An evaluation of the performance of Sea-Bird Scientific's SeaFET™ autonomous pH sensor: considerations for the broader oceanographic community

Abstract

Abstract. The commercially available Sea-Bird SeaFET™ 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™ 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 in British Columbia, Canada) spanning a broad range of nearshore temperature and salinity conditions. Both the internal and external electrodes onboard the SeaFET™ were evaluated against robust benchtop measurements for accuracy using the factory calibration, an in situ single-point calibration, or an in situ multi-point calibration. In addition, two sensors deployed in parallel in Kasitsna Bay, Alaska, USA, were compared for inter-sensor variability in order to quantify other factors contributing to the sensor's 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™ electrodes.

The seasonal phases of an Arctic lagoon reveal the discontinuities of pH variability and CO<sub>2</sub> flux at the air–sea interface

“…A unique feature of ice-covered Arctic coastal waters is the negative relationship between pH T and salinity, which was observed here and in previous studies (Nomura et al, 2006;Miller et al, 2011;Fransson et al, 2013;Muth et al, 2021). In the open ocean, salinity is positively correlated with A T as higher salinity increases the difference between conservative cations to anions.…”

The Seasonal Phases of an Arctic Lagoon Reveal Non-linear pH Extremes

“…The reliability and accuracy of SeaFET sensors is dependent on estimating the total uncertainty attributable to an individual sensor's behavior and operator usage (Bresnahan et al, 2014;Gonski et al, 2018;McLaughlin et al, 2017;Miller et al, 2018;Rivest et al, 2016). A previous method for calculating the total uncertainty associated with SeaFET function has been previously proposed and was applied to this study (Miller and Kelley 2020 in review).…”

Perspectives on in situ Sensors for Ocean Acidification Research

“…Assessment of in situ sensor performance should incorporate sensor redundancy and include pre-and post-deployment laboratory calibrations where possible (e.g., Bresnahan et al, 2014;Miller et al, 2018). However, this approach requires that instruments are recovered in a condition suitable for such postdeployment calibrations to be meaningful, and assumes that drift during deployment is predictable.…”

“…Wanninkhof et al (2013) have provided further guidance on incorporating in situ sensors into the SOCAT QC framework, including details such as degree of in situ calibration necessary for desired confidence and levels of accuracy. A number of published papers address practical suggestions and demonstrations for Durafet R pH sensor QC (Bresnahan et al, 2014;Johnson et al, 2016;Rérolle et al, 2016;McLaughlin et al, 2017;Gonski et al, 2018;Miller et al, 2018). Similarly, there is considerable ongoing research into complications with in situ spectrophotometric pH analyzers, especially those resulting from indicator dye impurities and the moving parts within these systems (e.g., Liu et al, 2011;Lai et al, 2018).…”