Study Of Substrate Improvement Technic Synergistically Submerged Macrophyte Restoration And Its Microecological Effect In Shallow Lakes

Acting as the receiving water body of inland freshwater ecosystem,a variety of lakes has faced with severe challenge of eutrophication even hypereutrophication worldwide due to the sustained imputing and accumulation of nitrogen and phosphorus pollutants from point and non-point source pollution induced by anthropogenic activities.Caused by eutrophication,freshwater ecosystem biodiversity decreasing,submerged macrophyte community decline,and release of sediment internal pollutants jointly lead to the submerged macrophyte dominant and clean water state transformed into phytoplankton dominant and turbid state resulting in the aquatic ecological function of lakes decline or even lost completely.Because of the multiple ecological functions,reestablishing the ecosystem with submerged macrophyte as the core is an effective means to recover ecosystem of shallow lakes.West Lake is a typically urban shallow lake which is still faces eutrophication problems.This study was carried out that builds upon the successful implementation of the substrate sediment improvement and submerged macrophyte restoration of West Lake.1.Submerged macrophyte restoration is the key stage in the reestablishment of an aquatic ecosystem.Sediments of West Lake is characterized with high organic matter content,the special sediments conditions are not conducive to the field planting and propagation of submerged macrophytes.The community of submerged macrophytes have been restored through ecological substrate improvement technique.Previous studies have paid considerable attention to the effect of multiple environmental factors on submerged macrophytes.Meanwhile,few studies have been conducted regarding the spatial and seasonal characteristics of water and sediment properties and their long-term relationship with submerged macrophytes after the implementation of the submerged macrophytes restoration project.On a monthly basis,the spatial and seasonal variation in water parameters,sediment properties,and the submerged macrophyte characteristics of West Lake were investigated in Hangzhou from August 2013 to July 2019.From these measurements,the relationship between environmental factors and submerged macrophytes was determined.Results as follows:（1）Results showed that the submerged macrophytes characteristics exhibited an obviously improvement,the biomass of vegetation increased from 0.31 kg m^-2 in July2013 to 1.45 kg m^-2 in July 2019 in West Lake,and water NH₄⁺-N,NO₃^--N,TN and TP were declined from 0.15 mg L^-1,1.83 mg L^-1,2.75 mg L^-1 and 0.07 mg L^-1 in 2013to 0.06 mg L^-1,0.35 mg L^-1,1.51 mg L^-1 and 0.02 mg L^-1,respectively.Water nutrient concentrations continuously decreased with time as the submerged macrophyte communities developed on a long-term scale.The difference in water parameters was due to seasonal changes and land-use types in the watershed.The differences in the sediment properties were mainly attributed to seasonal changes.（2）Redundancy analysis demonstrated that the influence of water nutrients on the submerged macrophyte distribution was greater than that of sediment nutrients.The result also illustrated that the developed root system,high stoichiometric homeostasis coefficients of phosphorus（P）,and compensation ability of substantial leaf tissue may lead to a large distribution of Vallisneria natans in West Lake in Hangzhou.The correlation of water parameters and sediment properties with submerged macrophytes for a long time was very important as the restoration was achieved.2.In this study,temporal and spatial characteristics of water and sediment nutrients in a typical subtropical shallow urban lake Hangzhou West Lake were evaluated,meanwhile,quantitative analyses were investigated to determine the contribution of water nutrients,sediment properties and submerged vegetation on retention and release of sediment total phosphorus.Results as follows:（1）Results illustrated that the water nutrients characteristics are determined by the water flow field and season changes.Additionally,much lower Nitrogen/Phosphorus（N/P）ratio（6.29±3.25～16.82±1.54）of downstream locations compared to that（9.79±1.30～26.33±2.90）of upstream and midstream locations in lake water,which led to the inconsistent characteristics between water nutrients and water chlorophyll a（Chl-a）and trophic state index（TSI）.Temporal and spatial distribution of STP determined by season changes,water flow field types,submerged vegetation types,and deposition of aluminium salt（Al-salts）originated from water diversion project.Results of principal component analysis demonstrated that the effect of interception of submerged vegetation and flow velocities reduction induced by water flow field types changed are similar that both accelerate the phosphorus retention in sediments.（2）Results of structural equation modeling illustrated that the main effect on STP is water nutrients including water total nitrogen（TN）,Chl-a,and TSI,which implied the nitrogen limitation is the key factor.Water nutrients significantly and directly affect（coefficient is 0.254）STP while submerged vegetation exhibited indirectly effect induced by water nutrients（coefficient is 0.077）and sediment properties（coefficient is 0.062）.The loss of nitrogen and an increase in phosphorus mainly influence the submerged vegetation distribution,higher trophic level index and sediment phosphorus release.3.A mesocosmic system composed of water,sediment,substrate improvement materials and submerged macrophytes was established.Bentonite,maifanite and vermiculite were selected as substrate improvement materials.The experimental vegetations were Vallisneria natans,Hydrilla verticillate and Potamogeton malaianu.In this study,the improvement and microecological effects of sediment microenvironment by substrate improvement synergistically submerged macrophyte community recovery techniques were investigated from the aspects including sediment physicochemical properties,sediment denitrification rate,sediment CH₄ and CO₂ production rate and sediment bacterial community structure.（1）All mixing ratio treatment of the 3 materials slightly decreased sediment p H,and kept the sediment environment slightly acidic.The addition of bentonite and vermiculite can increase the water content of sediments,while the addition of maifanite decrease the water content of sediments.Compared with Hydrilla verticillate and Potamogeton malaianus,the treatment group of Vallisneria natans can enhance the water content and redox potential of sediments.Results exhibited the 30%and 40%mixing ratio treatment groups of 3 materials have better effect of controlling sediment ammonia（NH₄⁺-N）and nitrate（NO₃^--N）,and testified that the trend of NH₄⁺-N transforming to NO₃^--N.30%and 40%mixing ratio treatment groups of bentonite and vermiculite have better effect on removing sediment organic matter（OM）than other treatment groups.（2）The 3 kinds of materials have an obvious removal effect on sediment Fe/Al-P and IP,and all mixing ratio treatment groups of bentonite is better than that of maifanite and vermiculite.30%bentonite and vermiculite treatment groups had better removal effect on Ca-P.40%bentonite and 30%vermiculite have better removal of sediment Ca-P,higher mixing ratio materials have better removal of sediment total phosphorus（TP）.Except for Ca-P,all sediment P components exhibited that Vallisneria natans<Hydrilla verticillate<Potamogeton malaianus.（3）The sediment denitrification of denitrifying bacteria could be improved by adding materials,denitrification of vegetation types exhibited that Vallisneria natans>Hydrilla verticillate>Potamogeton malaianus.Addition of materials in sediment could improve the methane（CH₄）production rate.The CH₄ production rate of material types exhibited that bentonite<maifanite<vermiculite,and Vallisneria natans<Hydrilla verticillate<Potamogeton malaianus for vegetation types.The treatment of bentonite synergistically Vallisneria natans is the optimal group for controlling the release of CH₄and CO₂.（4）Results of microbial structure showed that main bacteria phyla were Chloroflexi,Proteobacteria,Firmicutes,Acidobacteria,Actinobacteriota,Desulfobacterota and Bacteroidetes,and the abundance of Chloroflexi and Acidobacteria could be increased by adding materials into sediment,while the abundance of Proteobacteria and Firmicutes could be decreased.The bacterial composition of maifanite and vermiculite groups were significantly different from that of control and bentonite groups,and there was some similarity between Vallisneria natans and Hydrilla verticillate.The abundance of certain functional microbial groups was enhanced due to adding materials although the diversity of bacteria communities decreased.Meanwhile,there is less humic organic matter in sediment and the organic matter decomposition is in the later stage which illustrated that the sediment microenvironment is in a relative better state that is benefit to microorganisms and submerged macrophytes.