| Due to the development of economy and the unreasonable utilizations of lakes, eutrophication becomes the major problem to many lake ecosystems. Studies show that there are two stable states in shallow lakes, the state of clear water dominated by submerged macrophytes and the state of turbid water dominated by phytoplankton. To reverse a turbid state to a clear state is the major aim of ecological restoration in many lakes. Presently, the combination of biomanipulation and macrophyte restoration is the major measure for ecological restoration. The successful applications are focused on the eutrophic shallow temperate lakes in European and Northern America, less studies about lake restoration on in subtropical lakes.Lake Wuli is a bay located on the north of Lake Taihu which has a subtropical monsoon climate. There were many submerged macrophytes in Lake Wuli in history. After1960s, with the development of economy, the water quality of Lake Wuli became worse and worse, and the submerged macrophytes disappeared. In order to improve the water quality, the Wu xi Municipal has thrown a lot of manpower and material resources. Although the water quality was improved but the water was still turbid. Ecological restoration demonstration area was established in Fisherman Island in Lake Wuli in May2010for further improve water quality. The major measures include submerged macrophyte re-establishment and fish removal. The submerged macrophytes planted include Elodea nuttallii, Vallisneria natans, Hydrilla erticillata, Ceratophyllum demersum, Potamogeton malainus, Potamogeton maackianus, Myriophyllum verticillatum. And we controlled the biomass of planktivorous and benthivorous fish by stocking piscivorous fish, for example, Siniperca chuatsi. In order to reveal the mechanism of water quality improvement of eutrophic shallow subtropical lakes, the water quality of the restored and the un-restored area was monitored from July,2010to July,2011.The concentration of nutrients in the restored area declined obviously. The average concentration of TN, TDN, PN, NO3-N, NH4-N, NO2-N, TP, PP were0.76mg/L,0.58mg/L,0.173mg/L,0.17mg/L,0.10mg/L,0.0046mg/L,0.037mg/L,0.018mg/L in the restored area respectively and were1.13mg/L,0.72mg/L,0.428mg/L,0.35mg/L,0.23mg/L,0.0261mg/L,0.051mg/L,0.037mg/L in un-restored area respectively. The average concentration of the former were32.7%,19%,59.5%,51.4%,30.4%,82.7%,29.4%,51.3%lower than the latter respectively. But the concentration of solubility active phosphorus in the restored area was significantly higher than the un-restored area and the average concentration was0.007mg/L in the restored area and was0.003mg/L in the un-restored area. The former was56.7%higher than the latter. The concentration of TSS and Chl-a also declined distinctly. The average concentration of TSS and Chl-a were4.55mg/L,5.34μg/L separately in the restored area and were16.78mg/L,13.70μg/L respectively in the un-restored area. The average concentration of TSS and Chl-a in restored area were72.9%and61.0%lower than the un-restored area. The transparency in restored area rose obviously. The average of transparency was1.19m in the restored area and was0.47m in the un-restored area. The transparency in the restored area was about two point five times higher than the un-restored area.Based on these results, we can conclude that the combination of fish removal and macrophyte re-establishment is an efficient method for restoring subtropical shallow eutrophic lakes, such as Lake Wuli. |
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