Spatiotemporal Characteristics And Health Assessment Of Ecological Environment For The Urban Lake

Urban lakes are an important part of the urban landscape and are particularly vulnerable to the effects of urbanization and human activities.The study on the temporal and spatial distribution characteristics of ecological environment and health status of urban lakes can provide more accurate scientific basis for their management,protection and restoration.It is of great significance to maintain the health and beauty of urban lakes,improve the comprehensive benefits and promote the harmony between human and water.In this paper,Daming Lake in Jinan City is selected as the typical research object.Through the field investigation and sampling in two typical periods,namely dry season and wet season,Studies on overlying water environmental factors analysis and water quality assessment,nutrient distribution characteristics and pollution assessment of surface sediments,plankton survey and diversity analysis,and ecological environment health assessment based on water-sediment-biological factors were carried out respectively.The main research conclusions are as follows:(1)In the index of overlying water of Daming Lake,the overlying factor is mainly total nitrogen(TN),whose average level exceeds the Class V standard of surface water by 3.56 times,and the spatial distribution difference is significant.Nitrogen pollution is concentrated in the west bank,and the concentration decreases gradually from west to east,which reflects that Yanboqiao water inlet is the main channel for pollution load into the lake.In dry season,the water body is at a medium trophic level,while in wet season,it is at a mild eutrophication level.The main influencing factors of eutrophication are TN and chlorophyll A(Chl.a),which are respectively controlled by foreign nitrogen nutrients and water temperature.Combined with the comprehensive nutrient state index of historical years,the water quality of the whole lake has gradually improved in recent years,but it still fluctuates in individual periods due to the influence of heavy rainfall and temperature conditions in flood season.(2)The pollution load of total organic carbon(TOC),TN and TP in the surface sediments in wet season was more serious than that in dry season,and the spatial distribution gradually decreased from the west to the east of the lake.The results of organic pollution index and comprehensive pollution index showed that the nutrient elements in surface sediments of Daming Lake were at the level of severe pollution as a whole.The index value in wet season was significantly higher than that in dry season,and the pollution level of Yanbo Bridge was the most serious in space.Comprehensive correlation analysis shows that there is a significant positive correlation between TN and TOC in the surface sediment,indicating that TN and TOC in the surface sediment have homology and both belong to organic pollution.The TN of surface sediments and overlying water is significantly correlated,indicating that the two have a high degree of homology,reflecting that the nitrogen load in lake water mainly comes from the release of organic nitrogen from the lake bottom sediments.(3)There were 159 species of phytoplankton in 8 phyla,and the diatom species had the largest number in dry season,with a density of 4.72×10~6 ind/L,accounting for 51.09%of the total density in dry season.The number of chlorophyta species was the largest,but the density distribution of cyanobacteria was the largest,which was 15.52×10~7 ind/L,accounting for 79.42%of the total density.As an important indicator species of eutrophic lakes,cyanobacteria overpropagation would significantly aggravate the enrichment process of lake water.The total number of zooplankton species reached 4 phyla and 45 species,mainly rotifers and protozoa.The density of protozoa in dry season was 10.02×10~2 ind/L,accounting for 93.49%of the total density in dry season;the density of rotifers in wet season was 1.59×10~2 ind/L,accounting for89.37%of the total density.The metabolic product of rotifers is ammonia nitrogen,which may lead to high ammonia nitrogen load in wet season,which will become a potential threat to the enrichment of lake water.The evaluation results of Shannon-Wiener diversity index showed that the macrophytoplankton was basically inβ-αstate of pollution,and the zooplankton in the lake was basically inα-αstate of pollution,and the whole Daming Lake was inβ-αstate of pollution.The redundancy analysis(RDA)of plankton and water environmental factors showed that SD was the most significant factor affecting the community structure of phytoplankton and zooplankton during dry season.In wet season,the most significant factors affecting both are T and TP.(4)Finally,the ecological environment assessment index system of Daming Lake was constructed,the weights of each index were determined by AHP,and the water ecological environment health status of Daming Lake was comprehensively evaluated by the established fuzzy comprehensive evaluation model.The results showed that the ecological environment health assessment status of Daming Lake in dry and wet period was"very healthy"and"sub-healthy"respectively.The main reasons for the decline of health were the high nitrogen load in overlying water,the enrichment of organic matter and nutrients in surface sediments,and the low species diversity of phytoplankton.The evaluation results are consistent with the results of field sampling analysis of the lake,which verifies the reliability and practicability of the typical urban lake ecological and environmental health evaluation system established in this paper.