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).

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 LI-7000 and the LI-840A in AS-P2 or LI-7815 and 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):

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

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-C5/C6L DIC analyzer uses a LI-COR 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). And the AS-D1 analyzer uses a Picarro CO₂ analyzer. A great advantage of this method is that within 2 minutes (or ~ 10 minutes in model D1), 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 (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

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 AS-ALK3 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