DIURNAL-VARIATION IN DISSOLVED-OXYGEN AND CARBON-DIOXIDE IN 2 LOW-ORDER STREAMS

This investigation aimed to establish the contribution of biological a ctivity, 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 199 3 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 h igh, indicating that although reaeration was high, changes In the two gases were greatly influenced by stream metabolism. In addition, diurn al changes in Ca2+ and DIC (dissolved inorganic carbon) also showed a clear daily pattern, indicating that the precipitation of calcium carb onate was directly or indirectly affected by biotic processes. On the other hand, no relation between the two gases was observed in Riera Ma jor, where turbulence was higher. In this stream, the concentration of dissolved oxygen was greatly affected by reaeration, but changes in c arbon dioxide showed a clear daily pattern. The lack of agreement betw een the diurnal variation of the two gases observed in this stream may be explained by the difference in solubility, which led to marked dif ferences in the exchange coefficient rate when turbulence is high. (C) 1998 Elsevier Science Ltd. All rights reserved.