Dissolved carbon dioxide and oxygen in the River Thames: Spring–summer 1997

Data on excess carbon dioxide partial pressures (EpCO2) and oxygen saturations in the River Thames 3 km upstream of Wallingford, Oxfordshire, are presented and related to general water quality. The data, which cover the period for the spring and summer of 1997, relate to: (1) weekly-spot sampling for determination of major, minor and trace elements, pH and alkalinity; and (2) continuous measurements of pH linked to the weekly alkalinity data. (Throughout the period major cation, major anion and alkalinity concentrations are relatively constant and the greatest changes in water quality are determined by the biological fluctuations changing the pH and dissolved oxygen levels). Large variations in EpCO2 occur, ranging between 0.3 and 11.5 times the atmospheric value. During April–May time, relatively high pH (8.7–9.4) and oversaturation with respect to dissolved oxygen (100–175% saturation) and relatively low EpCO2 of 0.3 to 2 times the atmospheric value, and low dissolved silica concentrations of <0.5 mg-Si/l occurs. This probably reflects a period of high photosynthetic activity. Later in the year, pH is about 0.7 lower and the water is approximately saturated with respect to dissolved oxygen (80–120%) and this probably reflects a period of lower photosynthetic activity. pH shows a negative relationship with EpCO2 and strong diurnal fluctuations in pH, dissolved oxygen and EpCO2 occur throughout the spring and summer as the balance between photosynthesis and respiration changes under differing light conditions. pH shows the highest and dissolved oxygen the lowest levels during the day and the opposite pattern is observed during the night in line with the changeover from net photosynthesis to net respiration. The largest variations occur during the spring period when photosynthetic activity is at its highest. The biological activity in combination with aeration not only determines the EpCO2 level but the pH as well and EpCO2 is directly proportional to the hydrogen ion activity. Thermodynamic analysis of the water quality data reveals that the waters vary between oversaturation and undersaturation with respect to the mineral calcite (CaCO3) although there is no evidence for active precipitation or dissolution. There is an approximate transition from undersaturation to oversaturation with respect to calcite around pH 8 and EpCO2 of seven times the atmospheric value with oversaturation occurring at higher pH and lower EpCO2. On a daily basis, EpCO2 is negatively and linearly correlated with the percentage of dissolved oxygen saturation in line with changing patterns of CO2 and O2 gaseous exchange during photosynthesis, respiration and transfers to and exchange with the atmosphere. EpCO2 and pH averages and ranges lie between those observed for the more pristine and the industrially polluted eastern UK rivers. The values are similar to those for eastern UK catchments with low but significant sewage inputs.