The OLIGOTREND project has compiled a comprehensive global database on long-term water quality trends in lakes, rivers and estuaries. Spanning nearly four decades (1986–2023), this publicly available database includes over 30,000 annual records of Chlorophyll-a (a proxy for algal biomass) and nutrient concentrations. These parameters are key indicators of ecosystem health, as excessive nutrients can lead to eutrophication, which primarily is the overgrowth of phytoplankton, harming biodiversity and affecting water use for drinking, fishing, and recreation. Making these long-term trends visible and accessible helps scientists, policymakers, and the public better understand how inland and coastal waters are changing and what interventions are needed to safeguard aquatic ecosystems for future generations.