Investigation of Biological Community and Assesment of Indicator Species in the Seagrass bed of the Dongsha Atoll

Project Duration	2011-01-02 ~ 2011-12-29
Research Title	Investigation of Biological Community and Assesment of Indicator Species in the Seagrass bed of the Dongsha Atoll
Contracted Organization	National Chung Hsing University
Principal Investigator	Lin Hsing-Chu
Co-Principal Investigator	Hsiao Shu-Chuan
Project Participants	Huang Yen-Hsun, Lee Cheng-Lu, Chung Chia-Yun, Tai Hsiao-Hsun, Lin Liang-Chin, Lin Wei-Jen, Cheng Hui-Yuan, Huang Cheng-Hao, Liao Ching-Chun
Keywords	Dongsha, seagrass, energy flow, carbon sink
Abstract	(I) Background The high primary productivity of Dongsha seagrass beds provides food and shelter for numerous organisms and serves as an important natural carbon sink. This project investigated fish and invertebrate resources within Dongsha seagrass beds, constructed food web energy-flow and carbon budget models, analyzed ecological roles of functional groups and their interactions with seagrass, and evaluated indicator species to provide baseline data for conservation strategies and resource management within the Marine National Park. (II) Results Key environmental factors affecting seagrass beds—including temperature, salinity, dissolved oxygen, and pH—were monitored. Water temperatures were higher in spring and summer and lower in autumn and winter. Salinity in the small lagoon fluctuated markedly due to high evaporation in summer and winter and frequent rainfall in summer and autumn. Dissolved oxygen and pH showed no significant seasonal or spatial variation. A total of 134 fish species from 45 families have been recorded in Dongsha seagrass beds. Species richness, abundance, and community composition were high and varied among sites. The west coast exhibited the highest species richness and fish density, while the small lagoon interior showed the lowest. Fish diversity peaked in summer and declined toward winter. Cluster analysis classified fish communities into three groups: inner small lagoon, lagoon mouth (west coast and outer small lagoon), and island periphery. Dominant families included Labridae, Lethrinidae, and Scaridae. Stomach content analysis indicated most fishes were carnivorous, primarily feeding on crustaceans; relatively few species fed directly on seagrass. Invertebrates were mainly polychaetes and small crustaceans, with relative abundances varying among sites. Three assemblage groups were identified: inner lagoon, outer lagoon, and island periphery (south and north coasts). Polychaetes dominated within and outside the lagoon, while either polychaetes or small crustaceans dominated around the island. Herbivory was higher in spring and summer and lower in autumn and winter, with parrotfishes as the primary grazers, followed by crustaceans. Grazing accounted for less than 10% of seagrass production, indicating that although herbivory was active, most seagrass biomass accumulated in plant tissues or entered the environment as detritus. Aboveground decomposition rates of seagrasses showed similar trends, with 50.87–81.96% biomass loss after 45 days. Species such as Halophila ovalis and Halodule uninervis decomposed more rapidly. Detritus export was higher in winter than in spring, averaging 0.74 g DW m⁻² d⁻¹, equivalent to 8.72 metric tons of detritus produced daily and approximately 3,183 metric tons annually across Dongsha seagrass beds. Carbon budget analysis showed that carbon production, herbivory, storage, and decomposition were higher in summer and lower in winter, while detritus export was higher in winter. With relatively few large herbivores, energy flow primarily occurred via detritus utilized by invertebrates and microorganisms, transferring energy through the food web. Energy-flow patterns were similar among the three habitat groups. The highest trophic levels consisted of transient carnivorous fishes, piscivorous fishes, and large benthic feeders (third- and fourth-level consumers). Seagrass exhibited relatively low ecological efficiency, indicating it was not the primary conduit of energy flow. The Lindeman energy pyramid showed comparable energy contributions from detritus and primary producers, with major transfers occurring between secondary and tertiary consumers. Keystone species identified from the food web models serve as indicator species for Dongsha seagrass beds. In all three habitat groups, top predators and large crustaceans (shrimp and crabs) were keystone species. Polychaetes were additionally important in the inner lagoon, while omnivorous fishes played key roles around the island, facilitating energy transfer within and between ecosystems. (III) Major Recommendations 1. Immediate Actions • Establish an internationally integrated seagrass ecological database and continue biodiversity research. Fifty unidentified species were recorded, many potentially new records or new species for Taiwan, and 25 species were newly recorded for Dongsha waters. • Implement long-term monitoring of seagrass beds to assess climate change impacts and support adaptive management. • Based on current inorganic carbon accumulation rates, sediment infilling of the small lagoon would take approximately 17,000 years; thus, short-term concerns about lagoon infilling due to excessive seagrass growth are minimal. 2. Long-Term Recommendations • Promote sustainable utilization of seagrass detritus, potentially integrating ecological education and regional特色 (特色) development, such as seagrass-based eco-architecture. • Support restoration of large protected herbivores. Dongsha’s relatively pristine environment makes it suitable for restoring species such as green sea turtles and dugongs. Further feasibility assessments for dugong restoration are recommended, with potential development of flagship species for future ecotourism initiatives in Dongsha Atoll National Park.