Fine sediments support the spread of the stony coral tissue loss disease in the USVI

Over the last eight years, the Stony Coral Tissue Loss Disease (SCTLD) has spread in the Caribbean and the Gulf of Mexico, leading to significant losses in coral cover and diversity. In the United States Virgin Islands (USVI), it has now become the greatest threat for shallow coral reefs (0 - 30m), even beyond climate change. In addition, the disease has recently been observed at low mesophotic depths (30 - 60m), thereby threatening the refuge potential of upper mesophotic coral ecosystems (UMCEs) for shallow species. However, the causative agent for this outbreak is currently unknown. Here we show how a three-dimensional high-resolution biophysical model coupled with a SIR epidemic model can characterize the potential causative agent(s) of the disease and its vector(s). Based on these findings, we assess the extent to which UMCEs are threatened by the spread of the SCTLD. We assume the disease agents to be transported within different materials (coral mucus, dead tissues, and/or suspended sediments) presenting specific buoyancy behaviors. The results of our simulations emphasize the importance of the disease vectors buoyancy on its propagation pattern. In particular, we find that the SCTLD may be transported by negatively buoyant materials referring to fine silts, and we conclude that the UMCEs are likely to be affected by the spread of the disease. Calibration of our epidemiological model with species-specific parameters gives a mean transmission time of 6.0 days, with a basic reproduction number above 1.1. This study follows a new connectivity-based approach to understand the spread of the SCTLD in the USVI, particularly between shallow coral reefs and UMCEs. More generally, this method could bring some insights into the characterization of other marine disease vectors and could provide a valuable complement to field experiments to support the management of coral disease outbreaks.