Ocean current, depth of water and cache of sea area of reef atoll are investigated in Dongsha

Project Duration	2008-03-31 ~ 2008-12-30
Research Title	Ocean current, depth of water and cache of sea area of reef atoll are investigated in Dongsha
Contracted Organization	National Sun Yat-sen University
Principal Investigators	Chen Yang-Yi, Wang Yu-Huai, Lee Chung-Pan
Co-Principal Investigators	Hsueh Hsien-Wen, Tien Wen-Min, Liu Chin-Yuan, Dai Chang-Feng, Tseng Jo-Hsuan
Project Personnel	Kuang Che, Liu Huang-Tsung-Lung, Fu Ke-Hsien, Lin Yu-Ju, Tsai Ya-Ju, Hsieh Yu-Chan, Lin Kai-Lun, Lin Sheng-Chin, Li Ying-Tsao, Huang Kang-Ming
Keywords	Ocean currents, habitat, water temperature
Abstract	Commissioned by the Marine National Park Headquarters to support environmental conservation and future development of Dongsha Atoll National Park, this project conducted surveys of ocean currents, bathymetry, and habitats within the inner lagoon of Dongsha Atoll. The study included historical data compilation, field observations, and data analysis. Unlike previous surveys focused mainly around Dongsha Island, this project conducted large-scale bathymetric measurements across the inner lagoon. From May 6 to August 26, survey work completed 1-km interval transects within the lagoon, 100-m interval transects within 3 km of Dongsha Island, and 100-m interval transects in both the northern and southern channels. The team deployed seven fixed temperature loggers, documented 296 habitat stations, collected 3,712 current measurements, 10,537 dynamic water quality records, and 573 static water quality records. Dynamic water quality results showed that average surface water temperatures within the lagoon were approximately 26.36°C in May, 29.3°C in June, 29.4°C in July, and 28.7°C in August, with relatively small spatial variation. Average salinity ranged from 34.21 to 34.61 psu. Surface pH values declined slightly from 8.23 in May to around 7.9 in July and August. Habitat classification identified six primary types: sand, macroalgae, seagrass, coral, reef rock, and coral rubble. Live corals were found in deeper areas; however, coral rubble dominated much of the inner lagoon. Seagrass distribution was more extensive along the southern coast of Dongsha Island than the northern coast. Surface current measurements showed an average velocity of 14.11 cm/s (SD = 11.04 cm/s). Ebb tide currents averaged 10.38 cm/s, while flood tide currents averaged 8.99 cm/s, indicating limited variation due to shallow water depth and lack of vertical stratification. Current circulation formed two gyres divided by the “Dragon’s Tail” area: north of this boundary, currents flowed northwestward and exited via the northern channel; south of it, currents flowed toward and exited through the southern channel, with greater discharge through the northern channel overall. Maximum observed velocity reached 50.27 cm/s. Tidal ellipses were dominated by M2, K1, and O1 constituents. Bathymetric analysis indicated depths of approximately -2 m within 1 km of Dongsha Island, with more reef structures to the north and sandy substrates to the south. At 3 km offshore, depths reached -5 m. The northern channel ranged from -5 to -6 m, while the southern channel was deeper at -7 to -8 m, making it more suitable for navigation. The deepest point within the inner lagoon reached -22.6 m. Survey findings suggest that the northern channel contains more reefs, shallower depths, and stronger currents, making navigation less favorable compared to the southern channel. For safety considerations, entry and exit via the southern channel are recommended. Based on bathymetry and current data, estimated seawater residence time within the inner lagoon is approximately two days. Fixed-point temperature records indicate daily inflow of cooler water near both channels. It is recommended that long-term monitoring stations for currents, waves, and temperature be established inside and outside the atoll, along with multipoint water quality monitoring, to better understand water exchange processes and nutrient dynamics, thereby supporting coral restoration and conservation within the lagoon.