pH and CO32− microsensors were calibrated with commercially available pH buffers (pH 7 and 9, Fluka) and in seawater that was adjusted to pH 6.5 to 9.5 in 0.5 pH units [National Bureau of Standards (NBS) scale] by adding HCl and NaOH. Seawater pH (pHSW [NBS]) was measured with a pH electrode (Mettler Toledo), which was calibrated with three commercially available pH NBS buffers (pH 4, 7, and 10; Fisher Chemical). For each pH step, subsamples were taken to determine total scale pH (m-cresol purple) and total alkalinity (AT, by titration). When alkalinity measurements were not possible, an average of all alkalinity measurements was taken for these datasets. For CO32− calibration, concentrations were calculated in CO2SYS v2.1 using the measured pH and AT with constants by Mehrbach et al. (47) refitted by Dickson and Millero (48), KHSO4 from Dickson, and total boron by Uppström (49). Ca2+ sensors were calibrated in NaCl-based buffers (490 mM NaCl, 54 mM MgCl2 × 6H2O and 10.5 mM KCl, 500-ml ultrapure water), with at least three concentrations from 1, 5, 10, and 20 mM CaCl2 × 2H2O.

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