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Performance of autonomous pH sensors is evaluated by comparing in situ data to independent bench-top measurements of pH and to co-located pH, O 2 , and p CO 2 sensors. While the best practice is always to deploy a properly calibrated sensor, the lengthy time period required for sensor conditioning and calibration often results in sensor deployment without comprehensive calibration. Quality control (QC) procedures are examined to determine the errors associated with different in situ calibration approaches and lay a framework for best practices. Sensor packages employing the Honeywell Durafet remained stable across multiple deployments for over nine months. However, sensor performance was often limited by biofouling. Regional empirical relationships for estimating carbonate system parameters are shown to enable identification of otherwise indistinguishable sensor offset and drift when multiple sensor types are co-located. Uncertainty is determined by calibration approach and must be quantified on a case-by-case basis. Our results indicate that the Durafet is capable of accuracy, relative to a chosen reference, of better than 0.03 pH units over multiple months. Accuracy is improved when a robust shore-side calibration is performed, an independent means of QC is available throughout a deployment, and effective biofouling prevention measures are taken. [ABSTRACT FROM AUTHOR]

Analysis of a global hydrographic data product showed a clear pH-dependent discrepancy between pH on the total scale measured spectrophotometrically (pH spec) using purified meta-Cresol Purple (mCP) and pH calculated from total alkalinity and dissolved inorganic carbon. However, this was based mainly on US cruises, and three recent Japanese cruises do not show this pH-dependent discrepancy. One potential explanation is that purified mCP batches obtained from different institutions lead to significantly different pH spec. Here, we tested this hypothesis by comparing the performance of purified mCP obtained from four different institutions. We demonstrate that consistent pH of ±0.0012 (95% C.I.) can be achieved regardless of the institution when impurities are properly removed. However, there was at least one batch from three of the four institutions that had significant pH-dependent errors that were as large as −0.008. The presence of impurities that led to pH-dependent errors was identified using HPLC and, for 8 out of the 9 cases, by spectrophotometry (although issues still remain for the latter). We conclude that pH-dependent errors due to impurities that remain after the purification process are, by themselves, too small to account for the differences observed between the recent set of cruises. Identifying the source of this difference should be a top priority. This study also highlights the importance of establishing robust quality assurance and quality control protocols to ensure consistent behavior with previously published equations to compute pH. We recommend a centralized system where one or a handful of institutions distribute purified mCP for the community, as this distribution approach will lead to lower prices and simplify quality assurance. • Consistent pH (±0.0012) can be obtained when impurities are properly removed. • Robust QA/QC protocols are required to prevent pH-dependent errors. • HPLC is an effective approach to identify the presence of impurities. • Impurities alone cannot explain different pH-dependent behavior in recent cruises. • Dye solutions are stable to ±0.001 for 2 years when stored properly. [ABSTRACT FROM AUTHOR]

Ocean acidification may shift coral reefs from a state of net ecosystem calcification (+NEC) to net ecosystem dissolution (–NEC). Changes in NEC are typically inferred from either measured or calculated total alkalinity (TA) or the dissolved calcium (Ca) to salinity ratio relative to a reference value. The alkalinity anomaly technique has historically been the primary method to estimate NEC due to the greater analytical challenges and uncertainty associated with dissolved Ca measurements in seawater. However, this method assumes that changes in salinity-normalized TA are exclusively the result of calcification and dissolution processes. In many cases, this assumption is valid, but in some environments additional processes can significantly influence seawater TA (e.g., nutrient fluxes and redox processes). Seawater Ca is unaffected or less sensitive to these processes, and therefore, Ca and TA anomalies can be used to estimate absolute or relative changes in NEC with greater confidence. Here, we present a two-year time series of monthly seawater Ca and TA measurements across the Bermuda carbonate platform and the nearby Bermuda Atlantic Time-series Study (BATS) location offshore. High precision Ca measurements (±6 μmol kg−1) were conducted using an improved spectrophotometric titration system and showed mostly good agreement with changes in TA over the same spatial and temporal scales. Ca and TA measurements across the Bermuda platform showed seasonal fluctuations relative to offshore waters, with +NEC during summer months and near-zero or possible –NEC (net dissolution) during winter months. These seasonal patterns were most pronounced at the inshore locations with the longest residence times (10+ days), which allow stronger biogeochemical signals to develop relative to the offshore source water. Although obtaining high accuracy and precision Ca measurements remains challenging, parallel measurements of Ca and TA from both inshore and offshore waters over a multi-annual timescale could strengthen the validity of predictions for when and where a reef system, such as the Bermuda platform, may shift from +NEC to –NEC. • Two-year time-series of seawater Ca and TA across the Bermuda carbonate platform • High precision Ca measurements using a novel spectrophotometric titration system • Seasonal fluctuations in Ca and TA measurements relative to offshore waters • Ca and TA measurements showed good agreement over similar spatiotemporal scales. • Joint Ca and TA anomalies may provide more robust estimates of NEC. [ABSTRACT FROM AUTHOR]