Résumé
Coral reefs are shaped by natural disturbances such as cyclones or unusual meteorological events that can drastically change the reef building coral’s composition and structure. Amongthese, extreme low tides may cause large-scale mortality events in shallow reefs, due to an unusually long direct exposure to air and solar stresses. However, research is limited on thisreef disturbance and few studies have documented the effects of expanded negative sea level anomalies (nSLA) on coral reef benthic communities.In this study, we report an extensive coral mortality event induced by a nSLA that occurred on the reef flats of Reunion island (South-West Indian Ocean, SWIO), during the austral winterseason between May and October 2015.We conducted a global analysis to ascertain the impact of the nSLA on reef building corals and other benthic organisms through in situ observationsduring the nSLA event and long-term Global Coral Reef Monitoring Network (GCRMN) data (18 years). Finally, we used high resolution remote sensing imagery and Lidar bathymetric datato assess the full spatial extent effects of this disturbance on coral reef flat.The nSLA corresponded to a rapid drop of 40 cm of the mean sea level, lasted from the 25th of May to late October 2015. The sea level was at its lowest in the past 18 years. Coral bleachingand mortalities increased along the SLA temporal gradient at various patches of the reef flat communities. The nSLA event affected mainly the upper part of the coral colonies, while somelower parts of corals, that remained immerged did not present any obvious damage. Other benthic organisms were also observed to rapidly bleach and die during the event such asseagrasses and crustose coralline algae. A total loss of 49.7% in mean coral cover was observed on reef flats when comparing the data collected before (January 2015) and after the nSLA event(November 2015), which corresponds to a decline in coral cover from 54.5 ± 12.7% to 27.4 ± 6.9%. We found no consistent changes in coral communities’ structures but important declinein coral cover following historical important nSLA events. High resolution remote sensing imagery and Lidar bathymetric data revealed the full 2015-nSLA spatial extent effects on coralcover of Saint-Gilles reef flats. We estimated that at least one fourth of all living corals died during the event, near half (48%) of living corals was affected in Saint-Gilles reef flat. Deeperbased-substrates along the bathymetric gradient demonstrated a higher resistance to 2015-nSLA.The drastic decline of Reunion island reef flats coral cover was concurrent with a vast nSLA throughout Mascarenes islands, suggesting that similar mortality could be widespread for reefflat communities in the SWIO. The historical records of altimetric data also suggest that such acute sea level falls were not unique in Reunion island, therefore these events could be moredeterminant in shaping Reunion island reef flat seascapes than formerly considered. Further analyses should be conducted to determine the full extent of consequences of these nSLA oncoral reef ecosystems. Particular attention should be given to predict these events and monitor their frequency and intensity in the light of rapid environmental changes for better reefmanagement.