Figure 3Biodiversity index of the phytoplankton in different sampling sites in Baiyangdian Lake in spring (a) and summer (b).3.2. Phytoplankton DensityThe number of phytoplankton in Baiyangdian Lake varied greatly between spring and summer. In spring, the density of phytoplankton varied from 496 �� 104 to 6256 �� 104cells/L with an average of 2384 �� 104cells/L; the LB42708? density varied greatly between different sampling sites: the density of phytoplankton cells in Sites 5 and 7 was high, while the density of Sites 1 and 8 was low. In summer, the density of phytoplankton varied from 318 �� 104 to 4630 �� 104cells/L with an average of 1785 �� 104cells/L (Figure 4); the density varied greatly between different sampling sites: the density of phytoplankton cells in Sites 4 and 6 was high, while the density of Sites 5 and 7 was low.

Generally speaking, the density of phytoplankton and dominant species can indicate the eutrophication degree of certain water. As species of high pollution tolerant, if Cyanophyta increases sharply and finally becomes the dominant species, it indicates that the water is eutrophic, that is to say, the higher the density of Cyanophyta cells is, the more serious the eutrophication is [18]. According to this survey, cyanophyta and Chlorophyta became dominant in the phytoplankton community, that may be caused by the increased organic matters after organic matters in industrial wastewater and domestic sewage came into Baiyangdian Lake [19] and resulted in the increase of varieties and number of phytoplankton, especially these species with high pollution tolerance.

Figure 4Comparison of phytoplankton density in different sampling sites in Baiyangdian Lake in spring and summer.To get a general understanding of the change of phytoplankton community in Baiyangdian Lake, data of the three times surveys of phytoplankton since 2005 were compared (Table 1), and dynamic variations of phytoplankton in Baiyangdian Lake in recent years were analyzed from the aspects of species composition, density, and dominant species. The data of the years 2005 and 2006 were averages from the 8 sampling sites in Baiyangdian Lake in spring and summer, and the data of the year 2009 were averages from the current survey in 8 sampling sites in Baiyangdian Lake in spring and summer. The survey methods were the same.Table 1Variations of phytoplankton density and dominant species.Table 1 shows the variations of phytoplankton density and dominant species in Baiyangdian Lake in recent years. Taking the year 2005 as GSK-3 the reference point, the average phytoplankton density decreased 0.22 times in 2006, whereas the average phytoplankton density increased 2.41 times in 2009. In the three surveys, Chlorophyta and Cyanophyta were the mainly dominant species.