| On the condition of Chinese economic development after reform and opening,it is a problem of eutrophication of lakes that becomes more and more noticeable in a large number of Chinese lakes,which has severely affected the people's lives and the sustainable development of society and economy.Moreover,China has become one of the countries which have a severe bloom phenomenon.Eutrophication of lakes owing to excessive input of nitrogen,phosphorus,and other nutrients are the proper environment for algae growing up at high speed as well as reproducing in large quantities,which contributes to destroy the stability of the entire ecosystem and gradually die the ecosystem.Therefore,controlling the loading of exogenous nutrients and the release of endogenous nutrients are the primary measure for preventing and controlling eutrophication in lakes.The main factors for bloom also include environmental ones(temperature,light,wind,etc.)and hydrodynamic conditions.Turbulent disturbances that the significant phenomenon is highly accessible to affect water environment variables can affect algae growing,moving and community succession directly or indirectly,which is commonly found in water systems.Previous studies have mostly studied the effects of water disturbances on the growth and transport of algae,through simulation experiments and field observation method.However,studies studies about the combined effects of flow disturbance and nutrients on algae are few,and the influences of wind-induced vertical disturbulences on the growth of algae in lakes are not in-depth and comprehensive.There are some errors in the research results of studies on the effects if wind-induced lateral disturbulences on the hotizontal transport of alage,due to the effects of algae proliferation and changes in the water environment.Besides,there are noticeable spatial and temporal differences between actual turbulent disturbances and the artificial turbulent disturbances in simulation experiments when study the impact of water disturbance on algae.Moreover,the simulation experiments method and field observation method all have the shortcomings of short observation frequency.The numerical simulation method is limited in the application process,such as without considering the buoyancy-related adaptability and nutrient limitations,assuming the water column in the model is completely mixed.To address the issues discussed above,the focus of this study is fourfold: first to coupe three-dimensional water quality model(EFDC)with a onedimensional ecological model(PCLake)to produce a water quality-ecosystem model to solve the problems of existing methods,second,based on the large shallow lake Nansi lake,the Nansi Lake water quality-ecological model was stablished:(1)to study the combined effects of flow turbulence and nutrients on algal biomass;(2)to analyze the effect mechanism of wind-induced vertical disturbances on algae growth in-depth;(3)to study the effects of wind-induced lateral disturbances on the horizontal algae transport and aggregation.The main research contents and results are as follows:(1)Establishing the Nansi Lake water quality-ecological model.The threedimensional water quality model(EFDC)and the one-dimensional ecological model(PCLake)are coupled to generate a water quality-ecosystem model for Nansi Lake.The model parameters are corrected and verified based on the water quality observation data from 2008 to 2010 and 2011 to 2013 on Nanyang Station located in the superior lake of Nansi Lake.The verification and sensitivity analysis results show that the model has good simulation accuracy and can reasonably reproduce the aquatic ecological processes in the superior lakes of the Nansi Lake,laying a foundation for subsequent research.(2)Effect of flow turbulence on algae biomass.The effects of flow turbulence disturbance on algae biomass are analyzed and want to figure out whether the effects are related to the nutrient concentration in the water body or not.Multiple regression analysis methods are used to establish a linear regression model between environmental variables and algae biomass,which aims at comparing the relationship between the values predicted in the regression model and the data in actual experiments.The results show that flow turbulence is an important factor affecting the growth of algae,which has a significant negative correlation with algal biomass,and the influence of flow turbulence on algae biomass is related to the nutrient concentration.The reason why there is a significant difference in the results of the regression model and the experiments is that the limitations of the model experiment cause the experimental conditions of the model to differ significantly from the actual situation.(3)Effect of wind-induced vertical disturbance on algae growth in lakes.The effects of wind-induced vertical disturbances on the changes of algae biomass and water quality indexes of lake under different wind field intensities were studied.And the PCA method is used to explore the influence of various influencing factors on algae biomass,and to analyze the effects of different wind field conditions on algae biomass.The results show that the algae biomass is inversely proportional to the wind field intensity,while there is a directly proportional relationship between variations of the physical,chemical,biological,and sediment flux indicators and the wind field intensity.However,the influence intensities of wind on them are proportional to the wind speed,and there are obvious spatial differences in these influences.Moreover,the influence of factors in water bodies on Chla,as well as the main factors affecting Chla,are related to the wind field intensity,which has spatial differences either.The results of PCA find that the standardization coefficient weights of the main influence factors and the correlation between the corresponding factors and Chla do not positively correlate,indicating the fact that it is not accurate enough to determine the main factors for chla by using the strength of the correlation.(4)Effect of wind-induced lateral disturbance on horizontal migration and accumulation of algae in lakes.Based on the hydrodynamic module in the water qualityecosystem model of Nansi Lake with tracers instead of algaes,the residence time,wind impact intensity,and connectivity indicators are used to evaluate the effects of different wind fields on the horizontal algae movement in the lake under breeze conditions.The study finds that under different wind directions,there are significant differences in the impact of the wind field on the transport of algae.And the wind field significantly changed the spatial distribution of algae residence time in the lake area,and these effects are most significant in the middle and lower reaches,but not significant in the upper and middle reaches.Also,the wind field effectively enhances the connectivity of algal migration in the lake.The connectivity of algal migration in the lake and the upstream of the source region increases as the intensity of the wind field increased,while the connectivity of the algal migration in the lake to the downstream of the source area became more evenly distributed.During the southeast wind,the connectivity of the algal migration in the lake to the upstream of the source area is higher than that in the westerly wind because of the opposite direction between the wind and the current in the lake.Throughout the study area,regardless of existence and the strength of the wind field,it is most likely for algae to accumulate and stay in sub7,indicating this area has blooms at a high probability.Meanwhile,it is not easy for algae to accumulate and stay to form blooms in sub1 ? sub3,where the retention time is relatively short. |
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