Research On The Recovery Of The Submerged Macrophyte In Eutrophicated Shallow Lakes

Since the 1980s,due to the rapid development of social economy and the intense human activities,most lakes in our nation were eutrophicated or eutrophicating.Based on "China Environmental Status Bulletin 2015",eutrophication of shallow lakes were particularly severe.One of the serious consequences caused by shallow lakes eutrophication was algal blooms and submerged macrophytes largely disappear.Reduction of nutrient concentrations is often insufficient to restore the vegetated clear state.The recovery of the submerged macrophytes in shallow eutrophic lakes was considered as an efficient measure to control eutrophication.But,for now,the recovery of the submerged macrophytes in large shallow eutrophic lakes seldom successed,more research should be done in this field.Given the facts above,we mainly study three aspects as follows:The first part of the dissertation was experimental study of submerged macrophytes to choose appropriate species and suitable initial plant biomass for recovery,while providing the field data for submerged macrophytes dynamics model calibration and validation.The second part of the dissertation was submerged macrophytes modeling.We built a submerged macrophytes dynamics model that could simulate several macrophyte species simultaneously.The model was calibrated and validated using pilot-scale experimental data,with satisfactory results.The model was then used to choose appropriate species and suitable initial plant biomass submerged macrophytes recovery in Gonghu Bay.The last part of the dissertation was effect evaluation of submerged macrophytes recovery.We built a new indices system for evaluating ecological-economic-social performances submerged macrophytes recovery and AHP were used to weighting each index.Then new indices system was applied to Gonghu Bay ecological restoration zone for dynamic effect evaluation of submerged macrophytes recovery(2014-2016).Our research use the recovery of the submerged macrophytes in shallow eutrophic lakes as entry points and was studied with experimental and numerical simulation methods.Conclusions obtained from the four-year study:The suitable submerged macrophytes pioneer species for recovery in Taihu basin including Ceratophyllum demersum,Myriophyllum spicatum,Potamogeton maackianus,Potamogeton crispus,Potamogeton malaianus,Potamogeton pectinatus,Najas marina,Vallisneria natans,Vallisneria spinulosa and Hydrilla verticillata.Filter-feeding silver carp deteriorated water quality by increasing nutrient and TSS concentrations,and decreased the biomass of submerged macrophytes.However,mussels promoted the establishment of a clear-water state by grazing on phytoplankton and detritus and stimulating submerged macrophyte growth.In terms of practical ecological engineering,we suggest that although silver carp has the potential to deteriorate water quality,the species can be used at low density(20 g/m3)as a biomanipulation tool.we constructed a submerged macrophytes dynamics model that can simulate several species simultaneously.The model was calibrated and validated using pilot-scale experimental data,and it achieved satisfactory results.The respective coefficient of determination(R2)and RMSE values were 0.86 and 0.03 for TP,0.71 and 0.38 for TN,0.93 and 6.78 for Chl a,and 0.99 and 27.61 for submerged macrophyte biomass.The model was then used to select the appropriate species and suitable initial plant biomass for the recovery of the submerged macrophytes in Gonghu Bay.The appropriate species at 1 m were V.spinulosa(1150g/m2),M.spicatum(525 g/m2),and V spinulosa-M.spicatum(600 g/m2;3:1).At 2 m,the appropriate species were M.spicatum(1450 g/m2)and V.spinuwosa-M.spicatum(1950 g/m2;12:1).Only M.spicatum(1608 g/m2)was recommended at 3 m.Meanwhile the model was used to determine the critical water transparency threshold for submerged macrophytes in Gonghu Bay,and a transparency value of 0.62 m at 2 m(WT/H value=0.31)can be regarded as the critical underwater light threshold for the recovery of submerged macrophytes in Gonghu Bay.A new indices system for evaluating ecological-economic-social performances submerged macrophytes recovery was built and applied to Gonghu Bay ecological restoration zone for dynamic effect evaluation of submerged macrophytes recovery(2014-2016).The performance of 2014 was not well,but in the following years,the community of submerged macrophytes optimized and biodiversity increased.The better natural environment attracts more tourists and the newly formed Gonghu Bay environmental management company improved the management efficiency.In 2016,the Gonghu Bay ecological restoration zone gained excellent grade in ecological-economic-social performance.The recovery of the submerged macrophytes in shallow eutrophic lakes were studied with experimental and numerical simulation methods.A series of key technical parameters for the submerged macrophytes recovery in shallow eutrophic lakes were given and applied to Gonghu Bay ecological restoration project.Compared with former models,the submerged macrophytes dynamics model we built in our study has a greater improvement and broad application prospects.Meanwhile,the ecological-economic-social performance of Gonghu Bay submerged macrophytes recovery was dynamic evaluated,making it easier for managers to adjust management plans and methods.The results obtained in this research provided a theoretical basis and technical support for the recovery of the submerged macrophytes in Gonghu Bay ecological restoration zone.In the meantime,it may provide a new idea for controlling the shallow lakes’ eutrophication and other water environmental researches.