Studies On Community Structure Of Plankton And Zoobenthos In Lakes Of Different Trophic Levels

The major contents of this paper included: comprehensive assessment on trophic state of lakes in Liangzihu water system; impacts of lake trophic levels on community structure and biodiversity of phytoplankton,protozoa,rotifers,crustacean zooplankton,zoobenthos. The results are as follows. Comprehensive assessment of trophic states was conducted by means of comprehensive trophic state index(TSIc) in four shallow lakes of Liangzihu water system. The principal component analysis (PCA) of 8 water-chemical variables in four lakes was also carried out. The results showed that Lake Yanglanhu and Lake Yanjiahu were in the state of eutrophication, Lake Wushihu and Lake Honglianhu were in the state of mesotrophication. The results indicated that PO4-P, TP and SiO2 were the main contributors to the first principal component.NO3-N, TN dominated the second principal component. The phosphorus content played the most important role in determining water quality of the four lakes. The first principal component was strongly related to the comprehensive trophic state index. Comparative studies on community structure of plankton and zoobenthos were carried out in four lakes with different trophic levels. The results showed that the higher the trophic level of the lake water, the lower the density and biomass of phytoplankton. No significant differences among phytoplankton species number were found between different trophic levels. The present study exhibited different percentage contribution of each algal class(phylum) to the total density and biomass in different trophic state of lake . The lakes having same trophic state rendered a higher similarity of the dominat species and seasonal changes. The biodiversity of phytoplankton did not decrease distinctly when the lake water changed from mesotrophication to eutrophication. No significant differences were observed in protozoan species number of four lakes, but the lake in different trophic state displayed a different protozoan community structure and a different functional-trophic group. Both protozoan density and biomass increased with lake trophic levels increasing.A significant difference in spatial heterogeneity and seasonal succession of dominant species were found in different water,s trophic station.It also seemed that the lake trophic state mayn,t significantly affect biodiversity of the protozoa. It is found that the higher the trophic state of lake, the smaller the number of species and biodiversity of rotifers, and that rotifer density increased with lake trophy levels increasing. There were significant differences in number of oligosaprobity and their distribution between nonpollutted and polluted lakes. There was a negative correlation between trophic state index and Margalef index, Shannon-Wiener index. The results indicate that eutrophication could result in a remarkable decline in spatial heterogeneity of rotifers. With the decrease of the trophic levels, there was an increase in species number of cladoceran. The copepods species number was similar among the four lakes. Significant differences in dominant species composition of cladoceran were observed in lakes of different trophic levels, but the copepods displayed a similar composition. No correlation existed between density or biomass of planktonic crustaceans and lake trophic level. The biodiversity of cladoceran in eutrophic lake were slightly lower than that in mesotrophic lake. Considerable differences in zoobenthos species composition, dominant species, abundance, biomass and diversity index were observed. The results showed the higher trophic levels of lake, the smaller species number and biodiversity. Inversely, zoobenthos density increased with the lake trophic state increasing on the whole. The density of the oligochaete Tanypus chinensis was positively correlated with the state of lake pollution. A negative correlation was apparent between the species number of mollusca and the pollution level. These results indicated that a clear decrease in biodiversity should be attributed to water pollution. The biomass of plankton yielded significant positive correlation with lake trophic state. The lakes having higher trophic state displayed a higher abundance and biomass of zooplankton . There were significant differences in species distribution of plankton and zoobenthos in different trophic level of lake. Total species number exhibited a strong statistical relationship with lake trophic state. The correlation between community structure of biota and lake trophic levels showed rotifers> protozoa > zoobenthos > cladoceran > phytoplankton > copepods. The lake trophic level had similar impact on plankton and zoobenthos.