LB 25-215 - Coral reef potential connectivity in the southwest Indian Ocean: network structure and variability

Coral larvae disperse through ocean currents between reefs, and the resulting flow of larvae is critical for establishing demographic and genetic connectivity. Connectivity is increasingly considered as a key parameter in marine ecosystem management, but the southwest Indian Ocean is one of the most data-sparse regions within the tropics. To help fill this data gap, we ran a 2km-resolution ocean model, and large-scale coral larval dispersal simulations incorporating mortality, competency, and settling dynamics. We simulate daily spawning events from 1993-2019 (for a total of almost 10,000 events), predicting connectivity between all coral reefs in the tropical southwest Indian Ocean.By identifying hierarchical clusters of reefs that tend to retain larval flow, we find that ocean current bifurcations and jets partition the reef network in broad agreement with previous studies from coral population genetics, and biogeographic ecoregions. Dispersal is strong but highly asymmetric along East Africa, very strong within the Chagos Archipelago, and non-negligible even across sparse archipelagos such as Seychelles. Seychelles plays a key role in connecting the Chagos Archipelago to the rest of the west Indian Ocean, with around 100 dispersal events required for larvae to cross the ocean between the two remote island groups.However, these patterns are highly variable in time. Although there is a significant seasonal cycle in potential connectivity at many reefs associated with the East Africa Coastal Current and Northeast Madagascar Current, day-to-day variability generally dominates. We find that stochastic oceanographic variability introduces considerable uncertainty to dispersal predictions, imposing fundamental limitations on what dispersal simulations can tell us about connectivity. This may partially explain the mismatch which is commonly found between observations of genetic differentiation between marine invertebrates, and numerical models. We also find that a small proportion of spawning events account for most successfully settling larvae, particularly at remote islands. In some cases, a once-in-a-century spawning event could be responsible for the majority of larvae that are able to settle. This demonstrates that a time-mean picture of dispersal may be inappropriate for predicting demographic and genetic connectivity.All data from this study, from the underlying hydrodynamics to the time-varying dispersal matrices, are openly available in full. Biological parameters are not hardcoded into our simulations, and our analyses can be rerun under different parameters using the data and scripts we have made available.