What is the partial pressure of carbon dioxide?
The world’s oceans readily exchange carbon dioxide (CO2) with the atmosphere. The CO2 dissolves in water to an extent determined by its partial pressure and the chemical reactions of the dissolved carbon dioxide with other solutes. The partial pressure of carbon dioxide (PCO2) is the gas phase pressure (i.e. in the air above a waterway) of carbon dioxide which would be in equilibrium with the dissolved carbon dioxide.
Figure 1. (a) PCO2 changes with distance upstream in the Mary River Queensland (data from Andrew Moss, Qld EPA). (b) Seasonal changes in PCO2 at a site in Wilson’s Inlet WA, a semi-closed estuary with significant anthropogenic nutrient and organic inputs (data: David Fredericks, GA).
What causes water column carbon dioxide to change?
Some processes that increase the CO2 concentrations of coastal waters include:
- the dissolution of atmospheric CO2 (i.e. exchange between air and sea) is affected by the PCO2 differential between the atmosphere and coastal water body (i.e. Henry’s Law), wind speed and water temperature. An increase in the CO2 concentration of the atmosphere (due to global warming) directly leads to an increase in the amount of CO2 absorbed by the oceans. This is called ocean acidification and it is a very serious and topical issue.
- the decomposition of organic matter by processes including oxygen reduction, nitrate reduction (a.k.a. denitrification), iron or manganese reduction and sulfate reduction;
- the precipitation of calcium carbonate (calcite or aragonite): e.g. Ca2+ + 2HCO3- = CaCO3 + H2O +CO2; and
- lowering of water temperature((Copin-Montegut, C., 1988. A new formula for the effects of temperature on the partial pressure of CO2 in seawater. Marine Chemistry 25, 29-37.)).
Some processes that decrease the CO2 content of coastal waters include:
- degassing of CO2 to the atmosphere (exsolution);
- he photosynthetic consumption of CO2;
- the dissolution of calcium carbonate (calcite or aragonite): e.g. CaCO3 + H2O +CO2 = Ca2+ + 2HCO3-;
- chemical weathering of alumino-silicate minerals; and
- increasing water temperature((Copin-Montegut, C., 1988. A new formula for the effects of temperature on the partial pressure of CO2 in seawater. Marine Chemistry 25, 29-37.)).
Significance of water column partial pressures of carbon dioxide
Water column PCO2 measurements provide a relative measure of trophic status because there is a delicate balance between the capacity of a coastal waterway to decompose organic matter coming in from the catchment, and its capacity to take up carbon dioxide through the process of photosynthesis. This balance between CO2 production (during decomposition) and CO2 assimilation (by photosynthesis) can be assessed by comparing the actual dissolved CO2 concentration of a sample with the concentration expected if it was in equilibrium with atmospheric carbon dioxide (e.g. Patmos = 360 uatmos (or ppm) and rising). If a coastal waterway (or section of a coastal waterway) consumes more organic carbon than it produces (e.g. PCO2 < Patmos), it is considered ‘net autotrophic’. By comparison, if a coastal waterway (or section of a coastal waterway) produces more organic carbon than it consumes, it will be over saturated with respect to the atmospheric value (e.g. PCO2 > Patmos), and is ‘net heterotrophic’. Typically, coastal waterways which are subject to human inputs of readily degradable organic matter (discharged through stormwater and sewage outfalls) usually fall into this category. In comparison, near-pristine estuaries are more likely to have PCO2 concentrations that are less than or equal to atmospheric values.
Phytoplankton species have different sensitivities to the amount of available CO2 and this might influence the succession and distribution of bloom-forming species((Rost, B., Riebesell, U., and S. Burkhardt. 2003. Carbon acquisition of bloom-forming marine phytoplankton. Limnology and Oceanography 48(1), 55-67.)).
Reactions that produce CO2 and/or carbonic acid consume free protons and affect water column pH. The presence of carbon dioxide in water contributes to the degree of hardness because it influences calcium carbonate solubility and therefore dissolved calcium concentrations.
Considerations for measurement and interpretation
PCO2 can be calculated from any two of the parameters (pH, Total Alkalinity and Total Inorganic CO2 (TCO2) measured simultaneously on a water sample together with the water temperature and salinity. Measurement of pH in marine waters is complicated by inherent uncertainties caused by poorly defined effects of the salinity on the activity coefficient of the hydrated proton. Recent work has shown that use of the NBS scale for pH together with Total Alkalinity (determined by Gran titration) produces results very similar to direct measurement of PCO2((Frankignoulle, M. and A. V. Borges, 2001. Direct and indirect PCO2 measurements in a wide range of PCO2 and salinity values (The Scheldt Estuary). Aquatic Geochemistry 7, 267-273.)).
The direct method for measuring PCO2 involves equilibrating air (or another carrier gas) with water and then measuring the PCO2 of the equilibrated air by either gas chromatography or infra-red spectroscopy((DOE ( 1994 ). Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water (Eds: A. G. Dickson and C. Goyet ). ORNL/CDIAC-74)))((Körtzinger, A., H. Thomas, B. Schneider, N. Gronau, L. Mintrop, and J.C. Duinker ( 1996 ). At-sea intercomparison of two newly designed underway pCO2 systems -encouraging results.Marine Chemistry 52:133-145.)). Some of the problems due to turbidity and the wide range of PCO2 found in actual systems and noted in these two papers, have recently been overcome((Frankignoulle, M., Borges, A.V. and R. Biondo, 2001. A new design of equilibrator to monitor carbon dioxide in highly dynamic and turbid environments. Water Research 35/5, 1344-1347.)).
Existing information and data
A Program Developed for CO2 System Calculations gives all the formulae and FORTRAN programs for calculating PCO2 using a very carefully evaluated set of constants together with a detailed overview((Lewis, E., and D. W. R. Wallace. 1998. Program Developed for CO2 System Calculations. ORNL/CDIAC-105. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee. (http://cdiac.ornl.gov/oceans/co2rprt.html))). There is also a thorough discussion of the sources of error and the uncertainties.
There is only very limited published data on PCO2 content in Australian estuaries but the general pattern of high PCO2 at the head of an estuary that then decreases toward the mouth (Figure 1) is commonly observed here and in America and Europe. Also evident, is a clear seasonality in PCO2 concentrations, reflecting heightened biological activity in summer months, and the assimilation of CO2 from anthropogenic organic loadings. Studies on the Scheldt, the Elbe, and the York River estuaries respectively provide good examples of how PCO2 concentrations have been used overseas in the assessment of coastal waterway health ((Frankignoulle, M. and A. V. Borges, 2001. Direct and indirect PCO2 measurements in a wide range of PCO2 and salinity values (The Scheldt Estuary). Aquatic Geochemistry 7, 267-273.))((S. Brasse, Nelle, M., Seifert, R. and W. Michaelis, 2002. The carbon dioxide system in the Elbe estuary. Biogeochemistry 59, 25-40.))((P. A. Raymond, Bauer, J.E., and J. J. Cole, 2000. Atmospheric CO2 evasion, dissolved inorganic carbon production, and net heterotrophy in the York River estuary. Limnology and Oceanography 45, 1707-1717.)).
Phillip Ford, CSIRO Land & Water