Scale-dependent patterns of hydrological, chemical, and ecological descriptors in an intensively managed Mediterranean river

ENG- This doctoral thesis uses a multiscale approach aiming to understand how climate change and anthropogenic pressures impact the hydrology of an intermittent river and how this, in turn, determines the patterns of nutrient concentrations and primary producers throughout the basin. The research combined long-term data analysis with field monitoring and hydrological modeling, taking the Onyar River basin (Girona, ES) as a case study. This basin is representative of the situation in other Mediterranean basins and provides us with insights into how they respond to the interaction between intermittency and human activities. The results of this thesis reveal that, due to climatic and anthropogenic factors, the studied river system experiences persistent low-flow conditions and an increase in nutrient loads. At the basin scale, flow contraction associated with these low flows decreases dilution capacity. Likewise, the spatial distribution of human pressures and groundwater contributions govern the cumulative export of nutrients and ecosystem functioning. The latter manifests in the generation of nutrient "hotspots" and in elevated concentrations of suspended chlorophyll. Crucially, it was demonstrated that at the sampling site scale (segment), the effects of low flows on nutrient concentrations can differ significantly from the patterns observed at the basin scale, highlighting variable responses. Consequently, scale emerges as a fundamental concept for understanding the complex response of intermittent rivers in water-scarce regions. These findings, obtained through an integrative approach, underscore that the explicit consideration of multiscale dynamics is crucial for developing more effective and sustainable water management strategies in Mediterranean basins subject to intense climatic and human pressure