| Summary: | 博士 === 國立中興大學 === 生命科學系所 === 103 === Tropical seagrass beds are important ecosystem which serves numerous key eco-functions in coastal regions. In this study, we evaluated what eco- functions did seagrass beds provide for fish resources around Dongsha Island, a remote island on South China Sea. We found the multi-species seagrass meadow around Dongsha had high primary production. However, seagrass shoots are poor-quality food for most reef fish. Only parrotfish and some invertebrate grazers (sea urchin, isopod and nerite snail) feed seagrass as main food sources. Significant spatial and seasonal herbivory differences were found. Grazers removed seagrass biomass about 3.6 times higher in subtidal zones than in intertidal zones and 2.6 times higher in warm season than in cool season. Leaf biomass losses were positively correlated with water depth, leaf production, and starch content. We found that only less than 10% of the leaf production was consumed by grazers at Dongsha. This indicated that most seagrass production was not directly used by consumers and most seagrass production will flow into detrital pathway in system. The large amount of seagrass detritus is the main energy sources to support various consumers at Dongsha. Deposit-feeding invertebrates are important secondary producers as they recycle energy from detritus in the systems. Most consumers (especially fish) in Dongsha seagrass beds are carnivorous and these invertebrate-feeding carnivores contributed about 70% of the total fish abundance.
Seagrass provide a perfect shelter for many juvenile reef fish. Fish assemblage around Dongsha seagrass beds are highly affected by tides and habitats. The deeper water during flood tides supports more space for many fish in intertidal zones. Moreover, the subtidal zones had more abundant fish than intertidal zones because of deeper water. The complex landscapes of seagrass and coral reef mosaics also tend to high fish diversity around Dongsha. We found that high coral reef dominant habitats, especially in the mixture habitats provide higher habitat complexity for more juvenile fish than pure seagrass beds. Although coral reefs have abundant juvenile assemblages, they supply shelter mostly for small-sized coral resident species. Seagrass, on the other hand, supply shelter for more diverse habitat niche groups and large-sized species. Coral reefs have high predator and competitor densities, and may lead to lower survival rate for many juvenile fish. However some juvenile fish may select coral habitat for more prey invertebrates or fish when under a low predation risk condition. Fish from different niche have distinctly habitat usage pattern among seagrass and coral mixture gradients. These differences in habitat selection may relate to the dietary variations which different species would optimize their growth benefit and competition avoiding on the ideal habitat. The seagrass/coral mixture habitat can provides flexible habitat choices for various juveniles, leading to high diversity within fish assemblages.
We revealed Dongsha seagrass bed ecosystem is considered to be a developing, but well-structured ecosystem. High seagrass production and low cycling rate suggest that most detritus may not be reused or exported and become benthic detrital storage around Dongsha. As a result, we considered the large part of unconsumed seagrass will accumulate into benthic organic mass sink in seafloor. The high trophic level fish and large crustaceans are the main key stones at tropical seagrass beds. Their predation affects the abundance of various compartments and lead to a strong top-down control in trophic cascade. High ecotrophic transfer efficiency (11.3) in system indicates the trophic structure at Dongsha is a relative unexploted sytem comparing with other overfishing reef systems.
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