Risk or relief? The influence of river plumes on coral bleaching and disease in marginal and extreme reefs

Climate change-driven stressors are increasingly affecting ecosystems, with shallow-water coral reefs among the most vulnerable. While the impact of river plumes on corals is well-documented in optimal reef environments, little is known about their effects in marginal and extreme settings, particularly during global bleaching events. Here, we assess how varying levels of river plume influence affect the prevalence and extent of bleaching and disease in Siderastrea stellata-a resilient and abundant reef-building coral inhabiting turbid coastal reefs in the South Atlantic. During the 2024 global bleaching event, we surveyed colonies across three sites with distinct levels of exposure to the Paraíba River plume. Our findings reveal that moderate plume influence reduced bleaching prevalence. However, once bleaching occurred, its extent was significantly higher closer (72 % of bleached colony area) than far from the river mouth (44 %). Colony size also played a role, with larger corals being less prone to bleaching. Moreover, average coral colony area affected by bleaching increased with proximity to the river mouth. In contrast, disease prevalence and extent showed little variation little among sites, suggesting a limited effect of the river plume on coral disease occurrence. These results underscore the complex interplay between environmental conditions and coral health, highlighting how larger colonies reduce bleaching prevalence, while extreme turbidity exacerbates bleaching extent during heatwaves.