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Tropical stream ecosystems are under increasing human pressure, making the development of effective restoration approaches and expanding knowledge in this field urgent. This study evaluated the impact of riparian vegetation restoration and environmental context on stream ecosystem functioning by measuring key ecosystem functions - gross primary production (GPP), ecosystem respiration (ER), and nutrient uptake of ammonium and soluble reactive phosphorus - across ten tropical streams in southeastern Brazil. The streams represented a gradient from clearcut areas (impacted reaches) to relatively pristine conditions (reference reaches), including intermediate stages of vegetation recovery (restored reaches). In the short-term (~15–20 years after restoration), restoration led to reduced GPP akin to reference reaches. Yet, ER did not show the anticipated increase, suggesting a longer timeframe is necessary for restored streams to emulate the functional characteristics of reference reaches. Additionally, the restored reaches did not achieve the nutrient uptake efficiencies observed in both impacted and reference reaches, pointing to a partial recovery of ecosystem function. This study suggests that while riparian vegetation restoration contributes positively to certain aspects of stream function, environmental variables less related to this type of restoration, such as discharge and hydromorphology, significantly influence stream ecosystem functioning, highlighting the importance of considering environmental context in restoration efforts. A more holistic approach, possibly encompassing broader hydromorphological and habitat enhancements, is needed to fully restore ecological processes in these vital ecosystems. These insights are critical for informing future tropical stream restoration projects, advocating the use of ecosystem function metrics as comprehensive indicators of ecological recovery and restoration success.

Wastewater treatment plants (WWTPs) have eliminated many problems related to sewage inputs to aquatic systems. However, the treated effluents still affect river and stream ecosystems. Yet, very limited information is available about the fate of treated effluents in tropical receiving water bodies. Here, we examined the longitudinal changes in ammonium (NH4-N), nitrate (NO3-N), and soluble reactive phosphorus (SRP) concentrations in reaches downstream from chronic WWTP inputs in three receiving rivers in São Paulo state (Southeastern Brazil). The studied WWTPs were not designed or operated to have specific tertiary treatment for nutrient removal at the time of the study. We conducted a total of five sampling campaigns between 2019 and 2021. We applied the nutrient spiraling approach to these high-magnitude nutrient additions to examine net nutrient uptake. Changes in nutrient loads along the reaches were also examined. Overall, nutrient concentrations and loads were considerably increased by WWTP discharges. Net uptake lengths (SW-net) for NO3-N, NH4-N, and SRP ranged from 1.2 to 13.6, 1.5–infinity (uptake coefficient < 0), and 1.2–7.5 km, respectively. Net uptake velocities (Vf-net) varied between 0.7–2.5, −0.4 to 4.1, and 0.2–10.2 mm/min, respectively. Areal net uptake rates (Unet) ranged from 0.78 to 10.7, −20.08–45.8, and 0.18–4.4 g/m2/min for NO3-N, NH4-N, and SRP, respectively. High export (long SW-net) and limited net uptake along the reaches (low Vf-net) indicated that nutrients were transported downstream for long distances without efficient removal (nutrient availability higher than demand) despite the high mass removal (high Unet). These results suggest that the export of nutrients is substantial, potentially creating water quality impairments to downstream ecosystems. Our study highlighted that WWTPs with limited nutrient removal have considerable effects on water chemistry, nutrient cycling and loading in tropical receiving freshwaters.