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Metabolism is an integrative measurement of stream and river ecosystem functioning, and thus, could be used to assess impairment. Stream metabolism is measured by different methods which often yield contrasting results. Furthermore, open-channel measurements of metabolism, which offer the best potential for continuous monitoring of stream functioning, rely on calculations of gas exchange with the atmosphere, for which a plethora of methods exists. Therefore, to incorporate metabolism in stream monitoring programs, it is necessary to determine which methods yield comparable results under a given set of environmental conditions. We studied 21 streams in the Basque Country (northern Spain), ranging widely in physical characteristics and water quality. We calculated reaeration during summer baseflows using three different approaches: the night-time drop in oxygen, the lag between noon and peak oxygen concentration, and ten empirical equations relating depth and velocity with reaeration coefficients obtained from the literature. Differences among methods were very large, especially at the shallower sites. The results obtained with most empirical equations were highly correlated, but showed little agreement with the night-time and peak lag methods. We then analyzed the response of reaeration rate to river stage: reaeration calculated by the nighttime method during 1 year of continuous monitoring was regressed against discharge at each site, and the resulting model was compared to the results of empirical equations, using software HecRas 2.2 to model hydraulic conditions at different river stages. The shape of reaeration-discharge plots differed greatly and in a sitedependent manner, and there was little agreement between methods. Finally, we investigated the effects of reaeration rate on estimates of metabolism. The choice of method greatly affected the estimates of both primary production and respiration. The empirical equations, except E 7 and E 10, yielded the most unrealistic estimations of stream metabolism. Overall, the night-time method, especially when regressed against discharge, seems to be the most robust and reliable among those tested, with the energy dissipation method (E 10 ) appearing to be a viable alternative when the nighttime method does not work.

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). Sampling sites showed large differences in cover, conductivity and nutrient contents, as well as some inter-annual differences, resulting from changes in discharge and in human activities. Most environmental variables showed low seasonality, main variations being produced by floods. Solar radiation changed little during 2001 -2002, because of the coincidence of a sunny winter followed by a cloudy summer, and seasonal differences in riparian cover resulted in more sunlight reaching the streambed of closed sites during winter, and small differences at the open ones. The growth and abscission of riparian leaves results in a strong light regime at the closed sites, with more sunlight during winter, but this pattern changes from year to year, depending on the weather. Spatial differences and highest values of periphytic biomass were much lower than those measured in 1990 -91, because of contrasting weather patterns. Variables controlling periphytic biomass in the Agüera change from reach to reach. At open sites flow is the main temporal controller, whereas at closed sites the effects of light availability prevail, thus giving more similar seasonal patterns from year to year. Nutrients seem to play a minor role in temporal variations.