Coral reef geomorphology as a function of seasonal prevailing currents and larval dispersal

Recent ecological and oceanographic evidence has demonstrated that coral recruitment can be highly localised and very successful in areas of high water residence time (eddy areas) on coral reefs. These areas occur in the lee of time-averaged and depth-averaged far-field currents. In this study, several reefs in the central region of the Great Barrier Reef, Australia, were examined to determine the relationship between asymmetry in reef geomorphology, wind direction, time- and depth-averaged currents, particularly during the coral spawning season. Detailed fine-scale bathymetric data were plotted in three dimensions for four reefs: Helix Reef, Wheeler Reef, Myrmidon Reef, and Rib Reef. The perspective from which each reef was viewed was rotated until maximum asymmetry (steep vs. gentle slope) from the floor of the continental shelf to the ocean's surface was achieved. The compass orientation of the reef's asymmetry was then compared with known directions of tradewinds and time- and depth-averaged currents. Asymmetry in overall reef geomorphology was found generally not to be related to tradewinds or resultant surface-driven currents. It was, however, found to be more closely associated with time- and depth-averaged seasonal currents, often prevailing at the time of coral spawning. We propose that far-field currents, near-field currents, and coral recruitment processes prevailing during the time of coral spawning have differentially enhanced lateral extension of the reef in a direction “down-current” far-field currents through geological time. We also propose that this relationship may be of assistance in hind-casting seasonal low-velocity, depth- and time-averaged palaeo-currents surrounding fossil reefs, in addition to tools already available for hind-casting wind-forced surface currents and waves.