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Volume 14, issue 4 | Copyright
Ocean Sci., 14, 751-768, 2018
https://doi.org/10.5194/os-14-751-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 02 Aug 2018

Research article | 02 Aug 2018

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

Cale A. Miller1,a, Katie Pocock2, Wiley Evans2, and Amanda L. Kelley1 Cale A. Miller et al.
  • 1College of Fisheries and Ocean Sciences, University of Alaska Fairbanks, Fairbanks, AK, USA
  • 2Hakai Institute, Heriot Bay, BC, Canada
  • apresent address: Department of Evolution and Ecology, College of Biological Sciences, University of California Davis, Davis, CA, USA

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.

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