| Landsat ETM image of Lake Taihu acquired on Aug.20, 2010 showed that there was black color water bloom (BCWB) near the shore of western part of the lake, the remotely sensed image was atmospherically corrected, several ROIs (Region Of Interest) were placed on BCWB area and other water bodies, the spectral reflectance statistics were extracted for these ROIs. Results showed that BCWB ROIs had very low reflectance at ETM visible bands 1,2 and 3, hence had very dark to black water color, but at ETM reflective infrared band 4,they showed slightly high reflectance than band 3,hence the Difference Vegetation Index (DVI) had positive value. The cause was that despite the rapid degeneration of cyano bacteria in BCWB, there still remain some live cyano-bacteria cells in the water bloom, the remnant chlorophylls and cell structure were able to reflect solar radiation at reflective infrared wavelengths. The criteria for identifying BCWB was proposed as ρ0.485＜0.05 and ρ0.56＜0.08 and ρ0.66＜0.065 and (ρ0.83-ρ0.66)＞0 and ρ0.83＜0.1. |
