CRUISE FICARAM II

PI: Aida F. Ríos and Fiz F. Pérez

R/V Hespérides

Cruise: He073

Dates: 5 March to 17 April 2001

Chief scientist: Aida F. Rios (IIM-CSIC)

Nutrients analysis responsible: Carmen G. Castro (IIM-CSIC)

During the cruise FICARAM II, 29 stations were performed (Figure 1).
Alkalinity and pH were measured on board. The variables of carbon
system, pH and alkalinity, in surface waters together with the fCO~2~
measured, were used to study the internal consistency of the
measurements.

**pH** was measured spectrophotometrically following Clayton and Byrne
(1993).

**Total Alkalinity (TA)** was measured using potentiometric titrations
with hydrochloric acid to a final pH of 4.40 (Pérez and Fraga, 1987).
The electrodes were standardised using a buffer of pH 4.42 made in CO2
free seawater (Pérez et al., 2002).

**Dissolved oxygen** was determined by Winkler potentiometric titration.
The estimated analytical error was 1 µmolkg^-1^. Oxygen saturation was
calculated following Benson and Krause equation (UNESCO, 1986).

**Nutrient** samples were frozen before the analysis. Nutrient
concentrations were determined by segmented flow analysis with Alpkem
system, following Hansen and Grasshoff (1983) with some improvements
(Mouriño and Fraga, 1985; Álvarez-Salgado *et al*., 1992). The
analytical error was 0.05 µmolkg^-1^ for nitrate, 0.05 µmolkg^-1^ for
silicic acid and 0.01 µmolkg^-1^ for phosphate.

**CT** was calculated from pH~T25~ and total alkalinity with the
Mehrbach et al (1973) constants refitted by Dickson and Millero (1987).

To check the accuracy of the pH and TA measurements, samples of CO~2~
reference material (CRM, batch 41 and 51 distributed by A.G. Dickson
from the Scripps Institution of Oceanography) were analysed during the
cruises. The uncertainties of pH and alkalinity were ±0.002 and ±1.4
µmol kg^-1^, respectively. The corresponding theoretical pH~T25~ value
(7.918) for this batch was calculated using the dissociation constants
from Lueker et al. (2000), which was in agreement with the theoretical
value (±0.0002).

To check de internal consistence of the carbon measurements the CO~2~
fugacity (*f*CO~2~) values measured at every station sampled during
FICARAM II cruise were compared with those calculated from pH~T25~ and
total alkalinity with the Lueker et al. (2000) dissociation constants.
The agreement between both *f*CO~2~ was high (r^2^ =0.94), confirming
the good internal consistency of our measurements. The average and
standard deviation of the differences between both calculated and
measured CO~2~ was -2.8 ±8 µatm.

![](media/image1.wmf){width="3.275in" height="3.683333333333333in"}

*FIGURE 1- Section of the cruise FICARAM II*

**References**

Alvarez-Salgado X.A., F. Fraga and F.F. Perez, 1992: Determination of
nutrient salts by automatic methods both in seawater and brackish
waters: The phosphate blank. *Marine Chemistry*, 39, 311-319.

Clayton, T.D., R.H. Byrne. 1993. Spectrophotometric seawater pH
measurements: total hydrogen ion concentration scale concentration scale
calibration of m-cresol purple and at-sea results. *Deep-Sea Res. I*,
Vol. 40, 10, 2115-2129.

Dickson, A.G., Millero, F.J., 1987. A comparison of the equilibrium
constants for the dissociation of carbonic acid in seawater media. *Deep
Sea Res*. 34, 1733-1743.

Hansen, H.P. and Grasshoff, K. Automated Chemical Analysis. In: K.
Grasshoff, M. Ehrhardt and K. Kremlig (Editors), *Methods of Seawater
Analysis*. Verlag Chemie, Weinheim, 419 pp., 1983.

Lueker, T.J., Dickson, A.G., Keeling, C.D., 2000. Ocean pCO~2~
calculated from dissolved inorganic carbon, alkalinity and equations for
K1 and K2: validations based on laboratory measurements of CO2 in gas
and seawater at equilibrium. *Mar. Chem.*, 70, 105-119.

Mehrbach, C., Culberson, C.H., Hawley, J.E., Pytkowicz, R.M., 1973.
Measurement of the apparent dissociation constants of carbonic acid in
seawater at atmospheric pressure*. Limnology and Oceanography*, 18,
897-907.

Mouriño C., F. Fraga, 1985: Determinacion de nitratos en agua de mar.
*Investigacion Pesquera*, 49, 81-96.

Pérez, F.F., Fraga, F., 1987. A precise and rapid analytical procedure
for alkalinity determination. *Marine Chemistry*, 21, 169-182.

Pérez, F.F., A.F. Ríos, T. Rellán, M. Álvarez. 2000. Improvements in a
fast potentiometric seawater alkalinity determination. *Ciencias
Marinas*, 26, 463-478.