For subtidal sites, discrete bottles samples were collected by a SCUBA
diver using a Niskin bottle, firing it manually. For intertidal sites,
discrete bottle samples were collected in a bottle, manually. These
data are intended to be benchmark samples for continuously recording
pH sensors (e.g., SeaFET), and so water samples were collected as
close to the pH sensor as possible and approximately in concurrence
with the sensor's voltage measurement. Water samples were poisoned
with saturated mercuric chloride immediately after collection
(following SOP1, Dickson et al. 2007), sealed, and stored in a cool,
dark place until the seawater was analyzed.
Salinity, pH, and total alkalinity in the water samples were measured
in the laboratory. Salinity was measured using a benchtop conductivity
meter (YSI 3100). Seawater pH was measured using a spectrophotometric
method with indicator dye, unpurified m-cresol purple (Sigma-Aldrich,
SOP 6b, Dickson et al. 2007). Total alkalinity (AT) was measured using
an automated, open-cell potentiometric titration (SOP 3b, Dickson et
al. 2007) with a Mettler-Toledo T50 titrator and a DG115-SC pH probe
(Mettler-Toledo). The probe was calibrated using a Tris buffer (A.
Dickson Laboratory, Scripps Institution of Oceanography). Titrations
were performed using certified acid titrant (~0.1M HCl, 0.6 M NaCl; A.
Dickson Laboratory, Scripps Institution of Oceanography). Certified
reference materials (CRMs) from A. Dickson Laboratory, Scripps
Institution of Oceanography, were used to determine the accuracy and
precision of the titrations daily before experimental samples were
measured. Analyzed CRMs were accurate within 10 micromol
kg-1. pH at 25°C, AT, temperature, and
salinity were used to calculate the pH and pCO2
of the treatments using CO2calc (Robbins et al. 2010), with
CO2 constants K1, K2 from Mehrbach et al. 1973
refit by Dickson and Millero, 1987 and pH expressed on the total scale
Robbins, L.L., Hansen, M.E., Kleypas, J.A., and Meylan, S.C., 2010,
CO2calc: A user-friendly seawater carbon calculator for Windows, Max
OS X, and iOS (iPhone): U.S. Geological Survey Open-File Report
2010-1280, 17 pp.
Rivest, E. B., M. C. O'Brien, L. Kapsenberg, C. Gotschalk, C. A.
Blanchette, U. Hoshijima and G. E. Hofmann. 2016. Beyond the benchtop
and the benthos: dataset management planning and design for time
series of ocean carbonate chemistry associated with Durafet®-based pH
sensors. Ecological Informatics, 36: 209-220.
Dickson, A.G., C.L. Sabine, and J.R. Christian (Eds.). 2007. Guide to
Best Practices for Ocean CO2 Measurements, PICES Spec. ed.