Supplemental Material
CO2 flux
Sea-air CO2 flux rates were calculated to establish a net sink or source behavior for the
ocean with respect to the atmosphere. This equation is given by:
F = ks (pCO2sw – pCO2air)
(Fagan and Mackenzie 2007) where k is the gas transfer velocity, s is the solubility of
CO2 (calculated as a function of temperature and salinity), and pCO2sw and pCO2air are
the partial pressures of CO2 in the surface water and air, respectively. The gas transfer
velocity was parameterized using the open-ocean wind-speed relationship established by
Wanninkhof (1992) and improved by Ho et al. (2006). Winds speeds (measured in knots)
were collected by the ship’s anemometer and converted to m/s for the CO2 flux
calculations.
pH
Underway pH was computed using the voltage output of a Honeywell Durafet,
calibrated with discrete samples measured spectrophotometrically on the total scale, and
assuming the electrode obeyed a Nernstian response (Martz et al. 2010). The Durafet
uses an Ion Sensitive Field Effect Transistor (ISFET) technology (Bergveld 2003), which
makes noise and drift much lower than in a glass electrode. Experimental results indicate
this sensor can operate with a short-term precision of ±0.0005 over several hours and
with a stability of ±0.005 over weeks to months (Martz et al. 2010). While Clayton and
Byrne (1993) have demonstrated that high accuracy (±0.004) and precision (±0.0004)
using the spectrophotometric method is possible, various studies (Hofmann et al. 2011;
Falter et al. 2012; Lantz et al. 2013; Thompson et al. 2014), including our own, still find
the reproducibility of these values to be difficult. Therefore, calibration of a precise
device such as an electrode with less precise spectrophotometric measurements remains
problematic.
In order to convert the electrode output to pH, calibration samples measured at 25
ºC in the lab were corrected to in situ temperature. This equation is given by:
pHin situ = pH25 - (Tin situ - 25) * 0.0149
where pH25 is the pH measured at 25°C in the lab, Tin situ is the output temperature given
by the temperature sensor, and -0.0149 is the slope for the decrease in pH per degree
increase in temperature (°C) (Millero 1995, 2007). This slope was derived from the
average ratio of the surface TA and DIC values (1.16) measured at Station ALOHA
(http://hahana.soest.hawaii.edu/hot/). All carbon system calculations were performed in
CO2SYS (Pierrot et al. 2006), using the equilibrium constants of Mehrbach et al. (1973)
refit by Dickson and Millero (1987), and the equilibrium constant for sulfate from
Dickson (1990).
Oceanographic Conditions
Although variability between cruises was observed, in general, SST and sea
surface salinity increased with latitude from approximately 21–25 °N. North of 25°N,
salinity remained relatively constant. Over the latitudinal extent of the two cruises,
temperature (Figure S2) and salinity (Figure S3) varied between ~25.1–28.0°C and
~34.9–36, respectively. During the August 2011 cruise, warmer temperatures and lower
salinities were observed than during the July 2012 cruise. Oxygen tended to be
oversaturated in the open ocean and undersaturated near the islands/atolls (Figure S4). In
order to estimate the magnitude of CO2 air-sea exchange, CO2 flux during August 2011
was calculated. The slight latitudinal decrease in the flux of CO2 (S5) to the atmosphere
is influenced, in part, by subsiding winds in the north and may contribute to the buildup
of CO2 in the surface waters of these northern atolls.
Table S1: Summary of measurements for accuracy of pH based on CRMs (Batch 122
pH=7.865 and Batch 97 pH=7.909). The average difference in our measured versus
standard values was 0.006 with a standard deviation of 0.003
Date
CRM
Salinity
Measured pH
Difference
12/10/2012
Batch 122
33.435
7.861
0.004
12/14/2012
Batch 122
33.435
7.863
0.002
12/21/2012
Batch 122
33.435
7.856
0.009
01/03/2013
Batch 97
33.142
7.891
0.018
01/10/2013
Batch 97
33.142
7.893
0.016
02/16/2013
Batch 97
33.142
7.899
0.010
03/30/2013
Batch 97
33.142
7.924
-0.015
Fig S1 Precision test for 33 duplicates indicate an average difference of 0.0002 (solid
line) and a standard deviation of 0.007 (dotted line)
Fig S2 Sea surface temperature (°C) plotted against latitude during August 2011, and
July 2012 cruises
Fig S3 Sea surface salinity plotted against latitude during August 2011 and July 2012
cruises
Fig S4 Oxygen (% saturation) plotted against latitude during August 2011 and July 2012
cruises
Fig S5 Air-sea CO2 flux calculated for August 2011 and July 2012
Fig S6 Ship track showing pCO2 for August 2011 (large circles) and July 2012 (small
triangles)