last day which is an effective monitoring day to find algae
bloom at a certain time is called as end blooming date (EBD)
in certain water region. The mathematical expression is as
the follow:
ASI=EBD-IBD+¢ (2)
ASI can reflect the length of algae bloom in some area at
certain time (in a year) (Duan H et al, 2009; Ma R er al,
2010). € is decided by the monitoring time interval. The
some area can be corrected to the pixel size.
2.3 The process of extracting the index and information
In order to achieve the extraction of temporal and spatial
distributions information about algal glooms, the space
distribution data of algal glooms in effective monitoring
days are stored as open storage formats, which use storage
framework of separating geographic code from data in
favour of data sharing and interoperability among various
sectors. The data include the original value and time
information (date and days in a year). According to the
method above, the IDL (Interactive Data Language) program
is compiled and executed to extract temporal and spatial
distributions information, obtaining the algae bloom
frequency index and the algae bloom sustainability index
map corrected to image pixel size based on MODIS data and
statistical information about the subregion in Taihu Lake.
3. RESULTS AND DISCUSSION
3.1 The Algae Bloom Frequency and Sustainability
Index
The figure 2(a, b and c) shows the annual algae bloom
outbreak situation in 2009, 2010 and 2011. It can be seen
that the area of highest of AAFI is mainly at the Western
Coast region (74096). The area of AAFI beyond 30% was
also located in Meiliang Bay and Southern Coast. In brief,
the area of higher frequency of booming focused on the
north and west of Taihu Lake. Conversely, it is very low in
the Gonghu Bay and extensive Central Lake. The booming
frequency decreased from the north and west to the East and
South of Taihu Lake, even if, in closer to the aquatic
vegetation area there are many blank area that has no
booming frequency in 2009. In 2010, the area of AAFI
beyond 40% is narrower than that in 2009, but the area of
AAFI beyond 30% or 20% is larger than that in 2009. The
highest of AAFI has moved from Western Coast region to
the border regions between the Western Coast and Central
Lake. It's worth noting that, in 2010, in the Central Lake;
area of AAFI beyond 20% is a common phenomenon but
rarely appears in 2009. These characteristics show that the
degree of booming is more serious than that in 2009. There
is a general trend that the outburst of booming is frequent in
the east part of Taihu Lake. From the picture of AAFI in
2011, it looks like that degree of bloom frequency is
significantly reduced compared with 2010. The area of
highest of AAFI is mainly at the Western Coast region,
which is as similar as the condition in 2009. It is obvious to
the trend of contraction from the Central Lake to the north
and west.
The figure 2(d, e and f) shows the annual algae bloom
duration in 2009, 2010 and 2011. The spatial distribution of
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
sustainability index was inconsistent with that of bloom
frequency index in these three years. There are rarely areas
that the duration of booming is beyond 200 days in 2009.
Such area is mainly located in Meiliang Bay. The regions
having the greater duration are the Western Coast, Meiliang
Bay and Southern Coast in which the blooming duration is
general more than 150 days. The blooming duration in
Central Lake is mostly less than 100 days. In 2010, there are
few regions that have the duration more than 200 days,
which located in Western Coast and Meiliang Bay. The
regions having the greater duration are border regions
between the Western Coast and Central Lake in which the
blooming duration is general more than 150 days and the
distribution area is smaller than that in 2009. The blooming
duration in Central Lake which is about 100-150 days is
much higher than that in 2009. Note that spatial distribution
of the blooming duration is stranger than the situation in
2009 and 2010. It is not like the duration that showed the
tendency of increasing from the East and South to the north
and west of Taihu Lake, but the messy distribution. The
regions having the greater duration are border regions among
the Central Lake, Meiliang Bay, and Gongshan Bay in which
the blooming duration is general more than 350 days and
significantly higher than maximum value of blooming
duration in 2009 and 2010. The spatial distribution area of
the other time slice of blooming duration is much smaller
than that in 2009 and 2010.
3.2 The Initial Blooming Date and End Blooming Date
The figure 2 (from g to 1) shows the annual initial blooming
date and end blooming date situation in 2009, 2010, and
2011. It can be seen that the area of the lowest value of IBD
(111th days) is located in Western Coast region in
2009(figure 2 g). It is a general trend that the date of initial
blooming is lower and lower from the north and west to the
Central Lake. The latest end blooming date is about 280th
days in 2009 and the area is located in the border between
the Gongshan Bay and Central Lake, in which the bloom
duration is very short(figure 2 h). In 2010, the area of the
lowest value of IBD (120th days) is also located in Western
Coast region in 2010(figure 2 i) and just closer to the Central
Lake. The initial time of the southern coast had been put off.
The latest initial blooming date is about 230th days in 2010
and the area is located in southeast part of the Central Lake.
The latest end blooming date went to the 350th days and that
spatial distribution is wide. The area with earlier end
blooming date is the small part of Zhushan Bay and
Gongshan Bay. In 2011, the area of the lowest value of IBD
(8th days) is also located in the border between the
Gongshan Bay and Central Lake and the date is obvious
earlier than that in others subregions in Taihu Lake. The
annual initial blooming date (about 120th days) in Western
Coast is as similar as the date in 2010 and 2009. The latest
end blooming date went to the 350th days and that spatial
area is Gongshan Bay and Central Lake. Combined with the
sustainability index (Figure 2 1), it shows that the blooming
appeared in January, 2011 and December, 2011.
3.3 Intermonth Variation
Here, it is necessary to discuss the overall trend of booming
in the 2009-2011, Taihu Lake. Because the visible-infrared
