Evaluation of the connectivity between shallow and mesophotic coral reefs in the United States Virgin Islands

Details

Supervisors

Hanert, Emmanuel

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en sciences et technologies de l'environnement, à finalité spécialisée

Abstract

Worldwide coral reefs being increasingly impacted by anthropogenic disturbances, identifying areas that can offer protection has become a priority. The Deep Reef Refugia Hypothesis states that mesophotic reefs (light-dependant reefs found between 30m and 150m) can act as substantial source of larvae for shallower reefs after a disturbing event. In this study, we assess the larval contribution of mesophotic O. faveolata habitats to shallow reefs in the northern USVI through late summer 2016, by the means of a biophysical model. In particular, we reproduce the study of Holstein et al. (2015) at a smaller habitat spatial scale with different modeling tools. To do so, we coupled the multi-scaled ocean model SLIM3D with a particle transport model. The resulting connectivity network was analyzed by the means of one-generational and multi-generational connectivity indicators. Our results suggest that more than half of the O. faveolata larvae arriving at shallow depths after the spawning event of August 2016 were spawned over mesophotic reefs. More specifically, most of these mesophotic larvae that could help repopulate shallow reefs originate from highly productive mesophotic reefs found between 30 and 40m depth. The deepest mesophotic habitat (40 - 50m depth) has the potential to act as a multigenerational corridor and thus increase the resilience of the entire coral reef network. All of this shows that mesophotic reefs can constitute significant sources of larvae for shallow reefs over several generations in the northern USVI. Despite some differences, Holstein et al. (2015) study and ours point towards the same direction, according to which mesophotic reefs have to be considered as a significant source of larvae for shallow reefs. While it has some limitations, the quantitative approach presented in this work has the potential to inform marine conservation in order to optimize resource allocation. We classified the reefs that constitute big exporters or that are part of multigenerational corridors as good candidates for restoration and localized them. The reefs north of St. Thomas have a high centrality value and are thus among the most important stepping stones for multigenerational larval connectivity. In the same way, big exporters found offshore at the south of St. Thomas have an important role regarding to the resilience of the network. Those reefs are however not part of any conservation program. Our results suggest that they should be a target of future management actions.