OPTIMIZATION OF THE 18O METHOD FOR DETERMINING PRIMARY PRODUCTIVITY OF SEAWATER USING MEMBRANE INLET MASS SPECTROMETRY AND ITS APPLICATION IN THE CHANGJIANG RIVER ESTUARY
The primary production of phytoplankton is crucial to the carbon cycle in ocean.Accurate measurement of primary production is important to assess marine carbon fixation rate.The gross primary production can be obtained accurately by the 18O method.However,the technical difficulties in handling and analysis of 18O-enriched samples limited in-situ application of the 18O method.Therefore,rapid determination of 18O-labeled gas(34O2)is crucial for the promotion of the 18O method.Therefore,the operating condition of analyzing the 18O-enriched samples by self-assembled membrane inlet mass spectrometry and incubation procedures for 18O method were optimized.Results show that the precision of measured 34O2/32O2 was 0.1％～0.2％under optimal operating conditions of inlet flow rate at 0.85～1.05 mL/min,sampling time at 3～4 min,and thermostat bath temperature at 15～20 ℃,which is suitable for the determination of 34O2.In addition,satisfactory results for gross primary production were obtained when the volume ratio of H218O to water sample in the incubation bottle was 2.1×10 3～3.4×10 3 and the incubation time was 6 h.Primary production in the estuary in spring was measured using the optimized method.Results show that the gross primary production and net community production ranged 0.11～1.20 mmol O2/(m3·h)and-1.60～0.75 mmol O2/(m3·h),respectively.The range of variation in depth-integrated gross primary production was(671.5±117.1)～(2 339.8±260.7)mg C/(m2·d),and the maximum carbon fixation rate was 1 056.9±662.4 mg C/(m2·d),which are consistent with the results from available published data.The high primary production area is located at the confluence of shallow seawater from the shelf and seawater from the open sea away from the Changjiang River estuary.
18O methodprimary productionmembrane inlet mass spectrometryChangjiang River estuary
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