Time Series Analysis of Chlorophyll-a Concentration considering Algae Class in a Eutrophic Water Body using Discrete Wavelet Transform

In the water environment evaluation of a eutrophic water body, it is important to conduct it from the viewpoint of seasonal change and how that affects the quantity and kind of algae in addition to its gross weight. In this study, consecutive observation of the Chl.alpha density according to the algae class was conducted, and the change of characteristics was examined by wavelet analysis. In this research, for In Vivo Chl.a, which is divided into blue-green algae, green algae, dinoflagellate genus/diatoms, and cryptophyte algae, total Chl.a and water temperature were measured by multiwavelength excitation spectrofluorometer (FluoroProbe). Additionally, the surface water was sampled about once a week, and algae were identified to the genus level with an optical microscope. Based on this observation, a lot of blue-green algae exist in the intended reservoir in summer, and as blue-green algae decreased, dinoflagellate genus/diatoms and cryptophyte algae also decreased. In the fluctuation component of the Chl.a density in each algae class, a slow frequency component from a few days to a week accelerated, and there was the contribution of a frequency component of about 1.2 × 10^(-5) (circadian rhythm) locally. The Chl.a density increased when the contribution of the fluctuation component of this circadian rhythm was high in all four kinds of algae. Furthermore, to examine the relevance between a change in the Chl.a density and the environmental factor, a cross-correlation coefficient of each Chl.a density for water temperature and amount of solar radiation was evaluated. As a result, because the correlation between each Chl.a density and the environmental factor could not be found, their fluctuation components were focused on, and the cross-correlation of the wavelet component interval for 24 hours was examined for both. In this way, good cross-correlation was accepted for blue-green algae and green algae, which should be considered mainly a result of eutrophication. From these, good responsiveness could be found by paying attention to the fluctuation component of a circadian rhythm when relevance of the quantity of the outbreak of algae and the environmental factor were examined.富栄養化水域の水環境評価において，藻類の総量に加え，種構成や現存量の季節的変化の観点から検討する意義は大きい．本研究では，Chl.alphaならびにTPがいずれも富栄養湖レベルにあり，夏季にアオコの発生が観測される農業用ため池を対象に，多波長励起蛍光光度計により藍藻類，緑藻類，渦鞭毛藻/珪藻類，クリプト藻類に区分した４種の藻類綱別Chl.alphaの連続観測を行った．ウェーブレット解析による各Chl.alphaの動態特性を検討することで，対象水域の藻類の種構成とその現存量の季節的消長について論じた．観測結果より，対象池では夏季に藍藻類が多く，秋季にかけて藍藻類が減少するにつれて渦鞭毛藻/珪藻類，クリプト藻類が増加した．ウェーブレットによる藻類綱別Chl.alphaの時間周波数解析の結果，各藻類綱に共通して，観測期間中の変動成分では，１週間以上の緩やかな周波数成分が卓越し，また約$1.2 \times $10^-5$$Hz（1日周期）以上の高い周波数成分の寄与が局所的に存在した．４種いずれの藻類において，この１日周期の変動成分の寄与が高い期間で，Chl.alpha濃度の増加が見られた．つぎに，Chl.alpha濃度の動態と環壇要因との関連性を検討するために，水温，日射量に対する各Chl.alpha濃度の相互相関係数を求めたところ，各Chl.alpha濃度と環境要因との間には明確な相関を見出せなかった．そこで，その変動成分に着目し，両者の１日周期のウェープレット成分間の相互相関について検討した．その結果，観測期間中に優占的に存在した藍藻類，緑藻類の１日周期の変動成分は，水温，日射量のそれに対して良好な相互相関を示した．以上より，藻類の発生量と環境要因の関連性を検討するにあたり，１日周期の変動成分に着目することで応答性を見出すことができた