Assessing water quality recovery begins with a global multi-decadal database of inland and coastal ecosystems

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 (phosphorus and nitrogen). 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.

The OLIGOTREND database was created by harmonising and cleaning data from hundreds of sources worldwide, making it possible to track and compare water quality trends across diverse ecosystems, climates, and regions. The findings indicate that while some aquatic ecosystems, particularly in parts of Europe and North America, have demonstrated clear signs of recovery due to pollution controls and restoration efforts, many others are not improving or are deteriorating. The lack of improved water quality is especially true in areas experiencing intensified agriculture, rapid urbanisation, or the growing impacts of climate change.

One of the striking contributions of OLIGOTREND is its ability to incorporate long-term changes in rivers and estuaries, which have been comparatively less studied than lakes because of its flushing dynamics. For example, some rivers are becoming clearer (a sign of oligotrophication) while others are suffering from increasing nutrient loads and algal blooms. These patterns vary by geography, climate zone, and river characteristics, such as water discharge and catchment use. Such information is critical for guiding river basin management, policy decisions, and conservation strategies in face of increasing human and global change impacts,  such as species invasions, flow regulations including dams, and warming.

Ultimately, OLIGOTREND is not just a scientific achievement but a tool for action. 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.