Application of membrane inlet mass spectrometry to measure aquatic gross primary production by the¹⁸O in vitro method

The¹⁸O technique is considered the most direct in vitro method for measuring gross primary production (GPP) in aquatic ecosystems. This method measures the¹⁸O enrichment of the dissolved O₂ pool through photosynthesis after spiking a water sample with a tracer amount of¹⁸O-labeled water (¹⁸O-H₂O) and incubating it under natural light conditions. Despite its advantages, the¹⁸O technique has only scarcely been used to measure GPP in the ocean. The lack of¹⁸O-based primary productivity measurements is most likely due to the technical difficulty associated with sample collection, handling, and processing, and to the need of an isotope ratio mass spectrometer (IRMS) for sample analysis, which is not available for the majority of research groups. The current procedure also precludes at sea measurements. In this manuscript, we demonstrate that the biological¹⁸O enrichment of dissolved O₂, after incubation of seawater enriched with¹⁸O-H₂O, can be precisely measured by shipboard or laboratory-based membrane inlet mass spectrometry (MIMS). The method was validated in the low-productivity oligotrophic North Pacific Subtropical Gyre, where the measured GPP ranged from 0.2 to 1.1 µmol O₂ L^-1 d^-1, with an approximate precision for surface waters of ± 0.02 lmol O₂ L^-1 d^-1. This new approach has the advantages of simple water sample handling and analysis, accurate dissolved gas measurements, capability of analysis on board of a ship, and use of relatively inexpensive instrumentation, and therefore has the potential to improve our understanding of primary production in the ocean and other aquatic environments.