Accurate pH and O2 measurements from Spray underwater gliders

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  • 1 Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039
  • 2 University of California San Diego, Scripps Institution of Oceanography. 9500 Gilman Drive, La Jolla, CA 92093
  • 3 University of Hawaii at Manoa, Department of Oceanography, 1000 Pope Road, Honolulu, HI 96822
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Abstract

The California Current System is thought to be particularly vulnerable to ocean acidification, yet pH remains chronically undersampled along this coast, limiting our ability to assess the impacts of ocean acidification. To address this observational gap, we integrated the Deep-Sea-DuraFET, a solid state pH sensor onto a Spray underwater glider. Over the course of a year starting in April 2019, we conducted 7 missions in Central California, which spanned 161 glider days and >1600 dives to a maximum depth of 1000 m. The sensor accuracy was estimated to be ± 0.01 based on comparisons to discrete samples taken alongside the glider (n=105), and the precision was ± 0.0016. CO2 partial pressure, dissolved inorganic carbon, and aragonite saturation state could be estimated from the pH data with uncertainty better than ± 2.5%, ± 8 μmol kg-1, and ± 2%, respectively. The sensor was stable to ± 0.01 for the first nine months, but exhibited a drift of 0.015 during the last mission. The drift was correctable using a piecewise linear regression based on a reference pH field at 450 m estimated from published global empirical algorithms. These algorithms require accurate O2 as inputs, thus, protocols for a simple pre-deployment air-calibration which achieved accuracy of better than 1 % were implemented. The glider observations revealed upwelling of undersaturated waters with respect to aragonite to within 5 m below the surface near Monterey Bay. These observations highlight the importance of persistent observations through autonomous platforms in highly dynamic coastal environments.

Corresponding Author: Yuichiro Takeshita, yui@mbari.org

Abstract

The California Current System is thought to be particularly vulnerable to ocean acidification, yet pH remains chronically undersampled along this coast, limiting our ability to assess the impacts of ocean acidification. To address this observational gap, we integrated the Deep-Sea-DuraFET, a solid state pH sensor onto a Spray underwater glider. Over the course of a year starting in April 2019, we conducted 7 missions in Central California, which spanned 161 glider days and >1600 dives to a maximum depth of 1000 m. The sensor accuracy was estimated to be ± 0.01 based on comparisons to discrete samples taken alongside the glider (n=105), and the precision was ± 0.0016. CO2 partial pressure, dissolved inorganic carbon, and aragonite saturation state could be estimated from the pH data with uncertainty better than ± 2.5%, ± 8 μmol kg-1, and ± 2%, respectively. The sensor was stable to ± 0.01 for the first nine months, but exhibited a drift of 0.015 during the last mission. The drift was correctable using a piecewise linear regression based on a reference pH field at 450 m estimated from published global empirical algorithms. These algorithms require accurate O2 as inputs, thus, protocols for a simple pre-deployment air-calibration which achieved accuracy of better than 1 % were implemented. The glider observations revealed upwelling of undersaturated waters with respect to aragonite to within 5 m below the surface near Monterey Bay. These observations highlight the importance of persistent observations through autonomous platforms in highly dynamic coastal environments.

Corresponding Author: Yuichiro Takeshita, yui@mbari.org
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