| Summary: | 碩士 === 國立臺灣大學 === 環境工程學研究所 === 101 === Algal blooming is the major causes of water-quality deterioration in subtropical reservoirs in Taiwan where Microcystis sp. is the most common bloom-forming genus of cyanobacteria, which is also the dominating species in Hsin-Shan Reservoir in the summer in recent years. The capabilities of both the diurnal vertical migration and colony forming are suggested to be the important reasons for the dominance of Microcystis in a wide range of aquatic ecosystems. Forming colonies is helpful to the vertical migration, nutrient storage, and to the defense against predation pressure. Microcystis can regulate their buoyancy and density by changing the amount of cell ballast, which is made possible by accumulating or consuming carbohydrate produced by the photosynthesis.
The purposes of this research was to modify the trajectory model developed previously in this laboratory and to simulate the daily vertical migration of Microcystis with different colony sizes by using the information of light intensity, temperature, algae concentration, and size distribution, collected from Hsin-Shan Reservoir. The influence of the response time on the density change was taken into consideration, and the formula of the density change rate in dark was also investigated.
The on-site 24-hours investigation on August 30–31, 2012 demonstrated that Microcystis was the dominant species in Hsin-Shan Reservoir and indeed migrating vertically. The observation of size distribution indicated that few colonies had size larger than 100 μm. Although most colonies had sizes ranging from 30 to 50 μm, the colonies with sizes between in 50 to 90 μm accounted for the major portion in terms of the cell numbers. Furthermore, the synchronized migration of colonies with sizes in the range of 50 to 90 μm is obvious. They dominated at 1m and then moved to 7m. The results of model simulation show that the main group of colonies migrating vertically has sizes in the range of 60 to 100 μm, which is consistent with the on-site observation. The colonies smaller than 50 μm often stays at depth between 2-3 m. The response time does help the model to simulate the migration of Microcystis more accurately. The modified model is able to describe the diurnal migration phenomenon of Microcystis well.
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