Changing coral disease dynamics with the emergence of stony coral tissue loss disease on reefs at Carrie Bow Cay, Belize Barrier Reef

Coral reefs in the Caribbean Sea continue to experience unprecedented declines driven by the combined impacts of emerging infectious diseases and increasingly frequent marine heatwaves. This study assessed temporal changes in coral health, disease prevalence, and community composition at nine sites on the Belize Barrier Reef surveyed before and after the emergence of stony coral tissue loss disease (SCTLD) and the 2023 Caribbean mass bleaching event. During our study period (2019-2024), disease became a major driver of mortality, albeit with an epidemiologic transition with shifts in the main diseases driving these patterns. Surveys in 2019 documented relatively healthy reefs with low disease prevalence (avg. 0.5%) characterized by numerous endemic coral diseases. Following the arrival of SCTLD, disease prevalence increased more than tenfold by 2022 (avg. 6.2%), with SCTLD affecting 12 coral species and a previously undescribed chronic tissue loss disease with bleached or purple discoloration (cTLD) emerging in Siderastrea siderea . By 2024, overall average disease prevalence was 4.2% but SCTLD prevalence had declined markedly while S. siderea cTLD became the dominant disease (90% of the diseased colonies), indicating a shift in disease patterns through time. Hard and soft coral cover declined, and sponge cover increased significantly from 1.3% in 2019 to 4.5% in 2024. Coral community structure changed substantially throughout the study, with reductions in colony densities (73.4% decline), and species richness (33% decline). The abundance of SCTLD-susceptible taxa declined following disease emergence, followed by additional losses of thermally sensitive corals after the 2023 bleaching event. Roving diver surveys documented SCTLD to be widespread along the Belize Barrier Reef. Histopathological analyses confirmed that Belizean SCTLD lesions resembled those previously described from the U.S. Virgin Islands and that bleached lesions on Orbicella annularis represented manifestations of SCTLD. Collectively, these findings show that coral disease outbreaks and bleaching events produce different patterns of coral mortality and community change, while their cumulative impacts are reshaping Caribbean coral assemblages. The emergence of cTLD in the increasingly dominant, stress-tolerant coral S. siderea highlights the need for continued disease surveillance, investigation of newly emerging coral diseases, and integrated monitoring programs that consider both disease and bleaching as interacting drivers of reef decline.