Calcification, photosynthesis and respiration of Halimeda opuntia at Racha island in the Andaman Sea, Thailand

The western coasts of coral islands facing the open Andaman Sea are exposed to large amplitude internal waves (LAIW) during winter to spring (January to April). LAIW entrain cold subpycnocline waters into shallow reef areas causing frequent (several events per day) and severe drops in temperature (up to 9 °C) and pH (up to 0.5 units) and increases in nutrients. The present study explored a possible impact of LAIW on the distribution, calcification, photosynthesis and respiration of the calcifying macroalgae Halimeda opuntia. The study was conducted at Racha island about 18 km south of Phuket in the Andaman Sea. Belt transect observations along the island revealed that H. opuntia occurs only at the east side of Racha. A combined transplant and incubation experiment was performed. The temperature at the west and east side of Racha was recorded for 6 months (October till March). These records indicated no difference in LAIW intensity between west and east coasts of Racha which is in contrast to other offshore islands in this region. The experiments showed that H. opuntia calcified faster and revealed higher photosynthetic rates after being transplanted for 2 months to the western side in contrast to their eastern counterparts. The higher rates in calcification and oxygen production of the west-transplanted algae were maintained as well under simulated LAIW-conditions with reduced temperature and/or pH during the incubations. Despite the natural absence of H. opuntia along the western side of Racha island, this algae seemed to benefit from the natural conditions at west in contrast to its common eastern environment. It may be concluded that factors other than LAIW such as less sedimentation and possibly higher light intensities at west favored the development of the algae which are naturally restricted to the east due to the lack of suitable settling substrate in west