP rates measured between CRDS and isotope ratio mass spectrometry (IRMS) in samples with different carbon content (30–160 μgC). The comparison revealed that 13C‐CRDS‐PP rates were highly correlated with those obtained by IRMS (Spearman correlation coefficient, ρ = 0.95, p