Change detection of Bunaken Island coral reefs using 15Â years of very high resolution satellite images: A kaleidoscope of habitat trajectories

In Bunaken Island (Indonesia), a time-series of very high resolution (2-4 m) satellite imagery was used to draw the long-term dynamics of shallow reef flat habitats from 2001 to 2015. Lack of historical georeferenced ground-truth data oriented the analysis towards a scenario-approach based on the monitoring of selected unambiguously-changing habitat polygons characterized in situ in 2014 and 2015. Eight representative scenarios (coral colonization, coral loss, coral stability, and sand colonization by seagrass) were identified. All occurred simultaneously in close vicinity, precluding the identification of a single general cause of changes that could have affected the whole reef. Likely, very fine differences in reef topography, exposure to wind/wave and sea level variations were responsible for the variety of trajectories. While trajectories of reef habitats is a way to measure resilience and coral recovery, here, the 15-year time-series was too short to be able to conclude on the resilience of Bunaken reefs.