Implications of diel fluctuations in streamflow on stream metabolism

Stream metabolic activity (e.g., respiration, primary production, or nutrient uptake) is highly influenced by hydrologic conditions, not only because they determine the supply rate of reactive solutes to the biota but also because they determine habitat availability. The conventional method to calculate stream metabolism (i.e., the open-channel method) requires the measurement of the atmosphere-stream gas exchange rate, discharge, travel time, and the wetted channel area, in addition to continuous measurement of dissolved oxygen concentration and water temperature.  All of these except oxygen and temperature are normally assumed to be constants. We hypothesize that transient effects might be important in small streams with a strong diel streamflow signal and that they become more relevant during baseflow conditions, when diel fluctuations in streamflow are more conspicuous. To test this hypothesis we measured ecosystem respiration and gross primary production using the open-system, two-station diel approach under different discharge conditions. Additionally, we performed injections of a metabolically-active tracer (resazurin which turns to resorufin in the presence of aerobic respiration), and a conservative tracer (NaCl). The study was conducted in WS01, a steep, 2nd-order stream located in the H.J. Andrews Experimental Forest in the western Cascade Mountains of Oregon.  This stream shows a strong diel variation in discharge, particularly in summer. Results show that metabolic rates during summer exhibit daily fluctuations that do not always coincide with the fluctuations in streamflow.  Additionally, ignoring the effects of diel fluctuations in discharge on water velocity and wetted channel area may lead to inaccurate estimation of metabolic rates using the open-channel method, especially during summer.