High-frequency monitoring reveals recurrent extreme events in a tropical estuary

Estuaries of the tropical Guiana Shield exhibit rapid short-term variability superimposed on strong seasonal forcing, yet high-frequency, multi-year observations remain scarce. A four-year record (Jan 2020–Mar 2024; 20-min resolution) of temperature, salinity, and dissolved oxygen from four sites along the Cayenne River estuary was assembled to assess cross-site coherence, diel ranges, seasonal signals, and extreme events—defined as hyposalinity (<5 ppt for ≥2 days), hypoxia (DO < 2 mg L−1 for ≥2 days), and thermal anomalies (daily maxima above month-specific 90th-percentile climatological thresholds for ≥2 days and ≥5 days). Temperature and salinity showed strong coherence among sites and clear seasonal cycles, with higher values in the dry season and pronounced freshening in the wet season. Oxygen was more heterogeneous, reflecting local biophysical controls. Extreme-event analysis revealed frequent hyposalinity and hypoxia, whereas thermal anomalies were less common and typically shorter. A predictive model driven by atmospheric temperature and river discharge allowed prediction of water temperature and salinity variability and generalized to independent observations, while skill for oxygen was weaker, indicating the need to include additional biogeochemical drivers. Collectively, these results establish a high-frequency baseline for a data-poor tropical estuary, demonstrate that hydrological and atmospheric variability dominate the physical template, and show both the utility and limits of statistical prediction for anticipating biologically relevant extremes. Sustained high-resolution monitoring, paired with expanded biogeochemical measurements, will improve detection of climate-sensitive events and support adaptive management of estuarine nursery habitats and fisheries resources across tropical systems. .