Culture pots were placed in large thermally insulated coolers in a temperature-controlled water bath at 15° C under saturating illumination of ~550 µmoles photons/m^2/s on a 12:12 L:D cycle. pCO2 treatments were supplied to closed culture pots by use of a gas mixing system combining Nitrogen, Oxygen and Carbon Dioxide to specific CO2 partial pressures, 20.9% oxygen and the balance being Nitrogen.

Carbonate chemistry parameters were measured by sampling pH and total alkalinity (TA) of water samples. Seawater samples from each culture pot during all 3 trials were collected 2-3 times per week in 50 ml Falcon tubes to monitor the average and variability of experimental treatment conditions. Water samples were measured usually within 1-2 hours of sample collection. pH was determined using the m-cresol indicator dye method in a spectrophotometer (Dickson et al. 2007). TA samples were analyzed by potentiometric titration coupled to a pH electrode calibrated using certified reference material (CRM) from the Dickson laboratory at Scripps Oceanographic Institute and the pH electrode calibrated using TRIS buffer (Dickson et al. 2007). TA and carbonate parameters were calculated from potentiometric titration data and spectrophotometric pH data.

Carbonate chemistry parameters (CO2 concentration, CO2 partial pressure, CO2 fugacity, HCO3, CO3, DIC, Omega Aragonite, Omega Calcite) and total scale pH at in situ temperature were calculated using the seacarb package (V3.0.14) in R (Lavigne et al. 2011).