Improved primary production and gross respiration measurements using the H 2 18 O technique with membrane inlet mass spectrometry and introduction of a small volume–uniform irradiance incubation chamber

Abstract An improvement in the H 2 18 O technique for phytoplankton gross primary productivity (GPP), net primary productivity (NPP), and gross respiration (GR) is described that reduces cost, labor, and time over the traditional isotope ratio mass spectrometer (IRMS) method. Membrane inlet mass spectrometry (MIMS) was optimized to measure small (3 mL) samples with low isotope enrichment (1% 18 O) and at high analytical precision (~1 per mil). A photosynthetron was used to measure up to 10‐point light curves of GPP, NPP, and GR using a total of 50 mL of sample water with an isotope cost of <$15 per experiment. A new incubation device is described, a Coronatron, that allows up to 12 samples with nutrient manipulation under uniform irradiance. Example light curves from highly contrasting natural water samples are presented that show elevated GPP relative to NPP and substantial light‐stimulated GR. To highlight the improved method, examples nutrient manipulation curves are also presented showing GPP, NPP, and GR as a function of changes in ambient nitrate concentrations and different temperatures under constant light.