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Ocean Science An interactive open-access journal of the European Geosciences Union

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Ocean Sci., 13, 427-442, 2017
https://doi.org/10.5194/os-13-427-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
24 May 2017
Measuring pH variability using an experimental sensor on an underwater glider
Michael P. Hemming1,2, Jan Kaiser1, Karen J. Heywood1, Dorothee C.E. Bakker1, Jacqueline Boutin2, Kiminori Shitashima3, Gareth Lee1, Oliver Legge1, and Reiner Onken4 1Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, UK
2Laboratoire d'Océanographie et du Climat, 4 Place Jussieu, 75005 Paris, France
3Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato, Tokyo 108-0075, Japan
4Helmholtz-Zentrum Geesthacht, Max-Planck-Straße 1, 21502 Geesthacht, Germany
Abstract. Autonomous underwater gliders offer the capability of measuring oceanic parameters continuously at high resolution in both vertical and horizontal planes, with timescales that can extend to many months. An experimental ion-sensitive field-effect transistor (ISFET) sensor measuring pH on the total scale was attached to a glider during the REP14-MED experiment in June 2014 in the Sardinian Sea in the northwestern Mediterranean. During the deployment, pH was sampled at depths of up to 1000 m along an 80 km transect over a period of 12 days. Water samples were collected from a nearby ship and analysed for dissolved inorganic carbon concentration and total alkalinity to derive the pH for validating the ISFET sensor measurements. The vertical resolution of the pH sensor was good (1 to 2 m), but stability was poor and the sensor drifted in a non-monotonous fashion. In order to remove the sensor drift, a depth-constant time-varying offset was applied throughout the water column for each dive, reducing the spread of the data by approximately two-thirds. Furthermore, the ISFET sensor required temperature- and pressure-based corrections, which were achieved using linear regression. Correcting for this decreased the apparent sensor pH variability by a further 13 to 31 %. Sunlight caused an apparent sensor pH decrease of up to 0.1 in surface waters around local noon, highlighting the importance of shielding the sensor from light in future deployments. The corrected pH from the ISFET sensor is presented along with potential temperature, salinity, potential density anomalies (σθ), and dissolved oxygen concentrations (c(O2)) measured by the glider, providing insights into the physical and biogeochemical variability in the Sardinian Sea. The pH maxima were identified close to the depth of the summer chlorophyll maximum, where high c(O2) values were also found. Longitudinal pH variations at depth (σθ > 28. 8 kg m−3) highlighted the variability of water masses in the Sardinian Sea. Higher pH was observed where salinity was  > 38. 65, and lower pH was found where salinity ranged between 38.3 and 38.65. The higher pH was associated with saltier Levantine Intermediate Water, and it is possible that the lower pH was related to the remineralisation of organic matter. Furthermore, shoaling isopycnals closer to shore coinciding with low pH and c(O2), high salinity, alkalinity, dissolved inorganic carbon concentrations, and chlorophyll fluorescence waters may be indicative of upwelling.

Citation: Hemming, M. P., Kaiser, J., Heywood, K. J., Bakker, D. C. E., Boutin, J., Shitashima, K., Lee, G., Legge, O., and Onken, R.: Measuring pH variability using an experimental sensor on an underwater glider, Ocean Sci., 13, 427-442, https://doi.org/10.5194/os-13-427-2017, 2017.
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Short summary
Underwater gliders are useful platforms for monitoring the world oceans at a high resolution. An experimental pH sensor was attached to an underwater glider in the Mediterranean Sea, which is an important carbon sink region. Comparing measurements from the glider with those obtained from a ship indicated that there were issues with the experimental pH sensor. Correcting for these issues enabled us to look at pH variability in the area related to biomass abundance and physical water properties.
Underwater gliders are useful platforms for monitoring the world oceans at a high resolution. An...
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