Detection limits of coral reef bleaching by satellite remote sensing: Simulation and data analysis

Monitoring o f coral reef bleaching has hitherto been based on regional-scale, in situ data. Larger-scale trends, however, must be determined using satellite-based observations. Using both a radiative transfer simulation and an analysis o f multitemporal Landsat TM images, the ability o f satellite remote sensing to detect and monitor coral reef bleaching is examined. The radiative transfer simulation indicates that the blue and green bands o f Landsat TM can detect bleaching i f at least 23% o f the coral surface in a pixel has been bleached, assuming a Landsat TM pixel with a resolution o f 3 0 x 3 0 m on shallow (less than 3 m deep) reef flats at Ishigaki Island, Japan. Assuming an area with an initial coral coverage o f 100% and in which all corals became completely bleached, the bleaching could be detected at a depth o f up to 17 m. The difference in reflectance o f shallow sand and corals is compared by examining multitemporal Landsat TM images at Ishigaki Island, after normalizing for variations in atmospheric conditions, incident light, water depth, and the sensor’s reaction to the radiance received. After the normalization, a severe bleaching event when 2 5 -5 5 % o f coral coverage was bleached was detected, but a slight bleaching event when 15% o f coral coverage was bleached was not detected. The simulation and data analysis agreed well with each other, and identified reliable limits for satellite remote sensing for detecting coral reef bleaching. Sensitivity analysis on solar zenith angle, aerosol (visibility) and water quality (Chi a concentration) quantified the effect o f these factors on bleaching detection, and thus served as general guidelines for detecting coral reef bleaching. Spatial misregistration resulted in a high degree o f uncertainty in the detection o f changes at the edges o f coral patches mainly because o f the low ( ~ 30 m) spatial resolution o f Landsat TM, indicating that detection o f coral reef bleaching by Landsat TM is limited to extremely severe cases on a large homogeneous coral patch and shallow water depths. Satellite remote sensing o f coral reef bleaching should be encouraged, however, because the development and deployment o f advanced satellite sensors with high spatial resolution continue to progress. © 2004 Elsevier Inc. All rights reserved.