Monitoring nutrient concentrations at fine-scale temporal resolution contributes to a better\ud understanding of nutrient cycling in stream ecosystems. However, the mechanisms underlying\ud fine-scale nutrient dynamics and its implications for budget catchment fluxes are still poorly\ud understood. To gain understanding on patterns and controls of fine-scale stream nitrogen (N)\ud18 dynamics and to assess how they affect hydrological N fluxes, we explored diel variation in\udstream nitrate (NO3-) concentration along a headwater stream with increasing riparian area and\ud channel width. At the down-stream site, the highest day-night variations occurred in early-spring\udwhen stream NO3- concentrations were 13% higher at night than during day time. Such day-night\ud variations were strongly related to daily light inputs (R2=0.74) and gross primary production\ud(GPP) (R2=0.74), and they showed an excellent fit with day-night NO3- variations predicted from\udGPP (R2=0.85). These results suggest that diel fluctuations in stream NO3- concentration were\ud mainly driven by photoautotrophic N uptake. Terrestrial influences were discarded because no\ud simultaneous diel variations in stream discharge, riparian groundwater level, or riparian solute\udconcentration were observed. In contrast to the down-stream site, no diel variations in NO3-\ud concentration occurred at the up-stream site likely because water temperature was colder (10 vs.\ud12 ºC) and light availability was lower (4 vs. 9 mol m-2 d-1). Although daily GPP was between\ud 10-100 folds lower than daily respiration, photoautotrophic N uptake contributed to a 10%\udreduction in spring NO3- loads at the down-stream site. Our study clearly shows that the activity\ud of photoautotrophs can substantially change over time and along the stream continuum in response to key environmental drivers such as light and temperature, and further that its capacity\ud to regulate diel and seasonal N fluxes can be important even in low productivity streams.
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