Diurnal and longer term patterns in carbon dioxide and calcite saturation for the River Kennet, south-eastern England.

Continuous pH, dissolved oxygen and temperature information for three sites on the upper portion of the River Kennet, a tributary of the Thames in southern England is presented. A thermodynamic model is developed which links this information to weekly water quality data allowing the estimation of dissolved carbon dioxide and calcite saturation levels within the river on a continuous basis. The results show a dynamic and variable system along the length of the river. Marked diurnal patterns are observed for pH, dissolved oxygen, dissolved carbon dioxide and calcite saturation and the highest fluctuations occur during the summer months. There is also a marked gradient downstream in average pH (7.5-8.0) and dissolved carbon dioxide (30-10 times atmospheric pressure). Dissolved oxygen levels average approximately 100% saturation across all sites, and the diurnal pattern occurring during the summer months declines downstream. The waters are permanently oversaturated with respect to calcite and the diurnal patterns of change are maximal during the summer months. The results are discussed in relation to: (a) the balance between photosynthesis and respiratory processes; (b) the relative interchanges between the sediment/plant interface with the water column; and (c) the rates of degassing of carbon dioxide and oxygen between the river surface and the atmosphere.