'Sticky water' enables the retention of larvae in a reef mosaic

Abstract

To study retention of waterborn larvae in a reef matrix we used a finite-element unstructured numerical model with a minimum horizontal resolution of 150 m that can capture variability of currents on a spatial scale relevant to coral reefs in the Great Barrier Reef (GBR). Areas of high reef density (i.e. closely aggregated reefs) are poorly flushed because the prevailing currents are directed around and away from these regions, which is an oceanographic process called the 'sticky water' effect. The model showed that the sticky water effect leads to decreased flushing and a high exposure time in high reef density areas in the southern and central regions of the GBR matrix. In turn this generated hot spots of high self-seeding, and these hot spots existed under both calm weather conditions and wind conditions typical of those during the coral spawning season. Away from these areas, self-seeding was less likely to occur and larval replenishment would result mainly from connectivity between reefs located kilometres to tens of kilometres apart. The location of sticky water areas varied spatially within the reef matrix according to tidal and mean currents, local bathymetry and reef density (defined as the degree of aggregation by reefs). A simple analytical formula is presented that explains ~70% of the variation in larval retention in both calm weather and windy conditions. Complex reef mosaics and the related sticky water effect may have significant implications on the fate of larvae, and thus on connectivity for coral reefs worldwide.