1 An Evaluation of the Performance of SeaBird Scientific ’ s Autonomous SeaFETTM : 1 Considerations for the Broader Oceanographic Community 2 3

Abstract. The commercially available Sea-Bird SeaFET TM provides an accessible way for a broad community of researchers to study ocean acidification and obtain robust measurements of seawater pH via the use of an in situ autonomous sensor. There are pitfalls, however, that have been detailed in previous best practices for sensor care, deployment, and data handling. Here, we took advantage of two distinctly different coastal settings to evaluate the Sea-Bird SeaFET TM and examine the multitude of scenarios in which problems may arise confounding the accuracy of measured pH. High-resolution temporal measurements of pH were obtained during 3- to 5-month field deployments in three separate locations (two in south-central, Alaska, USA, and one British Columbia, CA) spanning a broad range of nearshore temperature and salinity conditions. Both the internal and external electrodes onboard the SeaFET TM were evaluated against robust benchtop measurements for accuracy utilizing either the factory calibration, an in situ single-point calibration, or in situ multi-point calibration. In addition, two sensors deployed in parallel in Kasitsna Bay, AK, USA, were compared for inter-sensor variability in order to quantify other factors contributing to SeaFET TM intrinsic inaccuracies. Based on our results, the multi-point calibration method provided the highest accuracy ( 0.025 difference in pH) of pH when compared against benchtop measurements. Spectral analysis of time series data showed that during spring in Alaskan waters, a range of tidal frequencies dominated pH variability, while seasonal oceanographic conditions were the dominant driver in Canadian waters. Further, it is suggested that spectral analysis performed on initial deployments may be able to act as an a posteriori method to better identify appropriate calibration regimes. Based on this evaluation, we provide a comprehensive assessment of the potential sources of uncertainty associated with accuracy and precision of the SeaFET TM electrodes.