Reef island morphological change over the past two decades: Spermonde Archipelago, Indonesia

Low-lying coral reef islands are presumed to be highly vulnerable landforms to the effects of climate change. Rising sea levels, changes in wave regimes and reef degradation are all considered key threats to their future persistence and habitability. While a number of studies have examined morphological changes on islands over multidecadal timescales, there is a paucity of high-frequency data from recent years that discern variability in shoreline change trends at the local scale. In this study, we used frequently sampled high-resolution satellite imagery covering the past two decades and analysed the morphological evolution and dynamics of 22 reef islands of the Spermonde Archipelago at the southwest coast of Sulawesi, Indonesia - a location deemed as a climate change hotspot with sea-level rise rates higher than the global average, and anthropogenically affected reef ecosystems. Analysis of 4,329 transects cast across 192 recorded shorelines revealed a balance in erosional and accretionary response. Specifically, 32% of transects were characterized by statistically significant accretion, 29% by erosion and the remaining exhibited no significant change. The magnitude of shoreline changes showed high spatial variability across the archipelago, with marked differences between islands perched on patch reefs on the outer shelf and those in the mid-shelf and nearshore. Archipelago-wide, irrespective of a net gain or loss in land area on islands, accretion was predominant on the western margins, while the eastern margins experienced relatively high degrees of erosion, leading to a westward migration of 55% of the islands on their reef platforms. Collectively, this study provides the first high-resolution shoreline change record for the archipelago, explores contemporary patterns of island morphological change and highlights the importance of high-frequency sampling in reef island studies for understanding projections of island change and efforts towards developing robust adaptation strategies and decision-making.