湖沼生態系におけるアオコの異常発生現象には次のような特徴が見られる．(1)アオコをもたらす究極の原因である湖の富栄養化は10年，20年の歳月をかけて徐々に進行するが，異常発生はある年を境に，突然，勃発する(突然の出現)．(2)アオコの主成分であるミクロキスティスなどの藍藻類は冬から春にかけて湖底で越冬し，夏の訪れとともに湖面に上昇して｢水の華｣と呼ばれる異常発生現象を引き起こす(年周期の垂直上下運動)．(3)夏季の異常発生期間でもこれらの藍藻類は，午前中，水面に出て光合成を行い，午後になると水中に沈んで栄養分を吸収する(日周期の垂直上下運動)．本論文ではタイムスケールの異なるこれら3つの特徴を的確に説明するために，栄養塩と藍藻類から成る2つの2変数数理モデル(常微分系の基本モデルと偏微分系の垂直上下運動モデル)を作成する．そして，これらのモデルを用いて，神奈川県の『県営水道の水質』に記録された相模湖と津久井湖におけるアオコの異常発生現象を解析する．本論文の解析によれば，相模湖・津久井湖水系は1970年代前半に澄んだ状態から濁った状態にレジームシフトし，以後，現在まで濁った状態，すなわち夏季のアオコ異常発生が恒常化した状態が継続している．またアオコの発生量，発生パターンに関する年ごとの変動には日照量，水温，栄養塩濃度といった生態学的，生理学的要因とともに，台風の襲来，ダムの放流といった自然，人為による偶発的要因も深く関与していると考えられる．

Algal blooms in lake ecosystems are characterized by the following features. (1) Algal blooms break out abruptly at a certain time, although eutrophication, the ultimate cause of algal blooms, proceeds gradually over decades (abrupt outbreak of the phenomena). (2) Cyanobacteria such as Microcystis, the main component of algal blooms, overwinter at the bottom of the lake during the winter season, rising up to the water surface with the coming of summer (annual vertical migration). (3) During the summer season, cyanobacteria repeat vertical movement for photosynthesis at the surface from the midnight to the morning and for nutrient uptake at subsurface layers from the afternoon to the early evening (diurnal vertical migration). In this paper, we present two mutually correlated mathematical models, a fundamental model described by ordinary differential equations and a vertical migration model described by partial differential equations, both of which consist of nutrients and cyanobacteria. These models can properly explain the above-mentioned phenomena that differ in time scales. Then, we apply these aquatic models to the algal blooms in Lake Sagami and Lake Tsukui, referring to “Quality of prefectural tap water” published by Kanagawa Prefecture. According to our analyses, the aquatic system of these lakes has undergone the regime shift from the clear-water state to the turbid-water state at the beginning of the 1970s, with the turbid-water state continuing until now. In both lakes, the abundance of cyanobacteria and the seasonal algal blooming pattern differ considerably depending on years, indicating the significant influence of accidental factors of the natural and the anthropogenic origins such as the advent of typhoon and the water discharge from the dam as well as the ecological and the physiological factors such as the light intensity, the water temperature and the nutrient concentration.