Applications of in situ pH measurements for inorganic carbon calculations

This study examines the utility of combining pH measurements with other inorganic carbon parameters for autonomous mooring-based carbon cycle research. Determination of the full suite of inorganic carbon species in the oceans has previously been restricted to ship-based studies. Now with the availability of autonomous sensors for pH and the partial pressure of CO2 (pCO2), it is possible to characterize the inorganic carbon system on moorings and other unmanned platforms. The indicator-based pH instrument, SAMI-pH, was deployed with an autonomous equilibrator-infrared pCO2 system in Monterey Bay, California USA from June to August 2007. The two-month time-series show a high degree of short-term variability, with pH and pCO2 changing by as much as 0.32 pH units and 240 μatm, respectively, during upwelling periods. The pH and salinity-derived alkalinity (ATsalin) were used to calculate the other inorganic carbon parameters, including pCO2, total dissolved inorganic carbon (DIC) and CaCO3 saturation states. The calculated pCO2 was within 2 μatm of the measured pCO2 during the first day of the deployment and within 8 μatm over the first month. The DIC calculated from pH–ATsalin and pCO2–ATsalin were within 5 μmol kg−1 of each other during the first month. However, DIC calculated from pH–pCO2 differed by ~ 50 μmol kg−1 from the other estimates over the same period, reflecting the sensitivity of the pH–pCO2 calculation to measurement error. The data continued to diverge during the final month and this difference was likely driven by extensive biofouling. Because of the relative insensitivity of CO32− concentration to these errors, aragonite saturation calculated from the pH–pCO2 pair was within 0.15 of the pH–ATsalin values over the entire deployment. These results show that in situ pH, when combined with other CO2 parameters, can provide valuable insights into both data quality and inorganic carbon cycling.

Research Highlights
► In situ pH and pCO2 data are used to quantify marine inorganic carbon species.
► The pCO2 from pH and salinity-derived alkalinity closely matches the measured pCO2.
► Dissolved inorganic carbon is also accurately quantified.
► CaCO3 saturation states are also quantified for ocean acidification studies.