| With the rapid development of the economy and the increasing population, many of industrial wastes and domestic sewage, which are rich of nutrients, are discharged into the rivers and lakes, resulting in the grievous degradation of structure and function of the ecosystems. Eutrophication has become the main ecological environmental problem restricting our country's sustainable economy. Lake Taihu located in Yangtze River Delta area is the third largest freshwater lake, a typical shallow lake. It is an important drinking water source for Wuxi and Suzhou City and a water source supplies for Shanghai City and the eastern area of Zhejiang Province. Lake Taihu drainage area is not only one of the four most populated and urbanized regions, but also of the most developed districts in China. Eutrophication has been gradually deepened since 1980s, cyanobacterial blooms occurs with increasing frequency have seriously affected the local people's life. Consequently, the restoration of Lake Taihu ecosystem has been an imminent important matter.It is generally accepted that biomanipulation is one of the most effective methods for improving lakes eutrophication in the long run. Biomanipulation in shallow lakes employ stocking filter-feeding fishes, removing benthic fishes, growing aquatic plants or other ways to reconstruct biological community in order to decrease effectively the biomass of phytoplankton by means of cascade responses, and to restore clear water state dominated by macrophytes from the turbid state dominated by phytoplankton. Bivalve, a common macro-mollusc in freshwater ecosystems, feeds on phytoplankton, particulate organic matter and other suspended particles in water. We constructed microcosms to study the effects of filtration of Hyriopsis cumingii (a native bivalve in Lake Taihu) on cyanobacteria, and analyzed the effects of H. cumingii on the growth of submerged macrophyte Vallisneria natans under different conditions of eutrophic states. We summarized the mechanism that filter-feeding bivalve mediated the interspecies relation between phytoplankton and submerged macrophytes on the basis of results, and hoped that could provide theoretical basis for shallow lake restoration.To study the effect of different H. cumingii biomass on the growth of V. natans, we constructed four groups of microcosms (V. natans, V. natans+cyanobacteria, V. natans+ cyanobacteria+low H. cumingii biomass and V. natans+cyanobacteria+high H. cumingii biomass) in Meiliang Bay from Jul. to Aug.2010. The results showed that (1) cyanobacterial bloom significantly reduced the growth of V. natans, and the relative growth rate (RGR) of V. natans+cyanobacteria treatment and V. natans treatment were-7.65 mg·g-1·d-1 and 7.19 mg·g-1·d-1 respectively. (2) Reduction of the cyanobacterial biomass by H. cumingii depended on bivalve biomass, and such effect was significantly less under the condition of low biomass of bivalves. (3) H. cumingii mitigated the inhibiton on the growth of V. natan by cyanobacterial bloom, and the RGR of V. natans in low H. cumingii biomass and high H. cumingii biomass treatments were -5.31 mg·g-1·d-1 and 4.24 mg·g-1·d-1 respectively.We carried out another experiment to investigate the growth of V. natans in microcosms, in response to three concentrations of cyanobacterial chlorophyll a approximately 100,200 and 300μg/L respectively under the presence and absence of H. cumingii from Sep. to Oct.2010. The results showed that (1) increase of cyanobacteria in water column led to decrease of leaf number, tubers length and RGR of V. natans. Plants allocated more biomass to the root, resulting in higher root:shoot ratio. (2) H. cumingii improved the underwater light condition via filter-feeding on living cyanobacteria and suspended particles derived from it, especially in the treatment with low cyanobacterial concentration. However, there were no significant difference in dissolved nitrogen and dissolved phosphorus among treatments. (3) Compared with the control (without bivalves), the plants grown in treatments with bivalves showed high leaf numbers and length, tuber length and RGR.Our results suggest that the water body is more favourable for the growth of submerged macrophytes when filter-feeding bivalves are present, and improvement of light condition resulting from filter-feeding activities by H. cumingii contributes to the growth and biomass redistribution of V. natans. However, such positive effects of bivalves are closely interrelated with the biomass of bivalves and phytoplankton, and very poor when biomass of cyanobacteria is beyond the control by filter-feeding capacity. Therefore, we have to consider not just a long time scale but also spatial setting and regulation of the biomass/density of filter-feeding bivalves when filter-feeding bivalves are used to restore submersed macrophytes. Only in this way can it effectively promote the restoration of the submersed macrophytes. |
PDF Full Text Request