Assessment of ecological function indicators related to nitrate under multiple human stressors in a large watershed

Important ecological functions indicators, such as nitrate net balance (NNB), including nitrate removal (NR) and nitrate production (NP), seem to be impacted by human stressors and climate change, particularly during low flow periods associated with high nitrate pollution. NR is induced by in-stream processes such as nitrate uptake, denitrification, and transient storage. These processes are usually investigated by means of in-situ measurements, but the evaluation on a large scale is not yet developed. Hence, the objectives of this study are (i) to validate a model that quantifies NR indicator in a reach at a monthly time step in the Garonne watershed; (ii) use this model to quantify NR in all reaches at watershed scale; (iii) to analyze NR spatially and temporally at the reach scale, and finally (iv) to identify drivers influencing NR patterns. We used the Soil and Water Assessment Tool (SWAT) model to simulate the impact of human stressors such as land management and municipalities effluents on in-stream nitrogen cycles. The results show the seasonal variation of NR and NP ranging between −1.77 gN.m−2.d−1 and +1.62 gN.m−2.d−1. NR is stronger during the spring and summer periods (median of −0.1 gN.m−2. d−1). The hot spots of NP are located downstream in the main rivers whereas NR strongly occurs in small reaches of lowlands and intermediate streams defined by a Strahler order between 3 and 5 and a slope under 0.5%. NR is stronger in hillsides areas such as forests, wetlands and surprisingly agricultural areas and NP increases down in the municipalities due to the effluents. The spatio-temporal variability of NR makes the validation of the model for reaches with Strahler number more difficult than the measured one. However, this study shows that the NNB dynamics in time and space depend on a combination of influencing factors (slope, discharge and hydraulic condition explained by Froude number and nitrate concentration). Investigating a relationship between