The Determination of Carbon Dioxide System Parameters in Aquatic Systems

Carbon dioxide (CO2) is the dominant end product of organic carbon degradation in almost all aquatic environments and its variation is often a measure of net ecosystem metabolism (Hopkinson, 1985; Smith and Hollibaugh, 1993). Therefore, in aquatic biogeochemical studies, it is desirable to measure parameters that define the carbon dioxide system. CO2 is also the most important green house gas on Earth. Its fluxes across the air-water interface are among the most important concerns in global change studies and are often a measure of the net ecosystem production/metabolism of the aquatic system.

There are four readily measurable parameters of the aquatic carbon dioxide system:

  • pH,
  • partial pressure of CO2 (pCO2),
  • total dissolved inorganic carbon (DIC), and
  • total alkalinity (TA).

Partial Pressure of CO2

Surface water partial pressure of CO2 (pCO2) can be measured by a combination of a CO2 equilibration chamber and an infrared CO2 analyzer, such as the Apollo SciTech’s AS-P2. In this method, CO2 equilibrated air is sent into the CO2 analyzer (such as the Li7000 and the Li840A in AS-P2 or the Picarro analyzer in AS-P3) for quantification against a standard gas.

The CO2 flux across the air-sea interface is calculated by the following widely used one-dimensional thin-film model (Broecker and Peng, 1982):

CO2 Flux = kT KH (pCO2w – pCO2a),

where kT is the gas transfer velocity; KH (Henry's law constant) is the solubility constant of CO2 at given temperature and salinity (Weiss, 1974). pCO2w and pCO2a represent the partial pressure of CO2 in surface water and overlaying air, respectively. Most of uncertainty in this calculation results from estimation of gas transfer velocity (kT), which is empirically derived from sea surface wind speed. The most frequently used “k vs. wind speed” relationships was provided by Wanninkhof (Wanninkhof, 1992).

Dissolved Inorganic Carbon

DIC is the total dissolved inorganic carbon and is defined as:

DIC = [CO2*] + [HCO3-] + [CO32-],
where [CO2*]=[CO2aq]+[H2CO3].

Aquatic scientists measure DIC most often by acidification of water samples and subsequent quantification of the extracted CO2 gas by a Coulometer or by an infrared CO2 analyzer (Dickson and Goyet, 1994). DIC can be measured very precisely (0.05-0.1%) and accurately (0.1- 0.2%) using a coulometer when the sample volume is not limiting (~20 mL), and Certified Standard Reference Materials (CRMs) are used for calibration purposes. Apollo SciTech’s AS-C3 DIC analyzer uses a Li-Cor 7000 CO2 analyzer and achieves a precision of 0.1% or better using only less than 1 mL of water sample (see products for more information). A great advantage of this method is that within 2 minutes, 100% of the CO2 is extracted from the water. Thus samples with a wide range of DIC values (i.e., sediment porewaters) can be measured precisely. In addition, because of the small sample volume, time series analysis can be performed on volume-limited systems (i.e., measuring respiration in culture or incubation).

Total Alkalinity

Total alkalinity (TAlk or TA) is the deficiency of H+ or the excess base with respect to the zero proton level at the CO2 equivalence point (about pH = 4.5) (Cai et al., 1998; Dickson, 1981). Alkalinity by this definition can be determined by HCl titration of the water sample to the CO2 equivalence point, the Gran titration (Gran, 1952), or by a curve fitting method. Mathematically, it is defined as

TA = [HCO3-] + 2 [CO32-] + [OH-] - [H+] + [B(OH)4-] + 2[PO4 3-] + [HPO4 2-] - [H3PO4] + [SiO(OH)3 -] + ...contributions of other minor acid or base species.

With water sample volumes greater than 20 mL, TAlk in seawater can be measured to a precision of ±0.1% or better. Certified Standard Reference Materials for alkalinity have been in use since 1996. With these standards, the reported accuracy of analyses can be ±0.1%. Apollo SciTech’s AC-A2 Alkalinity Titrator provides automated Gran titration and result calculation. It achieves a precision of 0.1% or better in laboratory conditions (see products for more information).



References

Broecker, W.S., Peng, T.H., 1982. Tracers in the Sea, 690 pp., Eldigio, Palisades, New York.
Cai, W.-J., Wang, Y., Hodson, R.E., 1998. Acid-base properties of dissolved organic matter in the estuarine waters of Georgia. Geochimica et Cosmochimica Acta, 62: 473-483.
Dickson, A., Goyet, C., 1994. DOE Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water, Version 2.
Dickson, A.G., 1981. An exact definition of total alkalinity and a procedure for the estimation of alkalinity and total CO2 from titration data, Deep-Sea Res., 28, pp. 15-21.
Gran, G., 1952. Determination of the equivalence point in potentiometric titrations. Part II. Analyst, 77: 661-671.
Hopkinson, C.S., 1985. Shallow-water and pelagic metabolism: Evidence of heterotrophy in the near-shore Georgia Bight. Mar. Biol., 87: 19-32.
Smith, S.V., Hollibaugh, J.T., 1993. Coastal metabolism and the oceanic organic-carbon balance. Reviews of Geophysics, 31(1): 75-89.
Wanninkhof, R., 1992. Relationship between wind speed and gas exchange over the ocean. Journal of Geophysical Research-Oceans, 97: 7373-7382.
Weiss, R.F., 1974. Carbon dioxide in water and seawater: The solution of a non-ideal gas. Mar. Chem., 2: 203-215.