Study On Bio-optical Properties Of Microcystis And Its Remote Sensing Identification

With the rapid development of economy, massive algal (blue-green algae in particular) blooms are often observed in Chinese eutrophic lakes in recent years. There is an emerging need to predict the occurrence of blue-green algae blooms for minimizing the adverse effects. The research proved that the predominant algal species in algal blooms was Microcystis. So the ability of measuring the content of Microcystis in water precisely plays a very important role in warning of algal blooming. This paper selected Microcystis aeruginosa, chlorella vulgaris and Cyclotella meneghiniana Kiits as research objectives, and their bio-optic properties were investigated. Through cultivating in laboratory, bio-optical parameters of single type of algae along with mixed algae in different proportions were measured and the difference of apparent optical properties and inherent optical properties between Microcystis aeruginosa and the other two was analyzed. Meanwhile. A method of identifying Microcystis aeruginosa through its absorption features based on characteristic bands was proposed, and the model was used to identify the proportion of Microcystis aeruginosa and to analyze its distribution pattern in Taihu Lake.The conclusions are as follows:1. The difference on bio-optical parameters among Microcystis aeruginosa, chlorella vulgaris and Cyclotella meneghiniana Kiits.The difference on absorption features among Microcystis aeruginosa and other two was detailed analyzed. The result showed that the absorption characteristics of Microcystis aeruginosa is different from that of chlorella vulgaris and Cyclotella meneghiniana Kiits with an absorption peak at620nm, which was caused by the absorption of phycocyanin.In the aspect of scattering properties, the position of scatter peak and valley of Microcystis aeruginosa and other two types of algae varies considerably. There is a peak near410nm when the presence of Microcystis aeruginosa was enough in the scattering spectral of Microcystis aeruginosa, while chlorella vulgaris has a scatter peak at525nm, scatter valleys at435and675nm, and Cyclotella meneghiniana Kiits has a scatter valley at690nm.In the case of backscattering feature, There are clear peaks in wavelength440nm、532nm and676nm in both Cyclotella meneghiniana Kiits and chlorella vulgaris backscattering spectral, while Microcystis aeruginosa only has a remarkable peak in440nm and a sub-peak in510nm which decreases with the increase of wavelength in510-715nm.In addition, there are lots of differences among three algae in reflectance spectral. Valleys at440nm and675nm, and peaks at550nm and700nm appeared in the reflectance spectral of Microcystis aeruginosa and Cyclotella meneghiniana Kiits, which is similar to the reflectance of chlorella vulgaris. But the reflectance peak of Cyclotella meneghiniana Kiits appeared at wavelength700nm and800nm, and the valley appeared near500nm and660nm. The most obvious difference among Microcystis aeruginosa, chlorella vulgaris and Cyclotella meneghiniana Kiits is that reflectance of Microcystis aeruginosa has a remarkable valley at620nm.2. Recognition model of Microcystis aeruginosa under laboratory conditionsFirst, the average standardized absorption curve of Microcystis aeruginosa and chlorella vulgaris according to their absorption features were got. Second, the characteristic bands of Microcystis aeruginosa that was used to recognize was determined. Finally, the recognition model of Microcystis aeruginosa by means of multiple linear regression analysis based on absorption spectrum of mixed algae was developed.3. Application of recognition model of Microcystis aeruginosa in Taihu Lake and its distribution pattern analysis.The proportion of Microcystis aeruginosa based on measured absorption curve in Taihu Lake and in different depths of the water was conducted using our recognition model.The results proved that the minimum proportion of Microcystis aeruginosa in Taihu Lake appeared at southwest of the coastal area, and the maximum value appeared in the center of the lake. In addition, the mean proportions of Microcystis aeruginosa in Zhushan Bay region and Gong Lake region are relatively high, which are up to90%. But the percentage of Microcystis aeruginosa in Meiliang Bay is pretty low with the value of56.41%. The proportions of Microcystis aeruginosa in coastal areas, southern coastal and eastern coastal are very low, which are28.59%and34.44%, separately, while in the west part, the proportion reaches to77.93%.Meanwhile, The distribution of Microcystis aeruginosa in vertical section was also discussed. The minimum proportion of Microcystis aeruginosa of most sampling sites emergeed at120cm below the water surface, while the maximum value appeared at40cm or80cm below the water surface. The average proportion of Microcystis aeruginosa first increase and then decrease from the surface to the bottom of water, reaching to the maximum proportion in40cm and the minimum value at120cm below the water surface.