High-frequency variability dominates potential connectivity between remote coral reefs

Abstract. Coral larval dispersal establishes connectivity between reefs, but larval fluxes vary over timescales from daily to multidecadal due to oceanographic variability. Using a 2 km-resolution ocean model, we simulate daily spawning events from 1993–2019 (for a total of almost 10,000 events) across all coral reefs in the tropical southwest Indian Ocean. Although there is a significant seasonal cycle in potential connectivity at many reefs, day-to-day variability generally dominates. The importance of this day-to-day variability depends on the local geography and oceanography. Stochastic oceanographic variability introduces considerable uncertainty to dispersal predictions, imposing significant limitations on what dispersal simulations can tell us about connectivity. Protracted spawning over only a few days can significantly reduce variability associated with the likelihood of a larva settling successfully. The duration of spawning is therefore likely a more important parameter in modelling coral connectivity than the exact timing of spawning onset. Finally, we find that a small proportion of spawning events account for the majority of successfully settling larvae, particularly at remote islands, and demonstrate that a time-mean picture of dispersal may be inappropriate for predicting demographic and genetic connectivity.