Massive wastewater discharge severely impacts a whole eutrophic coastal lagoon ecosystem.

Coastal lagoons are productive ecosystems threatened by human-driven nutrient over-enrichment, both through runoff and wastewater discharge, ending in increasing eutrophication and dead zones. Water quality monitoring remains key to assess biogeochemistry impacts and changes, but in situ sampling can be limited and sparse. To evaluate the influence of a massive wastewater discharge occurred in January 2021 in a eutrophic Brazilian coastal lagoon, Bayesian Dynamic Generalized Additive Models (DGAM) statistical analysis are applied to assess non-linear trends of in situ chlorophyll-a series (2001-2024) and Sentinel-2 archive (2017-2024, 359 satellite images) has been processed through ACOLITE to study temporal and spatial trends. The Bayesian DGAM reveals a change in chlorophyll-a dynamics from 2021, inducing higher fluctuations from the wastewater episode until nowadays. The Sentinel-2 series reveals a mean validated chlorophyll-a maximum peak of 108.4 mg/m in the southern lagoon three weeks after the wastewater episode as a phytoplankton biomass response, as well as significantly altered K and Secchi disk depth values yet in 2024 showing a slow recovery rate, which might affect photosynthetic benthic habitat degradation. Bottom effects of shallow waters in satellite-derived water quality parameters have been also evaluated, highlighting the need of in situ radiometry for their correction and calling for a coordinated in situ sampling planning in accordance to Earth Observation acquisitions. This double approach in assessing the impact of massive wastewater discharges in eutrophic coastal lagoons may quantify its severity and recovery capacity, as well as support decision-making for urgent coastal ecosystem conservation and restoration.