亞熱帶離槽水庫微囊藻取得優勢之機制分析及利用軌跡模式建立動態消長模式之研究

Analysis of the mechanism of Microcystis domination and modeling its succession in a subtropical off-channel reservoir with a trajectory approach

簡鈺晴(Yu-ching Chien)

指導教授 ： 吳先琪

國立臺灣大學/工學院/環境工程學研究所/博士(2013年)

https://doi.org/10.6342/NTU.2013.03080

摘要

在亞熱帶地區水庫水質惡化的原因以藻類優養問題最為普遍，而其中又以藍綠藻產生的問題最為棘手。藍綠藻所分泌之有機物質為消毒副產物之前質，而且有多種藍綠藻有產生藻毒素的能力。微囊藻即為台灣地區最常見之產毒藍綠藻種之一，以新山水庫為例，近年來所觀察到之藻華優勢藻種幾乎均為微囊藻（吳等，2010）。2005年環保署調查台灣11座優養水庫之結果也顯示均有微囊藻藻華出現（郭等，2005）。因此微囊藻華成為最為急切亦最需要重視的問題。
本論文研究以新山水庫為例，利用時間及空間藻濃度分佈探討微囊藻季節消長之原因。調查資料包括2005至2009年之藻類、水質及氣象等背景資料，及庫內分層採樣之各水質及藻類濃度。研究工作首先以統計分析方法探討微囊藻和各環境因子間之關係。進而以新山水庫的生態系為例，將微囊藻浮力調控機制與體內營養鹽影響生長速率之Droop生長模式納入，以軌跡模擬方法建立之微囊藻動態生長的預測模式，以期能準確描述在環境因子影響下藻類生長與水體物理化學變化的關係，提供微囊藻動態消長現象較合理之解釋。
由研究的結果發現，微囊藻數量與水體穩定度有密切關係。在穩定度漸增時，可觀察到微囊藻數量逐漸增加之情形。水體水質剖面分佈結果得知混合時期水體之各項水質及藻類因子與深度無顯著相關性，而分層水體與轉型過渡水體之各項水質及藻類因子與深度相關性較佳。為了將水體分層及微囊藻生長特性納入藻類消長模式中，本研究發展一利用軌跡模擬的方法來模擬微囊藻之日週期性移動及動態消長的模式。此模式較平流擴散模式更能代表實際自然水體中微囊藻團移動及生長情形。軌跡模式詳細記錄藻團經歷不同光線、營養鹽等環境歷史，並模擬伴隨而來之後續生理及移動路線之個體間差異。模擬結果證實微囊藻在分層水體中存在一最佳之藻團大小分佈。較小之藻團不易克服水中之密度梯度及渦流，故易散佈在水體中某一深度，沒有明顯的集中；較大顆粒藻團其垂直移動速度較大，可取得日照量及營養攝取量兩種生長要素，具有較大生存優勢。但是500 um以上之藻團，因停留強光區受光抑制現象過長，若考慮生長及營養鹽攝取反應時，反而不具有絕對生存優勢。模擬之結果大致符合實際觀察所得到之微囊藻日夜週期性之垂直分佈。
研究亦以模式模擬分為上層及底層操作進水方式情境來探討營養鹽在不同進水位置對微囊藻生長差異。模擬結果顯示底層進水可將光線與營養鹽隔絕，不失為控管水庫微囊藻生長的好方法。
水庫現地模組試驗，係將水庫10公尺之上層水混合，使微囊藻停留在光區之時間減少，探討此混合方式對現地微囊藻消長之影響。結果顯示藻類總數量增加，但微囊藻減少，藻相組成有顯著改變。混合作用可以有效減少微囊藻數量，但是將營養鹽帶至上層，造成可能適應低光強度之藻類形成優勢。未來軌跡模式可以很容易擴大成為多藻模式，用於現地實驗案件之模擬驗證及藻華控制方案分析。

關鍵字

微囊藻 ； 藻團 ； 優勢機制 ； 軌跡模式 ； 動態消長模擬 ； 亞熱帶水庫 ； 光線 ； 密度 ； 垂直移動 ； Droop生長模式 ； 上層水混合試驗

並列摘要

Eutrophication is the most common cause of water-quality problems; in subtropical regions while the eutrophic conditions caused by cyanobacteria are particularly of most concern. Several cyanobacterial species release toxins and most species excrete dissolved organic matters which are the precursors of disinfection by-products (DBP). Microcystis is the most common bloom-forming genus of toxic cyanobacteria in Taiwan, which is also the dominating species in Hsin-shan Reservoir in recent years (Wu et al., 2010). Taiwan EPA has investigated 11 eutrophic reservoirs in 2005. Microcystis has appeared in all of those reservoirs (Kuo et al., 2005). Hence, Microcystis bloom is the most concerned and urgent problem.
In this thesis, Hsin-shan Reservoir is the reservoir studied to investigated the Microcystis seasonal succession. The temporal and spatial field data including Microcystis concentration (cells/L) over depth, water quality parameters over depth and climate records from 2005 to 2009 were collected. At the beginning, relationships between Microcystis succession and environmental factors were be identified with statistic approaches. Then, a model was developed to simulate the dynamic change of Microcystis population by using a trajectory approach, in which the ability of diurnal migration due to density change and growth rate caused by cell quota were taken into consideration. Better understanding on the relationship between the environmental factors and Microcystis successtion was obtained through the model simulation.
It was found that the abundance of Microcystis had positive correlation with water stability. There is no relationship among water parameters while water body was well mixing. Microcystis abundance and most water parameters vary over depth under stratified and transition stages. A trajectory approach was used to simulate the diurnal migration and succession of Microcystis, for the purposes of considering the stratified environment and the special growth behavior of Microcystis. This developed model is better than an advection-diffusion model for being able to reflect the migration and growth pattern of Microcystis, by recording all histories of surrounding light intensity, nutrient, and other environmental factors which each individual colony has experienced. The results of the simulation show that there exists an optimum distribution of Microcystis colonies which enable Microcystis obtain dominance in stratified water bodies. Smaller colonies are dragged by turbulent force and dispersed randomly over the mixing zone; whereas larger colonies obtain advantages byovercoming turbulent diffusion and getting higher frequency of cyclical shifts between the euphotic zone and the nutrient-rich deeper water layers. However, when taking the mechanism of the growth and that of nutrient-uptaking into consideration, the colonies with diameters larger than 500 μm will not gain dominance due to staying surface layer too longer to get enough nutrient. The results of the simulation are coincident with the field observation of the vertical profile of Microcystis concentration.
Furthermore, the effect of intake elevation on the Microcystis growth by the simulation of two intaking scenarios, inflow from surface or flow from bottom was investigated. The results of simulation show the operation by intaking from bottom was able to separate light and nutrient and suppress the growth of algae. The results suggest a solution for controlling Microcystis growth in a stratified reservoir.
The results of the investigation of the effect of reducing available light intensity by artificial mixing of water within 10 meters from surface showed that the total algal concentration become higher and the dominating species had changed. Although the abundance of Microcystis was reduced significantly, phytoplanktons with competitive ability of survival at lower light intensity had dominated due to that nutrients might be transported into the epilimnion by mixing. It will be easy to expand the trajectory model into multi-species succession models in the future, and the field data of operating and artificial mixing could be used to verify the model of the multi-species succession.

並列關鍵字

Microcystis ； colonies ； dominating mechanism ； trajectory model ； dynamic succession simulation ； subtropical reservoir ； light ； cell density ； vertical migration ； Droop’s growth model ； artificial mixing

參考文獻

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