Mapping Habitat of Dongsha Atoll by Using Remote Sensing Image

Project Duration	2013-12-31 ~ 2014-12-30
Research Title	Application of Remote Sensing Imagery to Habitat Change and Mapping of Dongsha Atoll
Contracted Organization	National Central University
Principal Investigator	Jen Hsuan
Co-Principal Investigator	Kao Hung-Ming
Project Participants	Sung Ke-Yi, Fan Chen-Shuo, Su Yen, Chang Yu-Ming
Keywords	Dongsha Atoll, reef crest habitat, satellite remote sensing, image classification, change detection
Abstract	1. Background The reef crest area of Dongsha Atoll is characterized by extremely shallow water (0–5 m), a vast spatial extent, and difficult navigation, which has limited comprehensive traditional field surveys. However, these same characteristics—shallow depth and large area—make the reef crest particularly suitable for large-scale and broad-extent investigation using satellite imagery. 2. Methods This study employed multispectral optical satellite images combined with ground truth data to survey bottom substrates and habitat distribution across the atoll reef crest. In addition, multispectral satellite images from 1994 to 2014 were integrated to analyze long-term changes in reef crest substrates and habitats. 3. Results Between 1994 and 2013, the various substrate types on the reef crest exhibited a generally stable pattern of reciprocal increases and decreases. Formosat-2 imagery from 2006 to 2013 revealed that seagrass coverage fluctuated on an annual cycle, likely related to seasonal variation. Long-term analysis of SPOT imagery (1994–2012) indicated a slight increase in seagrass distribution and a minor decrease in live coral coverage. These trends may be associated with global ocean acidification (favoring seagrass expansion) and ocean warming (contributing to coral decline). A significant event occurred between June and September 2014, when a large area (approximately 25 km²) of seagrass disappeared from the northern reef crest—an event not previously observed. This phenomenon may indicate increasingly extreme climatic variability. 4. Recommendations Immediate Recommendation Lead Agency: Marine National Park Headquarters A regular monitoring mechanism using satellite imagery should be established immediately. Given the 2014 event in which approximately 25 km² of seagrass disappeared within 2–3 months, it is recommended that the reef crest be monitored at least once annually. Mid- to Long-Term Recommendations Lead Agency: Marine National Park Headquarters Supporting Agencies: Ministry of Science and Technology; National Applied Research Laboratories It is recommended to implement zoned management planning for the shallow (2–3 m) reef crest area. Establishing fixed observation stations and conducting regular field visits would enhance familiarity among stationed personnel. Reef crest zoning should be incorporated into GIS mapping systems for operational use. Unique geomorphological features could be named for future interpretive and tourism purposes, and patch reefs within the lagoon could be cataloged and named to facilitate management. Given the dynamic nature of reef crest substrates, continued application of image processing techniques for habitat classification is recommended, along with the establishment of a long-term satellite-based monitoring database.