Samples were collected at 0 (initial) and 24 hours and parameters of carbonate chemistry were measured (DIC,CO2, HCO3,CO3-described below). pH drift assay experiments consisted of 2 independent variables; inhibitor treatment and algal presence, with 4 and 2 levels, respectively for each independent variable. The inhibitor treatments included: seawater only (control), Acetazolamide (A), 4,4’-Diisothiocyano-2,2’-stilbenedisulfonic acid (DIDS), and Ethoxyzolamide (E). Algae were either present or absent (control) in a vial. Two replicate water samples were collected from each of 10 experimental culture pots of algae grown for 3 weeks at different combinations of average and standard deviations of pCO2 level during each of 3 experimental trials and assayed after 24 hours with or without the presence of Ulva and combinations of inhibitor treatments. In addition, 3 different levels of pCO2 in seawater generated independently of the seawater in algal culture that represented the range of pCO2 levels assayed across the entire experiment were assayed over the same 24 hour period.

Carbonate chemistry parameters were measured by sampling pH and total alkalinity (TA) of water samples. 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.