Impacts of wind-driven hydrodynamics on the early stages of coral development

Coral spawning has evolved to occur during relatively calm hydrodynamic conditions. However, moderate to high surface winds are often present on spawning nights in specific regions, with many unknown impacts to the early life-history stages of corals. Here, we mechanistically examine the sensitivity of fertilisation and larval development to increasing wind speeds for four common species of hermaphroditic corals on the Great Barrier Reef: Acropora kenti, A. millepora, A. spathulata, and Platygyra daedalea. Wind intensities relevant to coral spawning seasons were simulated by downscaling location-specific, historical wind speeds to laboratory flumes. Generally, fertilisation success declined as duration of exposure increased and was often lowest at the highest wind intensity equating to 15 knots in situ. Embryo damage, deformity, and fragmentation increased with exposure time, but varied across species. Four days following spawning, damaged and fragmented embryos had the highest mortality rates (76-82% and 70-73%) compared to intact embryos (26% and 52%) for A. kenti and A. spathulata, respectively. Experimental parameters were used to fit a 3D Fluid Dynamics model with outputs confirming that water surface elevations, velocity, and turbulence energy increased by up to 10%, 40%, and 50% respectively as winds intensified, likely explaining the physical drivers of the deleterious effects observed. Overall, results from this study indicate that suboptimal spawning conditions reduce propagule production, which may diminish recovery potential, thus require consideration when planning management strategies to safeguard coral reproduction.