Shifting Sands in Warmer and Higher Seas: Quantifying Changes in GBR Islands

Coral reef cays are crucial habitat for many ecologically important species. Formed from the accumulation of biogenic calcium carbonate sediment that is continually eroding and dissolving, these low-lying landforms are highly susceptible to changing environmental conditions. Australia's Great Barrier Reef contains over 1000 islands, many being low-lying, relatively inaccessible, and therefore difficult to monitor. Here I investigate the use of Sentinel-2 satellite imagery to quantify the change in land area of vegetated coral cays and to assess the effectiveness of three water indices: the Normalised Difference Water Index (NDWI), Modified Normalised Difference Water Index (MNDWI) and Sentinel-2 Water Index (SWI). NDWI was determined the most appropriate water index when assessed across 12 coral cays. All available Sentinel-2 imagery from 2015 to 2023 was used to examine changes in Raine, Green, and North West Island land area using NDWI. Tide height was the biggest predictor of changes in island area, and all three islands experienced a decrease in tidally adjusted area. This study demonstrates the magnitude of variability across the islands, and future studies would benefit from including other satellite data with a greater historical presence. This research may assist management authorities in the assessment of cay stability and vulnerability.