Modelling coral larvae exchanges between the Great Barrier Reef and outer reefs

During these last years, while climate phenomena made tropical species extend their ranges poleward, a shift of a few tropical coral and fish species is being noticed. The subtropical regions shelter both tropical and temperate species and are called biogeographic transition zones. These huge natural laboratories giving a glimpse of the reaction of species from different natural habitat to climate change. This study intends to analyse the coral larvae exchanges between the Great Barrier Reef (GBR) and southern subtropical reefs located on the east coast of Australia. First the simulation of currents was devised with the multiscale ocean model SLIM (the 2D barotropic version). This model uses an unstructured mesh to increase the resolution close to the reefs and decrease it in deep uniform waters, allowing to lower the computational cost. It modelled rather well the real currents of the region even if the poor amount of data did not allow to fully validate it. A Lagrangian Particle Tracker was then used to simulate the larval dispersal during 30 days after the larvae spawning. This study used the biological parameters of Acropora Millepora and simulated the larvae exchanges between reefs from the night of December 4, 2012. This process filled a connectivity matrix listing all larval exchanges. This adjacency matrix described a directed graph where the points were the reefs and the weighted links were the number of larvae going from one reef to another. Since the matrix then constructed was very large (1329x1329), some predefined tools have been used to produce indices describing the reefs. These indices allowed to identify some patterns. First, under the conditions used in the simulations, it appeared that there were no exchange of larvae between the GBR and southern reefs. The analysis then continued with a particular focus on the southern reefs, concluding with the calculation of protection and restoration indices. The protection index helps to identify reefs that are useful to the reefs’ community but receive few larvae from other reefs in return while the restoration index points out reefs useful to the community and resilient enough to survive after the restoration. In conclusion this study provides a good understanding of subtropical reefs through their connectivity.