Hydrodynamic and Biogeochemical Responses of an Amazonian Estuary to Extreme Climatic Events and Anthropogenic Pressures

Estuaries along the Amazon coast are highly dynamic ecosystems whose biogeochemical functioning is driven by complex hydrodynamics, yet severely threatened by climate extremes and local urbanization. This study evaluated the spatiotemporal oscillations of hydrological, biogeochemical, and microbiological variables in the Caeté estuary (Northern Brazil) across 15 field campaigns (2013–2017). Extreme dry events reduced river discharge by 38% (Drought) and 31% (El Niño), causing increased surface temperature (up to 28.40 ± 0.90 °C), elevated salinity (up to 17.50 ± 15.00 PSU), higher pH (7.90 ± 0.50), and accumulation of dissolved inorganic nitrogen (DIN: 15.90 ± 11.90 µmol L−1) and chlorophyll-a (15.10 ± 27.90 mg m−3). Conversely, a weak La Niña event increased discharge by 19.00%, lowering salinity (7.20 ± 10.20 PSU) and pH (6.40 ± 0.80) while sustaining elevated DIN (17.20 ± 8.40 µmol L−1). Thermotolerant coliforms persistently exceeded safety limits (>1100.00 MPN 100 mL−1) in 100% of upper-sector samples during extreme rainfall periods, driven by raw sewage discharges from urban Bragança. While climate extremes dictate estuarine physical–chemical gradients, local sewage inputs offset the natural dilution expected from rainfall, demonstrating that urban pollution severely impairs water quality regardless of climatic variability and highlighting the need for sanitation infrastructure.