Pollution and climate change trigger seagrass losses in the southwestern Atlantic

Anthropogenic eutrophication has become a global concern due to the increasing spread of dead zones. This challenging scenario for coastal management has been further intensified by climate change, particularly the increased frequency and severity of extreme events such as marine heatwaves (MHWs). In this study, we evaluated the responses of seagrass meadows of Halodule wrightii and Ruppia maritima in a coastal lagoon before and after a severe eutrophication event combined with MHWs. We assessed meadow survival for both species and performed quantitative analyses of aboveground biomass (AGB), belowground biomass (BGB), total biomass, and shoot density of H. wrightii. Sediment grain size and water quality parameters-including the light attenuation coefficient (Kd), photosynthetically active radiation (PAR), and salinity-were also measured. PAR showed greater variability after the disturbance, and Kd in the southern region nearly doubled from 0.23 ± 0.03 m^-1 (2018) to 0.47 ± 0.14 m^-1 (2023), while other variables remained unchanged. By 2023, seagrass meadows had disappeared from ∼60% of the sites mapped in 2018, with complete loss in the southern region and only four H. wrightii and three R. maritima meadows remaining. Where H. wrightii persisted, shoot density declined by 35-44%, and biomass decreased by 47-84% in the most impacted areas. In contrast, sites farther from the disturbance source and with greater water renewal showed increased H. wrightii biomass and higher meadow persistence. These findings provide the first evidence of the combined influence of local and global stressors on seagrasses in the Southwest Atlantic.