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

Abstract 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 13 C‐uptake rates of marine phytoplankton by using the CRDS technique ( 13 C‐CRDS‐PP). We validated our method by comparing the 13 C‐PP rates measured between CRDS and isotope ratio mass spectrometry (IRMS) in samples with different carbon content (30–160 μ gC). The comparison revealed that 13 C‐CRDS‐PP rates were highly correlated with those obtained by IRMS (Spearman correlation coefficient, ρ = 0.95, p < 0.0001, n = 15), with a mean difference between the two estimates of ± 0.08 mgC m −3 h −1 . Moreover, the slope of the relationship between CRDS and IRMS results was not significantly different from 1 ( F = 0.03, p = 0.86), and the intercept did not differ from 0 ( F = 1.4, p = 0.24), indicating that there was no bias in the CRDS relative to the IRMS‐based measurements. A separate analysis also showed that despite the difference in volume and carbon content between samples (40 ± 10 μ gC and 160 ± 40 μ gC, respectively), the 13 C‐CRDS‐PP technique provides similar results (Mann–Whitney test, U = 30.5, p = 0.90, n = 8). In addition, 13 C‐CRDS‐PP rates measured along the Red Sea (∼ 176 mgC m −2 d −1 ) agreed with 14 C‐based PP rates previously reported for similar locations. Thus, this study evidenced that the 13 C‐CRDS‐PP method is sensitive enough to quantify carbon fixation rates in oligotrophic regions.