Can optical water types be used as ecological indicators? Insights from a temperate estuary

• OWT classification is a useful stand-alone product for environmental monitoring. • Specific OWT classes were identified as proxy for key estuarine processes. • Distinct OWT patterns were associated to different tidal phases. • Environmental drivers explained the occurrence of OWT groups. • Time-series indicate an increasing frequency of marine waters inside the estuaryAbstract. Monitoring water quality and understanding how estuarine ecosystems respond to environmental changes is essential to sustain their ecological integrity and associated ecosystem services. In this study, we demonstrate that Optical Water Type (OWT) classification from Sentinel-2 MSI can be used as a stand-alone product rather than an intermediate tool for algorithm refinement, providing valuable insights for environmental monitoring, using the Tagus Estuary as an example. In-situ measurements (Chl- a , temperature, salinity, pH, and dissolved oxygen) were employed to characterize environmental conditions associated with OWT classes and to analyse the spatio-temporal and tidal variability during 2017–2024. We identified distinct water quality profiles among groups of OWTs, which were associated with marine, transitional and riverine waters with different physico-chemical characteristics that were related to different environmental drivers. Tides significantly influenced the distribution of OWTs, with high and neap tides favouring the occurrence of clearer marine waters. Specific OWT classes emerged as indicators for key environmental processes, including marine water intrusion, freshwater inputs and phytoplankton-rich waters. Time-series analysis revealed a trend of increasing marine waters inside the estuary alongside interannual variability driven by hydro-climatic forcings. Our findings highlight OWT classification as a valuable stand-alone satellite product for water quality monitoring, providing a powerful and scalable tool for estuarine science, policy, and management.