Assessment of Water Quality Regulation Functions in Southwestern Europe Watersheds

Estimation at large scale of the water quality regulation services is still lacking. It is essential to develop methodological approaches to quantify nutrient-related functions' distribution. The present study aims to quantify nitrate-related ecological functions through nitrate net balance (NNB), nitrate removal (NR), and nitrate production (NP). This study explores the spatiotemporal dynamics of these indicators in South-Western Europe (SUDOE, 216 subsystems over 81 basins) at a monthly scale from 2000 to 2010. We use the Soil and Water Assessment Tool model to simulate nutrient transfer at the subsystem scale (~3000 km(2)) and calculate ecological functions. The modeled NNB is validated at the subsystem scale by comparing with NNB predicted at the water body scale (~60 km(2)) over the Garonne watershed (France). Hot spots of NR are located in the south of SUDOE, characterized by a warmer and dryer climate, whereas NP hot spots are located in the most anthropized streams. The mean NNB (the balance between NP and NR) at the subsystem scale for the SUDOE territory reaches -3.5 and -2.8 gN m(-2) day(-1) during the most active seasons. The results highlight drivers influencing NR such as streamflow, river slope, and hydrological alteration. Getting an overview of where and when these nitrate regulation functions (NP and NR) occur is essential for socio-ecosystem sustainability and this study highlights its sensitivity to anthropogenic stressors.