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Increasing interest in the deployment of optical oxygen sensors, or optodes, on oceanographic moorings reflects the value of dissolved oxygen (DO) measurements in studies of physical and biogeochemical processes. Optodes are well-suited for moored applications but require careful, multi-step calibrations in the field to ensure data accuracy. Without a standardized set of protocols, this can be an obstacle for science teams lacking expertise in optode data processing and calibration. Here, we provide a set of recommendations for the deployment andin situcalibration of data from moored optodes, developed from our experience working with a set of 60 optodes deployed as part of the Gases in the Overturning and Horizontal circulation of the Subpolar North Atlantic Program (GOHSNAP). In particular, we detail the correction of drift in moored optodes, which occurs in two forms: (i) an irreversible, time-dependent drift that occurs during both optode storage and deployment and (ii) a reversible and pressure-and-time-dependent drift that is detectable in some optodes deployed at depths greater than 1,000 m. The latter is virtually unidentified in the literature yet appears to cause a low-bias in measured DO on the order of 1 to 3µmol kg⁻¹per 1,000 m of depth, appearing as an exponential decay over the first days to months of deployment. Comparisons of our calibrated DO time series against serendipitous mid-deployment conductivity-temperature-depth (CTD)-DO profiles, as well as biogeochemical (BGC)-ARGO float profiles, suggest the protocols described here yield an accuracy in optode-DO of ∼1%, or approximately 2.5 to 3µmol kg⁻¹. We intend this paper to serve as both documentation of the current best practices in the deployment of moored optodes as well as a guide for science teams seeking to collect high-quality moored oxygen data, regardless of expertise. 

The field of oceanography is transitioning from data-poor to data-rich, thanks in part to increased deployment ofin-situplatforms and sensors, such as those that instrument the US-funded Ocean Observatories Initiative (OOI). However, generating science-ready data products from these sensors, particularly those making biogeochemical measurements, often requires extensive end-user calibration and validation procedures, which can present a significant barrier. Openly available community-developed and -vetted Best Practices contribute to overcoming such barriers, but collaboratively developing user-friendly Best Practices can be challenging. Here we describe the process undertaken by the NSF-funded OOI Biogeochemical Sensor Data Working Group to develop Best Practices for creating science-ready biogeochemical data products from OOI data, culminating in the publication of the GOOS-endorsed OOI Biogeochemical Sensor Data Best Practices and User Guide. For Best Practices related to ocean observatories, engaging observatory staff is crucial, but having a “user-defined” process ensures the final product addresses user needs. Our process prioritized bringing together a diverse team and creating an inclusive environment where all participants could effectively contribute. Incorporating the perspectives of a wide range of experts and prospective end users through an iterative review process that included “Beta Testers’’ enabled us to produce a final product that combines technical information with a user-friendly structure that illustrates data analysis pipelines via flowcharts and worked examples accompanied by pseudo-code. Our process and its impact on improving the accessibility and utility of the end product provides a roadmap for other groups undertaking similar community-driven activities to develop and disseminate new Ocean Best Practices. 

Abstract In 2021, the Ocean Thematic Centre of the European Research Infrastructure “Integrated Carbon Observation System” conducted an international partial pressure of carbon dioxide (pCO₂) instrument intercomparison. The goal was to understand how different types of instrumentation for the measurement of oceanpCO₂compare to each other. During the two‐week long experiment, we installed various instruments in a tank facility using natural sea water (North Sea). These included direct air–water equilibration systems and membrane‐based flow‐through instruments along with submersible sensors and instruments that are normally installed on buoys and autonomous surface vehicles. In situ instruments were installed inside the tank and the flow‐through instruments were fed the same water using a pumping system. We changed the temperature (between 10°C and 28°C) and the seawaterpCO₂(between 250 and 800μatm) to observe instrument responses over a wide range. Since there is no reference for surface oceanpCO₂measurements, we agreed on a set of instruments serving as intercomparison reference. All data from the different instruments were then compared against the intercomparison reference during periods of stable temperature andpCO₂. The study provides important information to enhance future ocean carbon monitoring networks, but makes no direct recommendation for the use of any specific sensor. A major finding is that equilibration through direct air–water contact appears to be more consistent and independent of external factors than equilibration through a membrane or photometric detection. We found several instruments with no temperature measurements at the location of equilibration or with uncalibrated temperature sensors introducing significant uncertainty in the results.