Botanica Marina 2014; 57(5): 367–377 Thidarat Noiraksar*, Shuhei Sawayama, Sophany Phauk and Teruhisa Komatsu Mapping Sargassum beds off the coast of Chon Buri Province, Thailand, using ALOS AVNIR-2 satellite imagery Abstract: Sargassum species grow on rocks and dead coral, forming dense seaweed beds in their growth sea- sons. Sargassum beds play a range of ecological roles in coastal waters, but their sustainability is threatened by pollution and reclamation within ASEAN countries with rapid economic growth. Before conservation initiatives can be implemented, it is necessary to establish their pre- sent distribution. These beds can be mapped using the noncommercial satellite, ALOS, which can provide multi- band images using high spatial resolution optical sensors (sensitive to 10 m -2 plots), AVNIR-2. These images are of suf- ficient quality for examining coastal ecosystems and, in this study, for mapping the Sargassum beds in the waters off the coast of Sattahip, Chon Buri Province, Thailand. This coastal zone is a natural marine park reserve and protected from human activities such as fishery and recla- mation. Biomass data were obtained in January, February, March, and December 2009 from quadrat sampling. The biomass of Sargassum aquifolium (Turner) C.Agardh at a bottom depth of 1 m and 1.5–2 m was determined to be 7.73 and 92.75 g dw m -2 , while that of Sargassum oligocystum Montagne was found to be 44.05 and 87.97 g dw m -2 , respectively. Ground truth data were obtained in February and October 2012 from serial images taken by manta tow. Supervised classification is a procedure for identifying spectrally similar areas on an image by identifying “train- ing” sites of known targets and then extrapolating those spectral signatures to other areas of unknown targets. By applying this methodology, the Sargassum beds off Sat- tahip can be detected with an accuracy of about 70%. It is estimated that the error is caused by mixel effects of the bottom substrates in individual pixels, each of which covers an area of 10× 10 m. Our results indicate that the images captured by ALOS AVNIR-2 are informative and useful for mapping the Sargassum beds in Southeast Asia. Keywords: AVNIR-2; mapping; remote sensing; Sargassum beds; Thailand. DOI 10.1515/bot-2014-0015 Received 16 March, 2014; accepted 3 September, 2014 Introduction Seaweed beds are typically found in coastal waters, growing on a range of shallow rocky or hard substrates. They serve not only as materials for human consumption and for chemical industries (e.g., alginic acid) but also as spawning, feeding, and nursery grounds for a diverse range of fish and molluscs. Among coastal seaweed beds, Sargassum is one of the most productive seaweeds in temperate and tropical coastal waters. Many commer- cially important species spawn within Sargassum beds (e.g., sea urchins, abalones, cuttlefish), with the larvae and juveniles using the beds as nursery grounds. Sargas- sum species detaching from the substratum form drifting seaweed mats that serve as a habitat for fish and other aquatic animals that attach to it (Komatsu et al. 2007, 2008). Providing an important habitat for aquatic animals, Sargassum beds support a rich diversity of species in both coastal and offshore waters. Sargassum beds create unique environments equivalent to forests on land due to their luxuriant growth. They influence downward radia- tion from the sun through their canopy (Komatsu et al. 1990) and profiles of water temperature inside the Sargas- sum forest (Komatsu et al. 1982, 1995, Komatsu 1985). Their stipes and fronds buffer water motion inside the forest (Komatsu and Murakami 1994) in the same way as in sea- grass beds (Komatsu 1996, Komatsu and Yamano 2000, Komatsu et al. 2004). Seaweed beds, including those of Sargassum, however, are as susceptible as seagrass beds *Corresponding author: Thidarat Noiraksar, Institute of Marine Science, Burapha University, Bangsaen, Chon Buri, 20131 Thailand, e-mail: sargassum2005@yahoo.com Shuhei Sawayama: Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashaiwa 277-8564, Japan Sophany Phauk: Faculty of Science, Royal University of Phnom Penh, Khan Toul Kork, Phnom Penh, 120000 Cambodia Teruhisa Komatsu: Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashaiwa 277-8564, Japan; and Japan Science and Technology Agency, CREST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan Authenticated | sargassum2005@yahoo.com author's copy Download Date | 9/30/14 3:59 PM