Intercomparison of carbon‐13 and carbon‐14 tracer approaches for estimating marine calcification and net primary production

Accurate measurements of phytoplankton net primary production (NPP) and calcification are essential for quantifying marine carbon cycling, yet in situ calcification rates remain poorly constrained. We compared 13 C‐ and 14 C‐based tracer methods for NPP in laboratory cultures of the diatom Chaetoceros tenuissimus and the coccolithophore Gephyrocapsa huxleyi across two incubation durations (8 and 24 h). Calcification was quantified only in G. huxleyi , for which experiments were conducted across a range of cell concentrations (10 3 –10 5 cells mL −1 ) while diatom incubations were performed at a single concentration (10 5 cells mL −1 ). 13 C‐ and 14 C‐based NPP and calcification estimates converged in 24‐h incubations across all cell concentrations. There was no significant difference between rates measured with either tracer across all incubations, although short (8‐h) incubations showed greater variability in NPP estimates, particularly for G. huxleyi . In situ application of the 13 C method at the Bermuda Atlantic Time‐Series Study site revealed strong vertical structure and seasonal declines in calcification from July to September, while NPP remained comparatively stable. Particulate 13 C enrichments consistently exceeded analytical detection limits in both laboratory and field settings, demonstrating that low biomass in oligotrophic regions would not limit method sensitivity. These results validate 13 C spike incubations as a practical alternative to radiocarbon for simultaneous measurements of NPP and calcification. To support the adoption of this method, we provide a detailed step‐by‐step standard operating procedure for the 13 C spike incubation method in the Supporting Information.