Changes of Tiahura (Moorea Island) reef flat habitats using 67 years of remote sensing observations

The resilience of Moorea Island's coral reefs has been extensively discussed. In this debate, and despite their greater exposure to anthropogenic disturbances, shallow reef flats have often been overlooked compared to oceanic forereefs. This study assesses changes in the reef flat habitats over 67 years (1955-2022) in Tiahura in the northwest of Moorea, using a combination of modern satellite imagery, vintage aerial photographs and a habitat mapping survey conducted in 2023. Remotely sensed data provided time series of coral habitat surface area changes could be in some cases corroborated by historical literature reports and in situ observations from 1972 to the present, although historical data were scarce, heterogeneous, and informed at a different level of descriptions (e.g., coral cover) than the habitat scale. Contrary to expectations, the period from 1955 to 1977, perceived as pristine, exhibited a significantly lower extent of coral habitat compared to the subsequent period marked by coastal development and anthropogenic disturbances. To discuss resilience of coral habitats across the 67-year period, we relied on habitat surface changes. To understand the changes and evaluate the coral habitat resilience of the Tiahura reef flats, it was crucial to differentiate the barrier from the fringing reefs due to their distinct exposure to physical disturbances during the 1970s. The fringing reef, unlike the barrier reef, experienced substantial human-induced structural and hydrodynamical modifications. Concluding or not on coral habitat resilience depends on the period considered: 1955-2022 or 1977-2022. Beyond the Tiahura case study, we highlight the importance of defining the temporal reference for any resilience analysis, by taking into account the stability of hydrodynamic conditions that affect sedimentation and lagoon water levels.