An integrated assessment of groundwater controls on streamflow and ecosystem functioning in the Guarani aquifer outcrop zone

Groundwater–surface water interactions sustain river ecosystems and regulate biogeochemical processes. However, despite growing interest in hyporheic zone functioning, interdisciplinary assessments remain scarce in tropical regions. Here, we present a scalable interdisciplinary framework to evaluate groundwater influences on streamflow and ecosystem functioning at the reach scale in the Guarani Aquifer System (GAS) outcrop area, São Paulo, Brazil. The approach combines (i) temperature as a natural tracer of groundwater discharge, (ii) physicochemical and nutrient analyses, (iii) macroinvertebrates as bioindicators, and (iv) cotton-strip assays to estimate organic matter decomposition. Two years of quarterly monitoring were conducted across three creek reaches (S1, S2, and S3) with contrasting degrees of groundwater connectivity. The reach with stronger groundwater influence (S2) exhibited significantly lower thermal variability than the upstream (S1) and downstream (S3) reaches, whereas no significant differences in phase lag were detected. Nutrient concentrations showed contrasting longitudinal patterns between S1–S2 and S2–S3, particularly for NH 4 + -N. Furthermore, S2 had the highest macroinvertebrate diversity, and the highest organic-matter decomposition rates, with a significant difference between S2 and S1, whereas biofilm respiration was higher in S3. The overall consistency among indicators suggests that river-aquifer interactions not only sustain streamflow but also regulate relevant biological communities, ecosystem functions and biogeochemical processes. Although spatially limited relative to the overall extent of the GAS, its outcrop areas are critical recharge zones and particularly susceptible to anthropogenic contamination. The proposed approach may be adapted to other local creeks in GAS outcrop area, or tropical regions with similar climate and hydrological conditions