Abstract
The amount of live and dead coral is related to recruitment, but differentiating them by remote sensing techniques is difficult. We measured change in the amount of live, bleached, and recently (<6 months) dead coral cover (CCA6) for an island archipelago in the South China Sea. Six Landsat 4/7 ETM/8 OLI images from 1989–1990, 2005, and 2014 were analyzed to assess changes in CCA6 at 14 coral reefs in the western Xisha Islands. Satellite images were georectified and calibrated to remote sensing reflectance. Models for three shallow water (<6 m) geomorphic zones throughout the western Xisha Islands are proposed based on ground-truthed data collected in 2014 and satellite-determined spectral values for 2014 images. Nonlinear models based on Landsat image blue spectral bands for reef slope and lagoon habitats, and green bands for reef flat habitats, are determined as optimal models. Significant changes in CCA6 from 2005 to 2014, and changes in coral reefs and geomorphic zones, correlate with increased numbers of both crown-of-thorns starfish, and sea surface temperature in 2007. Detecting change in CCA6 by remote sensing can provide large-scale information of value for coral reef management, restoration, and protection.
Highlights
Worldwide, coral reefs are under threat from climate warming [1], rising sea levels [2], increased frequency and intensity of El Niño-Southern Oscillation (ENSO) events [3], and local anthropogenic disturbances such as overfishing [4]
Compared with previous studies that have focused on identifying changes in coral reefs combining remote sensing and in situ data [20,21,22,23,25], our study demonstrates the usefulness of geomorphic segmentation for discriminating habitat
We deem nonlinear optimal models for reef slope and lagoon habitats based on Landsat image blue bands, and Landsat green band images for reef flat habitats, to be valid and useful for estimating CCA6 in shallow water(depth
Summary
Coral reefs are under threat from climate warming [1], rising sea levels [2], increased frequency and intensity of El Niño-Southern Oscillation (ENSO) events [3], and local anthropogenic disturbances such as overfishing [4]. These stressors contribute to decreased live coral cover on many coral reefs [5,6,7], for which the long-term outlook is “poor, and getting worse” [8]. Because the amounts of live and dead coral are related to coral recruitment [10], dynamic mapping of CCA6 facilitates planning of coral reef management, protection, and recovery
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