Abstract
Sentinel-2 mission has been shown to have promising applications in coral reef remote sensing because of its superior properties. It has a 5-day revisit time, spatial resolution of 10 m, free data, etc. In this study, Sentinel-2 imagery was investigated for bleaching detection through simulations and a case study over the Lizard Island, Australia. The spectral and image simulations based on the semianalytical (SA) model and the sensor spectral response function, respectively, confirmed that coral bleaching cannot be detected only using one image, and the change analysis was proposed for detection because there will be a featured change signal for bleached corals. Band 2 of Sentinel-2 is superior to its other bands for the overall consideration of signal attenuation and spatial resolution. However, the detection capability of Sentinel-2 is still limited by the water depth. With rapid signal attenuation due to the water absorption effect, the applicable water depth for bleaching detection was recommended to be less than 10 m. The change analysis was conducted using two methods: one radiometric normalization with pseudo invariant features (PIFs) and the other with multi-temporal depth invariant indices (DII). The former performed better than the latter in terms of classification. The bleached corals maps obtained using the PIFs and DII approaches had an overall accuracy of 88.9 and 57.1%, respectively. Compared with the change analysis based on two dated images, the use of a third image that recorded the spectral signals of recovered corals or corals overgrown by algae after bleaching significantly improved the detection accuracy. All the preliminary results of this article will aid in the future studies on coral bleaching detection based on remote sensing.
Highlights
Coral reefs have long supported healthy coasts and thousands of businesses
The present study was conducted at the Lizard Island group, which is a part of the Northern Great Barrier Reef, located 241 km north of Cairns and 92 km north east off the coast from Cooktown, Australia (Figure 1)
To further assess the sensitivity of Sentinel-2A mission to reef signals, the environmental noise equivalent delta reflectance (NE RE) (Wettle et al, 2004; Brando et al, 2009) was calculated based on the image taken in November 2015 and compared with signal difference of bleached and healthy corals at different water depths
Summary
Global warming is emerging as a major threat to coral reefs It has resulted in coral bleaching, i.e., stressed corals expel their symbiotic algae, increasing the possibility of subsequent coral morbidity and mortality. Many studies have documented regional or global coral bleaching patterns and impacts using field observations data or reef environmental conditions retrieving from satellite data over the past several decades (Barkley et al, 2018; Hughes et al, 2018a). These studies emphasize the understanding of the Remote Sensing of Coral Bleaching relationship between environmental factors and coral bleaching. There is an urgent need for better mapping of coral bleaching in a time- and cost-effective way
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