Choosing the appropriate spatial resolution for monitoring coral bleaching events using remote sensing |
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Authors: | S Andréfouët R Berkelmans L Odriozola T Done J Oliver F Müller-Karger |
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Institution: | Remote Sensing and Biological Oceanography Laboratory, College of Marine Science, University of South Florida, St. Petersburg, Florida 33701, USA, Great Barrier Reef Marine Park Authority, P.O. Box 1379, Townsville, Queensland 4810, Australia, Australian Institute of Marine Science, PMB 3, Townsville, Queensland 4810, Australia, International Centre for Living Aquatic Resources Management, P.O. Box 500, GPO 1060, Penang, Malaysia,
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Abstract: | Bleached corals provide a strong optical signal that suggests that remote sensing investigations of major bleaching events are feasible using airborne or satellite sensors. However, patchy coral cover, varying intensities of bleaching, and water depths are likely to limit the application of remote sensing techniques in monitoring and mapping coral bleaching. Today, satellite multispectral sensors routinely provide images of reefs from 4 m (Ikonos) to 30 m resolution (Landsat); however, the adequacy of these sensors for monitoring and mapping bleaching events remains unclear. To clarify these issues, scanned aerial photographs acquired during the 1998 bleaching event over the Great Barrier Reef (Australia) were analyzed at various spatial resolutions, from 10 cm to 5 m. We found that the accuracy of mapping bleaching is highly sensitive to spatial resolution. Highest accuracy was obtained at 10 cm resolution for detection of totally bleached colonies. At 1 m resolution, as much as 50% of the 10-cm resolution signal is lost, though the spatial patterns remain correctly described. Partially bleached (pale) corals are difficult to detect even in aerial surveys, leading to an underestimation of overall bleaching levels (total and partial bleaching) in aerial photos compared to in-situ surveys. If data volume and processing time are limiting factors, local variance analysis suggests that the optimal resolution necessary to capture spatial patterns of bleaching is in the range 40-80 cm. |
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