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 ionsensitive 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 (σ θ ), and dissolved oxygen concentrations (c(O 2 )) 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(O 2 ) values were also found. Longitudinal pH variations at depth (σ θ > 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(O 2 ), high salinity, alkalinity, dissolved inorganic carbon concentrations, and chlorophyll fluorescence waters may be indicative of upwelling.

Accurate pH and O2 Measurements from Spray Underwater Gliders

“…9). Light sensitivity for ISFETs have been previously reported (Hemming et al 2017), although their pH bias under solar irradiation was negative. Based on our experience, the light sensitivity is specific for each Honeywell ISFET, causes a positive pH bias, and can be as large as .0.02 under ambient room light conditions and .0.35 pH in sunlight for this ISFET.…”

“…2). Integrating the sensor into a pumped flow stream has the benefit of improved sensor response, combating biofouling, and protection from light, as ISFETs are light sensitive (Hemming et al 2017). A flow cell for the DSD was machined out of black Delrin that could be easily removed to enable access for sensor inspection and cleaning.…”

“…pH sensors have been operated on underwater gliders previously. For example, an ISFET and Cl-ISE pH sensor was operated in the Mediterranean Sea on a Seaglider for 12 days down to a depth of 1000 m (Hemming et al 2017). This ISFET was not manufactured by Honeywell and was described as a custombuilt ''experimental'' ISFET (Shitashima 2010;Shitashima et al 2013).…”

High-resolution observations in the western Mediterranean Sea: the REP14-MED experiment

“…Two dual pH/p(CO 2 ) sensors were mounted on Fin -one stand-alone pair and another one integrated into the glider electronics. However, due to quality issues, only the data of the stand-alone pH sensor could be used, and the article of Hemming et al (2017) is restricted to that one. During the deployment, pH was sampled over a period of 12 days.…”

Quality assurance and control on hydrological data off western Sardinia (2000–2004), western Mediterranean

“…In 2014, NATO STO-CMRE based in La Spezia organized a 2-week experiment with a large use of ocean instruments and two research vessels in a limited area (110x110 km 2 ). Its aim was to improve local ocean numerical simulations and forecasts and study the local ocean variability and structures (Onken, 2017a,b;Knoll et al, 2017;Hemming et al, 2017;Onken et al, 2018;Hernandez-Lasheras and Mourre, 2018).…”