Nest exposure to marine flooding and impacts on green turtle (Chelonia mydas) embryos under rapid sea level rise: The role of beach geomorphology and hydrodynamics

Climate change-induced sea-level rise increases the risk of sea turtles' nests being flooded which could threaten their populations. However, impacts on embryonic development remain limited, and the mechanisms driving nest flooding are still poorly understood. Our multidisciplinary study aimed to determine the geomorphological and hydrodynamic factors exposing nests to flooding, and to better understand the effects of flooding on embryonic development under natural conditions. The research was conducted on Mayotte Island (France, Indian Ocean), where a seismic crisis between 2018 and 2020 caused subsidence, resulting in a relative rise in sea level. Thirty green turtle nests on a primary nesting reef beach were monitored from March to June 2022 using pressure sensors to measure flooding. The hydrodynamics were measured, and the beach morphology was mapped. Approximately 30 % of the nests were flooded, mainly due to their relatively low position on the beach, with the flooding likely to have been influenced by water level fluctuations related to the tidal cycle. The early and late incubation phases were particularly sensitive to flooding. Emergence success was 1 % for flooded nests, significantly lower than 85 % for non-flooded nests. This study provides new insights and reveals that under natural conditions, nests are more sensitive to flooding than previously reported in controlled laboratory studies. However, flooding does not always directly impact embryonic mortality; other related factors may be involved, requiring further research.