A calibrated YSI 6920 multisonde was used to obtain in-situ temperature at both the surface (~0.5 m) and the bottom (within 0.5 m from the sediment-water interface) of the water column, and a Van Dorn water sampler was used to take water samples from both the surface and bottom of the water column. 

Following the standard OA operating protocol, unfiltered water samples were collected in 250 mL borosilicate glass bottles for carbonate system characterization. 100 µL saturated mercuric chloride (HgCl2) was added into the samples bottles and bottle stoppers were replaced after the application the Apiezon® L grease and secured with a rubber band and hose clamp. These preserved samples were used for salinity, total alkalinity (TA), total dissolved inorganic carbon (DIC), and pH analyses within two months of sample collection. 

Salinity was measured using a benchtop salinometer (Orion Star™ A12, Thermo Scientific) which was calibrated using MilliQ and know salinity certified reference material (CRM). TA was analyzed at 22±0.1°C using Gran titration on an automated titration system (AS-Alk2, Apollo Scitech Inc.). DIC was analyzed using infrared detection on a AS-C3 DIC analyzer (Apollo Scitech Inc.). CRM was used to ensure the quality of the analysis and optimal performance of the instruments. Total scale pH for salinity ≥20 samples was measured at 25°C using purified m-cresol purple. The equation in Liu et al. (2011) was used in the calculation of pH values. For low salinity waters (<20), a potentiometric method was used to measure pH. A Orion® Ross™ high precision glass electrode was used and three pH standard (4.01, 7.00, and 10.01) were used to calibrate the electrode. Calcium concentration ([Ca2+]) was measured (from non-preserved water samples) using an automatic titration with an ethylene glycol tetraacetic acid (EGTA) titrant on a Metrohm Titrando. The end-point was detected using a Metrohm calcium ion-selective electrode.