Temporal Dynamics of Rocky-shore Macroalgal Assemblage Structures in Relation to Coastal Construction Threats in Orchard Island (Taiwan): Impacts of turbidity and nutrients on the blooms of Galaxaura oblongata and a red alga-sponge symbiose Ceratodictyon/
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概要
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Ecosystems in coastal areas of islands around Taiwan have faced construction threats in the past 10 years. A study was conducted from 2001-2004 to monitor the effects of disturbances on macroalgal assemblage structures on a nearshore rocky reef in Orchard Island (off southeastern Taiwan), where seashore road and jetty construction took place during 2002-2003. Outdoor laboratory experiments were used to confirm the factors responsible for changes of macroalgal compositions. Macroalgal cover and biomass increased markedly in 2002-2003 mainly due to the blooms of a red alga-sponge symbiose Ceratodictyon/Haliclona and a calcified rhodophyte Galaxaura oblongata. Hierarchical cluster analysis and non-metric multidimensional scaling ordination analysis of species similarities between different sampling times showed there are 3 clusters corresponding to 2001, 2002/2003, and 2004. The results of ANOSIM tests showed that species structure was different not only from year to year but also between seasons, and the results of SIMPER analysis showed that the blooms of G. oblongata and Ceratodictyon/Haliclona in 2002 and 2003, the decline of Halimeda opuntia in 2002-2004, and the appearance of Amphiroa fragilissima and Gelidiopsis repens in 2004 contribute to annual differences. Stepwise regression analysis indicates that temporal variations of Ceratodictyon/Haliclona biomass is negatively related to monthly maximum temperature and soluble-reactive phosphorus (SRP) concentrations but positively related to turbidity and DIN concentrations, while Galaxaura oblongata biomass is positively related to turbidity but negatively related to monthly minimum temperature and monthly cumulative precipitation. Halimeda opuntia biomass is negatively correlated with monthly maximum temperature but showed a positive relation to salinity and SRP concentrations. The comparison of macroalgal compositions with environmental variables shows that turbidity and SRP are the best combination of environment variables to explain the yearly changes in algal compositions. The data from outdoors laboratory culture experiments suggest that low SRP/high dissolved nitrogen (DIN) concentrations and reduced irradiance are the factors which led to the blooms of both Ceratodictyon/Haliclona and Galaxaura oblongata in 2002-2003. In conclusion, the coastal construction threats are reflected in increasing turbidity and high nitrogen/low phosphate loading, which result in the blooms of Ceratodictyon/Haliclona and Galaxaura oblongata and in turn, the modification of macroalgal assemblage structures around Orchard Island off southeastern Taiwan.
- 2009-03-25
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