Biomass Threshold Of Lake Aquatic Plants Targeting Clear Water Steady State

One of the objectives of ecological restoration of eutrophic shallow lakes is to develop a stable and healthy ecosystem.The core approach,is the restoration of the submergent mycrophytecommunity.However,there are some problems on the restoration of aquatic mycrophytes.For example,unreasonable planting density,explosive growth of submerged plants in summer and decaying and decomposing during winter and next spring period.In this paper,the eutrophic shallow water lake Datong Lake is taken as the research area.Based on the hydrological,meteorological and water quality data of Datong Lake,PClake model is constructed from the perspective of suitable biomass threshold of aquatic plants.Based on this,a set of suitable research methods of the optimal biomass of lake aquatic plants to improve water quality was established.We simulated the dynamic changes of TP,TN,ammonia nitrogen,Chl-a concentration and biomass of aquatic plants in the water,and analyzed the impact of nutrient increase and decrease on lake ecosystem,the results of this study can provide important theoretical basis and practical guidance for ecological restoration of shallow lakes.The main conclusions are as follows:(1)Building PCLake model.Based on the survey of the ecosystem in this study area,Datong Lake's hydrological,meteorological,and water quality data from 2016 to 2019,We provided the initial conditions,boundary conditions,and external environmental conditions for the construction of the PCLake model,and used this to build the PCLake model.(2)Sensitivity analysis and parameter calibration.Through the comparison and fitting analysis of the measured monitoring and simulation values,we finally determined the calibration values for 19 sensitivity parameters.We used the water quality data monitored in 2018-2019 to verify and analyze the model,and the simulation value and monitoring value in a clear water state have a better convergence effect.The RMSE values of TP,Ammonia nitrogen,TN,and Chl-a are 0.094,0.199,0.822,and 16.246 respectively.The PCLake model can accurately predict the relationship between nutrient salts in water bodies and the growth cycle of aquatic plant biomass.(3)Indoor experiments on the decay of plants in spring.The experiment was set up with 0%,20%,40%,60%,80%,100% coverage of different life forms of aquatic plant community decomposition experiment.The experimental results show that in the mud-water-plant system,plants are the main source of nutrient salt release.After being released from the plants into the water,they were adsorped intohe sediment,and part of them were converted into gas and emitted into the air.By comparing the nitrogen and phosphorus contents in the terminal water body,and considering the economy,harvesting 60% of the biomass during the harvesting period showed the best effect on maintaining the water quality of Datong Lake.(4)Model application.The results show that the existing submerged plant biomass is higher than the simulated value,and at the same time,there are only a few small emergent plants(Lotus)in the Datong Lake area.Based on the simulation of aquatic plants from 2020 to 2024,the biomass(wet weight)of submerged plants should be controlled to be 5.5 kg/m2.Lake managers should increase the cultivation of hydroponic plants and optimize the distribution of aquatic plants,and homogenize the range of plant growth.Avoid over-concentration of aquatic plants,so that the distribution of aquatic plant communities is reasonable.Also,if the nutrient load of the lake is increased by 5%,aquatic plants may die,and the water quality of Datong Lake will further deteriorate.If we reduce the pollution load of Datong Lake into the lake by 20%,we can meet the national surface water quality standard of Class IV.If we reduce the pollution load of Datong Lake into the lake by 50%,we can meet the national class III surface water quality standards.The effect of increasing the pollution load into the lake on the concentration of nutrient salt in the water body is much higher than the effect of reducing the concentration of nutrient salt into the lake.