Island accretion within a degraded reef ecosystem suggests adaptability to ecological transitions

Reef islands, elevated only a few meters above sea-level and restricted in area, are not only confronted with rising sea-levels, but the surrounding reef ecosystems, which are the only source of sediment maintaining those islands, are threatened by global (e.g. ocean warming and acidification) and local anthropogenic (e.g. pollution and destructive fishing methods) stressors affecting many tropical coastal areas. These stressors can increase coral mortality and lead to shifts from coral- to macroalgal-domination, likewise altering the production of skeletal carbonate sediment and ultimately endanger the physical persistence of reef islands. Here we study the evolution of an Indonesian reef island that has been inhabited since the 20th century. By analyzing the sedimentary record covering the last 5800 years from sediment cores taken on the island, we study the formation processes during the Holocene. For understanding the spatial dynamics, we compare the sediment record of the past decades with observations from satellite imagery data. Two shifts in the sedimentological composition over time point to alterations in the sediment-supplying reef ecosystems. The first sedimentological shift occurred from 3900 years BP on, shortly before the initial formation of the island, when the skeletal composition was diversified, presumably reflecting the modification of the reef ecosystem following a sea-level drop. A second sedimentological shift in the youngest sediments is marked by increased proportions of the calcifying green algae Halimeda, indicating that the reef ecosystem has shifted toward algal-domination, presumably reflecting increasing anthropogenic pressure. Of significance, shoreline change analysis reveals that the island is in an accreting state and has grown by 13 % in surface area over the past 24 years. Our findings suggest that the compositional alterations in sediment supply did not destabilize the reef island, and underline the adaptive potential of these landforms.