Measuring pH variability using an experimental sensor on an underwater glider

Abstract

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.

Net community production in the northwestern Mediterranean Sea from glider and buoy measurements

“…The glider completed 7 circumnavigations of the survey pattern, each in approximately 4 days, with each of the four sides of the pattern taking approximately 1 day to complete. The glider was equipped with a non-pumped SBE model CTD sensor, an Aanderaa model 4330F oxygen optode sensor, a WET Labs Eco Puck sensor measuring optical backscatter at two wavelengths (470 nm and 700 nm), and two paired prototype ISFET pH and p(CO 2 ) sensors (Shitashima et al, 2013;Hemming et al, 2017).…”

“…The deployment offered a second opportunity to trial prototype ISFET sensors, previously tested on a glider during the REP14-MED experiment in the Sardinian Sea (Hemming et al, 2017;Onken et al, 2018). ISFET pH was corrected (Fig.…”

“…for drift and pressure effects, similar to the steps undertaken by Hemming et al (2017). The drift-and pressure-corrected ISFET pH was calibrated by linear regression against ship pH on 7 March and 16 April 2016 (B2, C).…”

“…The deployment offered a second opportunity to trial two prototype ISFET pH-p(CO 2 ) sensor pairs, previously tested on an underwater glider in the Sardinian Sea during the REP14-MED experiment (Hemming et al, 2017;Onken et al, 2018).…”

Physical mechanisms for biological carbon uptake during the onset of the spring phytoplankton bloom in the northwestern Mediterranean Sea (BOUSSOLE site)

“…An iRobot Seaglider model 1KA (SG537) with an ogive fairing operated by the University of East Anglia (UEA) [see Hemming et al, 2017] was deployed during the BOUSSOLE monthly cruise (#169) [Golbol et al 2000] and recovered 1 month later with a ship of opportunity. Diving to 1000 m, 147 dives were completed between March 7 and April 5 2016 repeatedly-sampling a square-shaped path surrounding the BOUSSOLE buoy (Fig.…”