Study on the Lagoon Ecosystem on the Dongsh Atoll-Discuses on Physical,Chemical Hydrology and Basic Productivity

Project Duration	2011-03-31 ~ 2011-12-29
Research Title	Study on the Lagoon Ecosystem on the Dongsh Atoll-Discuses on Physical,Chemical Hydrology and Basic Productivity
Contracted Organization	National Sun Yat-sen University
Principal Investigator	Luo Wen-Tseng
Co-Principal Investigators	Hong Chia-Chang, Wang Yu-Huai
Project Participants	Hsu Pei-Kai, Liao Chen-Heng, Lao Po-Hsuan, Lin Hao-Chun, Yang Shun-Huai, Huang Chih-Ling, Hsin Wen-Fu, Lin Yi-Wen, Tseng Yi-Po, Peng Shao-Hung, Li Chen-Yu, Huang Mei-Hui, Lu Wan-Tzu, Yeh Yi-Ting, Chen Ying-Ting, Huang Wan-Chen, Shen Yi-Chieh, Lin Shao-Wei, Lin Yu-Ju, Li Min-Tzu, Wu Po-Feng, Huang Kang-Ming, Wei Hung-Ju, Chang Ya-Han, Tsao Po-Chi, Wan Yu-Chen, Hsu Chia-Ming, Lo Ho-Tsung
Keywords	Dongsha Atoll, water temperature, ocean currents, water quality, plankton
Abstract	I. Background This study aimed to understand the ecosystem within the Dongsha Atoll lagoon, focusing primarily on planktonic flora and fauna as fundamental components of the ecosystem. Plankton are highly sensitive to changes in hydrological conditions and strongly linked to ecosystem dynamics. Their fluctuations influence higher trophic levels, including nektonic and benthic communities. By integrating physical oceanographic and hydrological factors, this project explored the mechanisms driving ecological changes within the lagoon. II. Methods and Procedures Three field surveys were conducted between April and October 2011. Sampling included water quality measurements at 12 stations, plankton sampling at 10 stations, and deployment of one sediment trap station. These field observations provided data on water quality and plankton communities in the Dongsha Atoll waters. III. Results Seasonal variation in water temperature was evident, highest in July, followed by October, and lowest in April. Salinity variations were related to rainfall, lowest in October and highest in April. Nutrient distributions were comparable to surface waters of the northern South China Sea shelf. Nitrogen and phosphorus co-limitation occurred in April and July, while clear nitrogen limitation was observed in October. DOC and POC distributions were influenced mainly by chlorophyll-a distribution and detrital inputs. Suspended and sinking particles within the atoll exhibited distinct characteristics. Organic carbon flux showed significant seasonal differences, suggesting biological processes dominated flux variations. Notably, calcium carbonate sedimentation flux was approximately 30 times higher than organic matter flux, a characteristic feature of coral reef ecosystems. Biogeochemical parameters in the small lagoon differed markedly from those in the main atoll lagoon, likely due to differences in water volume and possible anthropogenic influences. A total of 82 phytoplankton species from 46 genera were recorded. Surface abundance was slightly higher than deeper layers, with an average of eight phytoplankton species per sampling. Dominant species included Thalassionema nitzschioides, Pseudo-nitzschia delicatissima, Nitzschia danysii, Ceratium furca, and Skeletonema costatum. Zooplankton comprised 28 major groups, with higher surface abundance. Dominant groups included copepods, crab larvae, fish eggs, foraminifera, and shrimp larvae. A total of 67 copepod species from 46 genera were identified, with higher average abundance in deeper layers. Dominant species included Acartia omorii, Oncaea media, Pontellidae copepodites, Acrocalanus gibber, and Farranula gibbula. Cluster analysis indicated clear seasonal variation in plankton community composition. Due to the shallow water depth, vertical differences between surface and deeper layers were not significant, nor were differences between inner and outer lagoon areas. IV. Recommendations Recommendation 1 – Immediate Actions Lead Agency: Marine National Park Headquarters Supporting Agency: National Sun Yat-sen University Seasonal sampling alone may not adequately reflect actual climatic variability. Establishing continuous monitoring stations is necessary to analyze long-term biogeochemical time-series changes. Although sedimentation flux data were collected for the first time within the atoll, limited study duration prevented assessment of long-term spatial and temporal trends. Continued long-term monitoring is recommended, pending funding availability. To fully evaluate lagoon productivity and biodiversity, surveys should expand beyond primary producers (phytoplankton and zooplankton) to include broader taxonomic groups. It is recommended to promote the establishment of an International Marine Research Station on Dongsha Island to facilitate long-term hydrographic and ecological monitoring and improve understanding of mechanisms controlling hydrological and ecological changes. Recommendation 2 – Medium- to Long-Term Actions Lead Agency: Marine National Park Headquarters Supporting Agency: National Sun Yat-sen University Following establishment of long-term monitoring stations, sustained funding and personnel will be required for data acquisition, analysis, and maintenance. Continuous long-term investigations are essential for understanding environmental change trends in the Dongsha Atoll waters, advancing marine conservation strategies, promoting scientific research, and ensuring sustainable ecological and resource management within Dongsha Atoll National Park.