Text S1 Methods Water column samples were collected with 10 m resolution from the surface to 50 m, with 25 m resolution from 50 m to 100 m, and with 50 - 100 m resolution from 100 m to the bottom. Conductivity-temperature-depth (CTD) data were collected on downcasts with a Seabird 911-plus system using dual temperature and conductivity sensors. Water velocity was measured with ship-mounted Acoustic Doppler Current Profiler (ADCP). Nutrient samples (nitrate/nitrite, phosphate and silicate) were collected at all CTD stations and immediately frozen. These samples were kept frozen until they were analyzed at UAF approximately 30 days later using an auto-analyzer following standard protocols. The TA of each sample was determined using the spectrophotometric method developed by Yao and Byrne, [1998]. Bromocresol purple was used as the indicator dye with a standardized acid (~0.1N HCl). Shipboard TA precision was determined by using Certified Reference Material provided by Andrew Dickson. Solution pH was determined by spectrophotometry on the total hydrogen scale using purified m-cresol purple (Liu et al., 2011). The underway sampling system for the partial pressure of carbon dioxide consisted of a container continuously flushed with seawater from the hull intake at 3 m depth. The partial pressure of carbon dioxide in the container was measured using a non-dispersive infrared (NDIR) sensor with a membrane equilibrator (Pro-Oceanus Sytems, Inc.). Derived oceanic values for dissolved gases account for any warming of the container's seawater. The partial pressure of carbon dioxide data from the underway system was used in the CO2 flux calculations. Water column partial pressure of carbon dioxide and carbonate mineral saturation states were calculated from pH and TA measurements. The carbonic acid dissociation constants (pK1and pK2) of Mehrbach et al. (1973), as refit by Dickson and Millero (1987), were used to calculate seawater the partial pressure of carbon dioxide and other carbonate parameters, using the equations of Zeebe and Wolf-Gladrow (2001). The CO2 solubility equations of Weiss (1974), and dissociation constants for borate (Dickson, 1990), and phosphate (Dickson et al., 2007) were used. The calculation of seawater pCO2 has an error of ~1-2 percent error depending on pK1 and pK2 used (i.e., Mehrbach et al., 1973, as refit by Dickson and Millero, 1987; Goyet and Poisson, 1989; Roy et al., 1993; Millero et al., 2006). References Dickson, A. G. and Millero, F. J.: A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media, Deep- Sea Res., 34, 1733–1743, 1987. A. G. Dickson, Thermodynamics of the dissociation of boric acid in synthetic sea water from 273.15 to 318.15 K. Deep-Sea Res. 37, 755-766, 1990. Dickson, A.G. 1990. Standard potential of the reaction AgCl(s) + .5H2(g) = Ag(s) + HCl(aq) and the standard acidity constant of the ion HSO4– in synthetic sea water from 273.15 to 318.15 K. The Journal of Chemical Thermodynamics 22:113–127, http://dx.doi.org/10.1016/0021-9614(90)90074- Z. Goyet, C. and Poisson, A. P.: New determination of carbonic acid dissociation constants in seawater as a function of temperature and salinity, Deep-Sea Res., 36, 1635–1654, 1989. Liu, X.; Patsavas, M. C.; Byrne, R. H., Purification and Characterization of meta-Cresol Purple for Spectrophotometric Seawater pH Measurements. Environ. Sci. Technol. 2011, 45 (11), 4862-4868. Mehrbach, C., Culberson, C. H., Hawley, J. E., and Pytkowicz, R. M.: Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure, Limnol. Oceanogr., 18, 897–907, 1973. Millero, F. J., Graham, T. B., Huang, F., Bustos-Serrano, H., and Pierrot, D.: Dissociation constants of carbonic acid in seawater as a function of salinity and temperature, Mar. Chem., 100, 80–94, 2006. Roy, R. N., Roy, L. N., Vogel, R., Moore, C. P., Pearson, T., Good, C. E., Millero, F. J., and Campbell, D.: Determination of the ionization constants of carbonic acid in seawater, Mar. Chem., 44, 249–268, 1993. Weiss, R. F.: Carbon dioxide in water and seawater: the solubility of a non-ideal gas, Mar. Chem., 2, 203–215, 1974. Yao, W and Byrne, R. H., Simplified seawater alkalinity analysis: Use of linear array spectrometers. Deep Sea Research Part I: Oceanographic Research Papers 1998, 45 (8), 1383-1392. Zeebe, R. and Wolf-Gladrow, D.: CO2 in Seawater: Equilibrium, Kinetics, Isotopes, Elsevier Oceanography Series 65, 2001.