Measuring pH variability using an experimental sensor on an
Michael P. Hemming1, Jan Kaiser1, Karen J. Heywood1, Dorothee C. E. Bakker1, Jacqueline Boutin2, Kiminori Shitashima3, Gareth Lee1, Oliver Legge1, and Reiner Onken41Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, United Kingdom 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
Received: 29 Sep 2016 – Accepted for review: 11 Oct 2016 – Discussion started: 11 Oct 2016
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 northwestern Mediterranean Sea. 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 pH for validating the ISFET 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 time-dependent, depth-invariant 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 away 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 physical and biogeochemical variability in this region. pH maxima were identified at 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 variability of water masses in this region. Higher pH was observed where salinity was > 38.65, and lower pH was found where salinity ranged between 38.3 and 38.65. It seemed that the higher pH was associated with saltier Levantine Intermediate Water. Furthermore, shoaling isopycnals closer to shore coinciding with low pH, high salinity, low c(O2) waters may be indicative of upwelling.
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. Discuss., doi:10.5194/os-2016-78, in review, 2016.