The measurement of the partial pressure of CO2 is carried by equilibration using an equilibrator designed for coastal environments: it can work without clogging in very turbid waters and has a fast equilibration time. The half-life time constant of equilibration is 30 seconds and 99% of equilibrium achieved in less than 2 min (Frankignoulle et al. 2001, Water Research, 35:1344-1347). The equilibrator is a Plexiglas cylinder (height: 80 cm, diameter: 10 cm) filled with marbles to increase exchange surface area. Pumped sample water runs (3 L min-1) from the top to the bottom of the equilibrator and air is pumped upwards (3 L min-1). The protocol was recently improved to allow measurements up to 10,000 ppm (Frankignoulle and Borges, 2001, Aquatic Geochemistry 7:267-273).
The measurements of CO2 can be carried out:
Recent improvements focused in making a measuring device as compact and as portable as possible.
The air-water CO2 are measured using the floating chamber method.
chamber is a plastic right circular cone (top radius = 49 cm; bottom radius
= 57 cm; height = 28 cm) mounted on a float, and, connected to a closed
air circuit with an air pump (3 L min-1) and an Infra-Red Gas
Analyser, both powered with a 12 Volts battery. Measurements are made from
a drifting rubber boat, in order to avoid the interference of water turbulence
within the chamber created by the passing water current, observed in earlier
measurements carried out from a fixed point. The floating chamber technique
provides gas transfer velocity estimates at short time scales (minute),
compared to the tracer mass balance approaches (hour to day). This is critical
in estuarine environments where short term processes such as tidal currents
can significantly contribute to the gas transfer velocity.
Recently, air-ice CO2 fluxes were carried out on the Antartic sea-ice in collaboration with the Laboratoire de Glaciologie of the ULB
pH is measured with a combined electrode either continuously or on discrete samples in the field or in the laboratory. Two pH scales are used. In fresh water or estuarine environments the electrode is calibrated on the National Bureau of Standards (NBS) scale according to Frankignoulle and Borges (2001, Aquatic Geochemistry 7:267-273). In seawater, the electrode is calibrated on the Total Hydrogen Ion Concentration Scale using the buffers prepared according to Dickson (1993, Deep-Sea Research 40:107-118).
TAlk is measured by Gran electrotitration. From the measurements of TAlk and pH, DIC and pCO2 can be computed. The agreement of the pCO2 values computed from pH-TAlk and those measured by equilibration is usually excellent both in estuarine and seawater conditions.
O2 is measured on discrete samples with a polarographic electrode or by the Winkler method with a potentiometric end-point determination, or, continuously using a polarographic electrode. The distribution of O2 allows to determine if biological activity is the main controlling factor of dissolved inorganic carbon. The diel variation of O2 allows to compute the rates of metabolic processes in the mixed layer.