A decade of Sentinel-2 observations for Water Framework Directive assessment in an anthropized coastal lagoon: The Berre Lagoon (France)

Coastal lagoons are productive yet vulnerable ecosystems exposed to strong anthropogenic pressures. The Berre Lagoon (southern France) illustrates this vulnerability, having undergone long-term degradation from large freshwater, nutrient and sediment inputs. Using a decade of calibrated and validated Sentinel-2 satellite observations (2015–2025), this study demonstrates the value of Earth Observation (EO) for Water Framework Directive (WFD) assessment and operational lagoon management. Satellite-derived chlorophyll-a concentration ([chl-a]), suspended particulate matter ([SPM]) and Secchi depth (Zs) were used to derive WFD-consistent indicators: the phytoplankton biomass Ecological Quality Ratio (EQR B ), an SPM-based hydropower discharge indicator, and a Zs-derived Potential Seagrass Habitat (PSH). Results show that eutrophication assessed from satellite is more severe than suggested by the current WFD monitoring network, which fails to capture major phytoplankton blooms occurring from late summer to early winter. Satellite-derived [SPM] time series also reveal a shift in hydropower operation from near-continuous discharges (2016–2021) to winter-restricted releases after 2022. This transition increased summer water transparency and expanded light-suitable seagrass habitat by about 15% (PSH ≈ 26 km 2 ), exceeding the 15 km 2 threshold required for “good” macrophyte status. Overall, this study shows how satellite observations can diagnose changes in anthropogenic pressures, monitor management-driven recovery, and provide actionable indicators for WFD implementation in coastal lagoons.