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).
Some processes that increase the CO2 concentrations of coastal waters include:
Some processes that decrease the CO2 content of coastal waters include:
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 .
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.
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 .
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 [3,7]. 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 .
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 . 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 [2,5,6].
Phillip Ford, CSIRO Land & Water