Coral microatoll partial mortality after multi-hour subaerial exposure with implications for relative sea-level studies

Some intertidal corals, known as microatolls, have a distinct morphology that reflects changes in local relative sea level. While past observations have shown that the top surface of these corals may be killed by subaerial exposure, little is known about the exact oceanographic or environmental conditions that cause a coral to die down to a particular level. Here, we combine field surveys, tide-gauge data and analysis of microatoll morphology to investigate the survival limits of Porites spp. microatolls on Singapore's intertidal reefs. Unponded Porites spp. microatolls on the Pulau Biola reef reach a 'highest level of growth' between mean low water springs and mean low water neaps. Diedowns on the highest microatolls during 2023 and 2024 suggest the survival of these corals depends on the duration of subaerial exposure. By comparing the estimated highest level of survival after a diedown to water levels recorded at local tide gauges, we show that intertidal corals on the Biola reef and nearby Siloso Point reef can survive more than 2 h of continuous subaerial exposure per day. However, Porites spp. corals may not have survived more than 3.5 h of daily partial exposure without dying down.