Severe ecological and functional simplification of coral communities following the combined impacts of the SCTLD and the fourth global bleaching event at Banco Chinchorro

The Banco Chinchorro Biosphere Reserve, an offshore reef complex in the western Caribbean, is recognized as one of the healthiest systems in the region due to its high biodiversity and isolation from land-based threats. However, in 2023, it faced two devastating events: an outbreak of stony coral tissue loss disease (SCTLD) and the strongest marine heatwave on record. This study investigates how these events impacted the ecological and functional integrity of the reef system by analyzing changes in coral communities at eight sites across the reef bank before, during, and after both events. Coral condition and composition of coral communities, as well as their overall calcification rates, were assessed using a minimum of six 10 m² belt transects for each survey period. Results revealed a halving of coral colony numbers over two years. While density losses were evident across almost all species, key reef-building species like Pseudodiploria strigosa , and Agaricia tenuifolia experienced the most severe reductions. Additionally, we report local extirpation of species such as Dendrogyra cylindrus and Meandrina meandrites . The disproportionate coral mortality has reshaped communities into simpler, more homogeneous assemblages, largely dominated by opportunistic and stress-tolerant corals. This decline indicates a critical loss of geo-ecological functionality, resulting in significant reductions in coral community calcification. Understanding the response mechanisms of coral communities to the cumulative effects of global change and disease outbreaks is essential for predicting how coral reefs will respond to rapidly increasing climatic pressures.