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

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 second-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 influenced by stream metabolism. In addition, diurnal changes in Ca2+ and DIC (dissolved inorganic carbon) also showed a clear daily pattern, indicating that the precipitation of calcium carbonate was directly or indirectly affected 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 affected 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 difference in solubility, which led to marked differences in the exchange coefficient rate when turbulence is high.