The carbon dioxide system in estuaries - An inorganic perspective

Abstract

The thermodynamics of the carbon dioxide system in estuarine waters has been re-assessed using the carbon dioxide solubility data of Weiss (1974) and the stability constants of Hansson (1973a) and Mehrbach et al. (1973) as summarised by Millero (1979). The end-members were assumed to be air-equilibrated with a pH of 8.2 at the seaward end and pH values of 7.0, 8.0 and 9.0 for the fresh-water end-members at 15°C. The influence of temperature on the calculated pH profiles was complex since the corresponding alterations in the ionisation constants and in the solubility of carbon dioxide tend to have opposing effects on the pH. Differences as large as 0.7 pH units were noted between equilibrium pH profiles calculated for estuarine systems which were respectively closed and open to carbon dioxide exchange with the atmosphere. The mixing of river water and sea water therefore tends to produce significant disequilibrium between the estuary and the atmosphere which may manifest itself as a deficit or as an excess of carbon dioxide, depending on the pH and alkalinity of the river water. The influence of air-water exchange characteristics on the equilibration of estuarine water with the atmosphere was considered with the aid of an advective analogue of the Tamar Estuary (S.W. England). The calculations indicate that in a real estuary the pH and pCO 2 profiles will lie between the characteristics of open and closed systems and are unlikely to approach equilibrium with the atmosphere. Chemical enhancement of carbon dioxide exchange had little effect on the degree of equilibration.