DIURNAL VARIATION IN DISSOLVED OXYGEN AND CARBON DIOXIDE IN TWO LOW-ORDER STREAMS HELENA GUASCH 1 *, JOAN ARMENGOL 1 , EUGE Á NIA MARTõÂ 2 and SERGI SABATER 1 1 Departament d'Ecologia, Fac. Biologia, Universitat de Barcelona, Avda. Diagonal 645, 08028 Barcelona and 2 Department of Zoology, Arizona State University, Tempe, AZ 85287-1501, U.S.A. (First received October 1995; accepted in revised form August 1997) AbstractÐThis 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 Ca 2+ 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 high. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐcalcareous, carbon dioxide, diurnal variation, Mediterranean, oxygen, primary production, siliceous, stream INTRODUCTION Based on the principle that photosynthesis, respir- ation and reaeration are responsible for the diurnal ¯uctuations of oxygen in streams (Odum, 1958), diel curves of dissolved oxygen have been used to measure community metabolism in many ¯uvial systems (e.g. Hoskin, 1959; Edwards and Owens, 1962; Prophet and Ransom, 1974; Fisher and Carpenter, 1976; Roos and Pieterse, 1992; Marzolf et al., 1994). Other studies used the same principle and measured CO 2 change to determine metabolic rates (i.e. Wright and Mills, 1967). Studies which include measurements of both gases in ¯uvial sys- tems are less abundant. Odum (1957) estimated the primary productivity of Silver Springs (Florida) using diurnal curves of dissolved oxygen and car- bon dioxide, and obtained similar values for the metabolism of both. However, this author observed that carbon dioxide and oxygen curves showed dierent patterns of diurnal variation during some periods, suggesting that carbon dioxide may be ®xed after sunset by dark reactions. Simonsen and HarremoeÈs (1978) obtained in the Havelse river (a rather polluted stream), lower values of oxygen pro- duction than carbon dioxide ®xation, and found that reaeration coecients for carbon dioxide were lower than those for dissolved oxygen. Investigations in the Gryde River, which is macro- phyte-dominated, revealed good agreement between the measurements of productivity based on diurnal variation in CO 2 and O 2 (Kelly et al., 1983) and similar values of the coecients of reaeration were also found (Thyssen and Kelly, 1985). Measurements of the rate exchange of oxygen and carbon dioxide between the water and the atmos- phere in the Amazon river (Richey et al., 1990) agree with the results obtained in the Gryde River. In this case, the mean invasion rate of oxygen essentially mirrored the mean evasion of carbon dioxide. Based on these results, it seems that in ¯owing waters, similar but opposite behaviour of the rate of change of the two gases should be expected. Most studies reported were performed in reaches with high productivity and low reaeration rates. However studies focused on the reaeration rates and diurnal variation of carbon dioxide and dissolved oxygen in turbulent low-order streams, are more scarce. Wanninkho et al. (1990) per- Wat. Res. Vol. 32, No. 4, pp. 1067±1074, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00 PII: S0043-1354(97)00330-8 *Author to whom all correspondence should be addressed [Fax: (343) 411 1438; E-mail: hguasch@porthos.- bio.ub.es]. 1067