Long Time Scale and Spatial Distributions of Freshwater Algae in
Taihu Lake, China Using MODIS Data
Abstract
Introduction
Taihu Lake (30°5'-32°8'N and 119°8'-121°55'E) is the third largest freshwater lake in eastern China, with surface
area of 2,338 km2 and a mean depth of 1.9 m. It locates in semi-tropical monsoon climate, and average multi-year
precipitation is about 1,100mm to 1,150mm, average temperature is 15℃ to 17℃(Jia et al., 1999). The lake is
traditionally divided in seven major parts including four embayment that are Zhushan Bay, Meiliang Bay, Gong Bay,
and East Bay (Mao et al., 2008), (see figure 1).
Fig.1. Location of Taihu Lake
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It is not only the main freshwater source of Wuxi citizens but also supports the irrigation of agriculture around the
lake. Unfortunately, from 1980’s the lake becomes a highly eutrophic lake (Ding et al., 2007), although local
government have enforced “ZERO” actions at the end of 1998 that requires all wastewater flowing into Taihu Lake
must reach the requested standard, and this actions have good effect on reducing the outside eutrophic water flowing
into Taihu Lake, it can not reduce the inside eutrophic water already contained in the lake. Research show that “ZERO”
actions play a very little effect on solving the eutrophic problem of Taihu Lake (Huang et al., 2002). During 1991 to
2007, tendency of total phosphorus, total phosphorus and suspended solids are increasing gradually, this tendency are
especially serious during 2001 to 2007, meanwhile the transparency of water decline during 1991 to 2007 (Zhu, 2008),
and average annual chlorophyll a concentration raise from 24.7 μg/l to 40.5 μg/l during 2000 to 2007, this lead to
sustained algae blooms (Jiang et al., 2001) and reduced the water quality of Taihu Lake. The most eutrophic area is the
Meiliang bay (Ma et al., 2005) which is water source of Wuxi city. During May to June 2007, algae bloom outbreak
makes a bad situation of water supply of Wuxi city, waterworks is enforced to close and nearly 1,000,000 people are
affected (Duan et al., 2008).
This serious situation caused a wide attention, and many studies have used satellite image to focus on the water
quality elements of Taihu Lake including color dissolved organic matter (CDOM), chlorophyll a and total suspended
matter (TSM). Spectral analysis has been shown that CDOM absorption at 440 nm, chlorophyll a absorption peak at
670 nm (Caren et al., 2008), TSM has high relevant at 830nm (Ma et al., 2005) so ratio method between these bands of
satellite data are used to set regression function on water quality index (Alicia et al., 2007), chlorophyll a and TSM(Wu
et al., 2005, Zhu et al., 2006)combining with fields observed data. Also other satellite data productions like Normalized
Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) are used to study blue algae by threshold
segmentation method (Xu et al., 2008) or by decision tree classification(Chen et al., 2008). However, there are still
some spaces need to improve, firstly these study usually lack of long time scale monitoring, that is very important for
us to trace the seasonal and spatial changing about the eutrophic water, secondly most functions they setup are based on
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single day’s field observed data that restrict the universal use for multi-date in the same water so common a model is
necessary.
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