Study on the Lagoon Ecosystem of the Dongsha Atoll(I)

Project Duration	2009-12-31 ~ 2011-05-16
Research Title	Study on the Lagoon Ecosystem of the Dongsha Atoll(I)
Contracted Organization	Coral Reef Society of the Republic of China
Principal Investigator	Chen Chung-Chi
Co-Principal Investigators	Hsia Fu-Kuo, Chan Sen, Hsu Shih-Chieh
Project Participants	Fu Ko-Hsien, Lin Yu-Ju, Lin Kun-Yi, Ho Chun-Chung, Lin Kai-Lun, Huang Kang-Ming, Cheng Ching-Wen, Shen Hsuan-Chih, Chang Han-Yu, Kao Ming-Hsiung, Chang Yu-Hung, Huang Wan-Chen, Shen Yi-Chieh, Huang Hu-Ching, Lin Ping-Hsien, Teng Cheng-Liang, Cheng Chun-Hsien, Yeh Ching-Huang
Keywords	Dongsha Atoll, Marine National Park, coral reef ecosystem, plankton community, atmospheric deposition
Abstract	This research project aimed to understand the water column ecosystem within the Dongsha Atoll lagoon and to investigate the factors influencing ecological changes within the lagoon. Major research components included measurements of physical parameters (temperature and salinity), chemical parameters (nitrate, nitrite, phosphate, and silicate), and biological parameters (chlorophyll concentration, heterotrophic bacterial biomass and productivity, phytoplankton abundance, zooplankton abundance, primary productivity, and plankton community respiration rates), as well as analyses of atmospheric dry and wet deposition rates and the characteristics of coarse and fine atmospheric particles in surrounding waters. Field investigations were conducted in May, July, and September 2010, and February 2011, representing seasonal variations of spring, summer, autumn, and winter, respectively. Results indicated that physical hydrographic conditions in the lagoon exhibited clear temporal (seasonal) and spatial (horizontal and vertical) variability. In addition to relatively low-temperature seawater near the southern and northern channels, comparatively cooler water masses were also observed in the eastern lagoon area. This suggests that, besides the known southern and northern channels, additional water exchange pathways between the lagoon and the open sea may exist along the eastern reef structure. In terms of biological and chemical parameters, although Dongsha Atoll is surrounded by the oligotrophic waters of the South China Sea, moderate eutrophic conditions were frequently observed within the lagoon. Various factors exhibited high spatial variability in three-dimensional structure. It is inferred that nutrients are transported into the lagoon from deeper external waters, subsequently assimilated by organisms within the atoll. Due to the semi-enclosed nature of the reef structure, nutrients are retained within the system and continuously recycled. Furthermore, slow water exchange between lagoon and offshore waters, combined with spatial heterogeneity, contributes to the high variability of biogeochemical parameters. Atmospheric observations suggest that seasonal patterns are related to the northeast monsoon and rainfall frequency. Elemental size distributions in atmospheric deposition showed limited seasonal variation, with crustal elements primarily concentrated in coarse particles and anthropogenic pollutants in fine particles. Dry deposition fluxes were consistent with monthly trends of suspended particulates, with higher fluxes observed in spring and winter compared to summer, indicating that atmospheric inputs to the northern South China Sea region are significantly greater during spring and winter.