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Global temperatures have increased markedly since the Industrial Revolution, with a clear breakpoint identified during this period. The most recent decade (2011–2020) has recorded progressively higher temperatures compared with the pre-industrial reference period (1850–1900). These increases, driven by anthropogenic climate change, are also reflected in the growing occurrence of extreme events, particularly heatwaves (HWs).Within this context, Portugal, characterised by a Mediterranean climate, is among the European regions most vulnerable to climate change. These alterations can impact multiple sectors, including water resources dependent on reservoirs, such as hydropower generation. A significant consequence is the intensification of chlorophyll a blooms during heatwave events, which can compromise water quality in reservoirs of national importance.This study aims to analyse heatwaves over mainland Portugal in 2025, with a focus on the country’s largest reservoir, Alqueva, and assesses the potential implications for chlorophyll-a dynamics. The year 2025 was the third warmest on record, following 2023 and 2024.The Alqueva Reservoir is located in southern Portugal, has a gross capacity of 4,150 hm³ and a flooded area of 250 km² at full supply level (FSL). Operational since 2002 and situated within the Guadiana River basin, a transboundary catchment of approximately 55,289 km² across Spain and Portugal with mean annual precipitation of 593 mm, the reservoir is a major source of irrigation, drinking water, and hydropower production, with an installed capacity of 520 MW.Heatwave analyses for 2025 were conducted using the ERA5-Land reanalysis dataset (0.1° × 0.1° resolution). Hourly 2 m air temperature data were used to derive daily maximum (Tmax) and minimum (Tmin) temperatures. Heatwaves were identified using ERA5-Land reanalysis data, with events defined as ≥3 consecutive days with Tmax ≥ 30 °C and Tmin ≥ 22 °C (definition of heatwave frequency). Three heatwaves occurred between June and August 2025 (Figure 1).Chlorophyll-a concentrations in the Alqueva Reservoir were estimated from Sentinel-2 Level-2A imagery, which provides atmospherically corrected surface reflectance at 10 m resolution. Images were selected from the Copernicus Data Space Browser with less than 10% cloud cover. To evaluate the effects of heatwaves on chlorophyll-a, with events selected based on data availability, concentrations were quantified before and during each event using the Three-Band Method (TBM), calculated from Sentinel-2 bands B4, B5, and B6.Figure 1. HWs identified in 2025 over the Alqueva Reservoir.The preliminary results showed a bloom of chlorophyll-a during the heatwaves (Figure 2), highlighting the impact of elevated temperatures on water quality. The methodology could be extended to identify other heatwave events based on pre-established definitions in order to assess whether blooms occur in a similar manner to those detected using the heatwave frequency metric.Figure 2. Chlorophyll-a in the Alqueva Reservoir before and during the three heatwaves of 2025 (HW1–HW3). Left panels: before each heatwave; right panels: during each heatwave. Acknowledgments: This research was fully funded by the Fundação para a Ciência e a Tecnologia (FCT) under Grant 2023.04248.BD and the CERIS research unit project UID/6438/2025. Additional support was provided by COST Action CA21104.