亞熱帶水庫中不同粒徑微囊藻日夜週期移動模式模擬

Abstract

優養化是造成台灣水庫水質惡化的常見原因之一，並且藍綠藻中的微囊藻屬為其中常見的優勢藻種。本研究之研究場址新山水庫即為一例，其為一亞熱帶離槽水庫，常在六月份到九月份發生微囊藻藻華現象。過去研究顯示，微囊藻於水體中的日夜週期垂直移動行為及形成藻團的特性，可能是其於水體中取得優勢的原因。微囊藻可藉由消耗及累積因光合作用所產生的細胞壓艙物含量來調整藻體於水體中的浮力及密度，進行日夜垂直的移動；而微囊藻形成藻團的特性則可能和其於水體中的垂直移動、營養鹽儲存及抵抗攝食壓力有關。 本研究之目的為修改及校正簡（2013）所發展之微囊藻移動軌跡模式。藉由於新山水庫水體分層時，現地採樣監測得到之光線強度、水體溫度、微囊藻藻濃度垂直分布及粒徑分布變化的結果，來模擬微囊藻日夜週期垂直移動，同時將藻體密度變化的延遲時間加入模式中，並且校正藻體於暗處之密度變化率常數。 新山水庫現地24小時觀測結果顯示微囊藻確實有日夜垂直移動行為。不同時間及深度的微囊藻粒徑分布結果顯示，粒徑大於100 μm的藻團甚少，粒徑介於30到50 μm的藻團雖然佔大多數，但以藻細胞數來說，粒徑介於50到90 μm的藻團佔最多；並且，由不同時間的粒徑分布可觀察到粒徑介於50到90 μm的藻團於1 m移動至7 m，此粒徑範圍的微囊藻移動行為較明顯。模式模擬結果也顯示，粒徑介於50到100 μm的藻團的垂直移動行為較明顯，而粒徑小於50 μm的藻團則是停留在水深2-3 m處，此皆和現地觀測結果相似。模式中加入藻體密度變化之延遲時間的加入確實有助於模擬微囊藻垂直移動現象，並且此軌跡模式的發展將微囊藻日夜週期垂直移動行為大致地模擬及描述出來。

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.