However, other unknown physical, chemical, and biological factors can probably directly or indirectly influence the number of phytoplankton in a certain way, because the growth of phytoplankton is also related to many other factors (e.g., water stability, climate, lake area, lake depth, spatial distribution of organic matter and heavy metals in wetland soils, the community sellckchem structure, and density of hydrophyte) [24�C35]. Therefore, it is difficult to do ��dose-effect analysis�� of the interaction between the number of phytoplankton and the abovementioned factors. It is suggested that in the future research, more efforts should be made to study lake type, climate characteristics, and surrounding environment, and simulated experiments should be also included to the impact factors on variation of phytoplankton number.

3.4. Eutrophication Degree AssessmentGenerally, the excessive growth of phytoplankton is the characterization of eutrophication. Chla, SD, and dominant species are usually regarded as the most important indicators for the assessment of eutrophication degree. In this paper, the index of TSIM and the dominant genus assessment were used to assess the trophic status of Baiyangdian Lake [36]. The index of Carlson nutritional status (TSIM) can elaborately describe the change of water trophic status and can also improve water quality monitoring and assessment. The method is to grade the lake trophic status with numbers from 0 to 100 according to the relation between SD, Chla, and TP. Index under 30 indicates oligotrophic water, index from 30 to 50 indicates mesotrophic water, and index from 50 to 100 indicates eutrophic water.

Under the same trophic status, the higher the index is, the more serious the eutrophication is. According to this assessment result (Table 4), water in Baiyangdian Lake is mesotrophic and eutrophicconsider the following:TSIM??(Chl??a)=10��(2.46+lnChl??aln?2.5),TSIM??(TP)=10��[2.46+(6.71+1.15��lnTP)ln?2.5],TSIM??(SD)=10��[2.46+(3.69?1.53��lnSD)ln?2.5].(4)In this formula, Chl a is the content of chlorophyll a; SD is secchi depth; TP is total Phosphorus.Table 4Assessment results of trophic status index of Baiyangdian Lake in 2009.According to the dominant genus assessment, dominant phytoplankton genus observed in this survey included not only indicator species for eutrophication, such as Chlorella sp., but also indicator species for serious eutrophication, such as Microcystis incerta Lemm. and Chroomonas Entinostat acuta Uterm. with indicator species for eutrophication as the dominant species.