**Materials and Methods**

Field Measurements

The survey was carried out on six cruises: 5-16 January 2005, 19-30
March 2005, 27 July - 5 August 2005, 7-17 October 2005, 16-21 December
2005, and 17-27 May 2006. In all of the sampling cruises except for the
one in December 2005, the research vessel transected the whole SAB from
coastline to about 500 m water depth. The survey focused on 5
cross-shelf transects that are named E-, D-, C-, B-, and A-transects
respectively from north to south (Figure 1). In December 2005 the ship
still transected the whole SAB, but did not cover D- and B-transects and
did not go beyond the 200 m isobath. Sea level pressure was recorded
using an onboard R.M. Young^®^ barometric pressure sensor. Sea surface
temperature (SST) and salinity were recorded continuously with an
on-board SeaBird^®^ flow through thermosalinography. Surface water and
atmospheric xCO~2~ were also measured underway during all cruises.

Surface water xCO~2~ was measured using a LI-COR 7000 infrared gas
analyzer coupled to a gas-water equilibrator (referred to as UGA system
hereafter). The equilibrator has a total volume of about 10 liters and
is equipped with a specially designed water-drain system, which
maintains the level of water inside the equilibrator and prevents
possible contact with the outside air (Figure 2). The LI-COR 7000 was
calibrated every 6 hours using certified gas standards, which had xCO~2~
values of 201.51, 350.30, and 499.60 ppm referenced against standards
traceable to those of the National Institute of Standards and Technology
(NIST). The water temperature right before the equilibration chamber was
recorded with a YSI^®^ temperature sensor.

The partial pressure of CO~2~ in seawater at the temperature of
equilibration \[*p*CO~2~ (eq), units: μatm\] is calculated as:

$p\text{CO}_{2}(\text{eq}) = \text{xCO}_{2}(\text{water}) \times \lbrack P_{b}(\text{eq}) - P_{w}(\text{eq})\rbrack$
(1)

Where, xCO~2~(water) is the mole fraction concentration of seawater
CO~2~ in the dried sample gas flow (units: ppm); P~b~(eq) is the
pressure of equilibration (atm); and P~w~(eq) is the water vapor
pressure of equilibration (units: atm) that is calculated using salinity
from the ship's thermosalinograph and temperature of equilibration that
is assumed to be equal to the temperature from the temperature sensor
immediately before the equilibrator. Since the pressure of equilibration
was not measured in the UGA system during the survey, P~b~(eq) is
approximated by adding 1 mb to the barometric pressure at sea surface.
The 1 mb offset is based on the average reading differences between the
pressure sensor in the AOML system and the one onboard the research
vessel in the October 2005 and May 2006 cruises. Then the partial
pressure of CO~2~ at the in-situ temperature \[*p*CO~2~(water), units:
μatm\] is calculated as *Takahashi et al.*, 2002\]:

$p\text{CO}_{2}(\text{water}) = p\text{CO}_{2}(\text{eq}) \times \text{exp}\lbrack 0\text{.}\text{0423} \times (\text{SST} - T_{\text{eq}})\rbrack$
(2)

Where, SST is the in-situ sea surface temperature (units: ^o^C); and
T~eq~ is the temperature of equilibration (units: ^o^C).

Atmospheric xCO~2~ was measured 30-50 times in all cruises using the
same UGA CO~2~ system. The inlet of the atmospheric CO~2~ pipe, made of
stainless steel, was set up on the highest platform in the front of the
ship. In order to avoid possible contamination from the ship's stack
gases, atmospheric xCO~2~ was measured only when the ship was moving and
the wind was blowing from the bow. The partial pressure of CO~2~ in the
air \[*p*CO~2~(air), units: μatm\] is calculated as:

$p\text{CO}_{2}(\text{air}) = \text{xCO}_{2}(\text{air}) \times \lbrack P_{b}(\text{sea surface}) - P_{w}(\text{sea surface})\rbrack$
(3)

Where, xCO~2~ (air) is the mole fraction concentration of atmospheric
CO~2~ in the dried sample gas flow (units: ppm); P~b~(sea surface) is
the barometric pressure at sea surface (units: atm); and P~w~(sea
surface) is the water vapor pressure at sea surface (units: atm). During
the cruises of October 2005 and May 2006, atmospheric xCO~2~ was also
measured every two hours with the AOML system.

**Figure captions**

Figure 1. The study area is the U.S. South Atlantic Bight from Cape
Lookout, North Carolina to Cape Canaveral, Florida. The thin dotted
lines on the continental shelf are the depth contours at 20, 40, and
100m, delineating the inner, middle and outer shelf. The gray arrows on
the land show the annual river discharge rates. The dashed lines on the
continental shelf are the typical ship track during each cruise. The
dark arrows close to the ship track show the direction of the ship
movement. The circles are the sampling stations, which start from the
shoreline and extend to about \~500 m water depth.

Figure 2. The diagram of the CO~2~ equilibrator. The equilibrator has a
total volume of about 10 liters. It's composed of an equilibrium
chamber, a shower head at the top, some beads to help the equilibration,
and a water drain at the bottom.

**Figures**

![](media/image1.wmf){width="5.647222222222222in"
height="6.389583333333333in"}Figure 1. Jiang et al.

Figure 2. Jiang et al.

![](media/image2.wmf){width="3.722916666666667in"
height="4.9527777777777775in"}