Diurnal variation in dissolved oxygen and carbon dioxide in two low-order streams

Abstract

This investigation aimed to establish the contribution of biological activity, physical and chemical processes, to the diurnal variation in dissolved oxygen and carbon dioxide in undisturbed streams. Changes in both gases were monitored during four surveys of 24 h duration in 1993 in two sec- ond-order Mediterranean streams: Riera Major, a siliceous shaded stream and La Solana, a calcareous open stream. In the latter, diurnal changes in carbon dioxide and dissolved oxygen showed similar, clear daily pattern when water discharge was low and algal activity high, indicating that although reaeration was high, changes in the two gases were greatly in¯uenced by stream metabolism. In ad- dition, diurnal changes in Ca2+ and DIC (dissolved inorganic carbon) also showed a clear daily pat- tern, indicating that the precipitation of calcium carbonate was directly or indirectly a ected by biotic processes. On the other hand, no relation between the two gases was observed in Riera Major, where turbulence was higher. In this stream, the concentration of dissolved oxygen was greatly a ected by reaeration, but changes in carbon dioxide showed a clear daily pattern. The lack of agreement between the diurnal variation of the two gases observed in this stream may be explained by the di erence in solubility, which led to marked di erences in the exchange coe cient rate when turbulence is hig