Green light: gross primary production influences seasonal stream N export by controlling fine-scale temporal N dynamics

Abstract:

The contribution of in-stream processes to nitrogen (N) exports remains unclear because stream water chemistry integrates biogeochemical processes occurring at different spatial and temporal scales within upland, riparian and aquatic ecosystems. Monitoring nutrient concentrations at fine-scale temporal resolution can provide insights on in-stream N processing. Yet, the mechanisms underlying fine-scale temporal nutrient dynamics and their implications for flux budgets at catchment scale are still poorly understood. Here, we investigated patterns and controls of diel variation in stream nitrate (NO₃^-) concentration and the influence of these on N fluxes along a stream continuum with increasing riparian area and channel width. We tested two alternative hypotheses: that diel variation in stream NO₃^- concentration is driven (a) by stream metabolism, or (b) by riparian groundwater inputs.

Although productivity in this stream was extremely low, patterns of diel variation in stream NO₃^- concentration emerged in early-spring and they were highly correlated with the daily regime of irradiance and gross primary production (GPP). In addition, diel NO₃^- patterns agreed with those predicted from GPP and biofilm C:N ratios, but not with diel variations predicted from changes in riparian groundwater inputs. These results point at in-stream photoautotrophic N uptake as the most likely driver of diel fluctuations in stream NO₃^- concentration. From a network perspective, the occurrence of diel variations in stream NO₃^- concentration, and thus their influence on stream N fluxes, increased along the stream continuum likely because of increases in light inputs and water temperature. This study evidences that monitoring of nutrient concentrations at fine-scale temporal resolution can provide mechanistic explanations about the relevance of in-stream and terrestrial processes on regulating stream N dynamics and their contribution to N export at catchment scale.