| Summary: | 碩士 === 臺灣大學 === 環境工程學研究所 === 95 === It has been demonstrated that phosphorus concentration is a limiting factor for the growth of planktons in reservoirs and lakes. High phosphorus concentration results in high rate of algae growth and eutrophic state of reservoirs. Bottom sediments have been identified as potential sink and source of phosphorus to the overlaying water. As a result, the understanding of the mechanisms and factors controlling phosphorus release flux from benthic sediment is needed. In the thesis, Hsin-shan Reservoir and Shih-men Reservoir were taken as our study sites for the investigation of sediment-water column interaction.
The results of fractional extraction analysis of bottom sediments and settling matters show that organic-P and calcium-P dominate in Hsin-shan Reservoir and Shih-men Reservoir, respectively. Therefore, in Hsin-shan Reservoir, dissolved oxygen has great influence on the release of phosphorus from sediments because of its rich of ferric content in sediment. In contrast, in Shih-men Reservoir, dissolved oxygen may not affect the release of phosphorus since aluminum and calcium minerals dominate in sediment. Flux of total settling phosphorus collected by sediment traps ranged from 1.36 to 1.68 mg/m2/day in Hsin-shan Reservoir and 6.04 mg/m2/day in Shih-men Reservoir. For the reason above, settling flux in Shih-men Reservoir is influenced by external input significantly.
Through the investigation of the phosphorus release flux under oxic or anoxic condition, the following results were observed: The release flux in oxic condition was about 0.10 and 0.13 mg/m2/day in Hsin-shan Reservoir and Shih-men Reservoir, respectively. Under anoxic condition, it was about 3.57 and 0.10 mg/m2/day in Hsin-shan Reservoir and Shih-men Reservoir, respectively. Mass balance for the annual external and internal phosphorus loading shows that released internal loading from bottom sediment has exceeded external loading in Hsin-shan Reservoir in 2006. However, internal phosphorus loading only accounts for 1.7% of the external loading in Shih-men Reservoir in the same year. So it is concluded that internal loading from sediments may have considerable impact on water quality in Hsin-shan Reservoir but minor impact in Shih-men Reservoir.
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