Sampling points along the track St.Nazaire-Vigo

For calibrating the different sensors of our unattending system underway sampling for pH, alkalinity, nutrient, oxygen, salinity and chlorophyll are also collected at to 12 locations along the transit every two cruises. Additionally, nutrient samples are also collected.

pH is measured spectrophotometrically following Clayton and Byrne (1993). Roughly, this method consists on adding a dye solution to the seawater sample, so that the ratio between two absorbancies at two different wavelengths is proportional to the sample pH. The indicator was a solution of m-cresol purple prepared in seawater. After sampling collection, the batch is stabilised at 25ºC. All the absorbance measurements are obtained in the thermostatted cell compartment of a BECKMAN spectrophotometers. The accuracy of pH estimated using CO2 certified reference material (CRM), is ± 0.002.

Alkalinity is determined by automatic potentiometric titration with HCl at a final pH of 4.40 (Pérez and Fraga, 1987b). The electrodes are standardised using a buffer of pH 4.42 made in CO2 free seawater (Pérez et al., 2002). This method has an accuracy of ±1 µmol/kg estimated using CRMs.

Nutrient samples are stored in the freezer prior to lab analysis. Nutrient concentrations are determined by segmented flow analysis with Alpkem systems, following Hansen and Grasshoff (1983) with some improvements (Mouriño and Fraga, 1985; Álvarez-Salgado et al., 1992). The analytical error is ±0.05 µmol/kg for nitrate, ±0.05 µmol/kg for silicic acid and ±0.01 µmol/kg for phosphate.

Dissolved oxygen is determined by Winkler potentiometric titration. The estimated analytical error is ±1 µmol/kg. Oxygen saturation is calculated following Benson and Krause equation (UNESCO, 1986).

Chlorophyll a is measured using 90% acetone extraction in a 10,000 R Turner fluorometer (Yentsch and Menzel, 1963). The precision was ±0.05 mg/m3.