| Urban lakes are a landscape complex composed of natural and human elements in cities and an important part of urban ecology.Wuhan is one of the cities with the largest number of lakes in China.However,with the acceleration of urbanization,Wuhan suffered from shrinking areas of lakes and declining water quality.In this study,we selected 49 lakes in Wuhan to analyze the spatial and temporal variations in lake areas from 1987 to 2017,and the effects of aquatic plants on water quality.Landsat TM/OLI image and normalized water body index(NDWI)were used to extract the area and perimeter of lakes.Then,all the lakes were divided,according to its location,into three groups: downtown,suburban and countryside.We compared the similarities and differences in the variations of lakes from different groups.Based on the field sampling of lake water in autumn of 2017 and spring of 2018,the seasonal variations of water quality within and between the groups were compared.Combined with climatic factors,statistical yearbook data and lake morphology data,the causes for variations in lake area and water quality were analyzed.The main results of this paper are as follows:(1)From 1987 to 2017,the areas of 49 lakes in Wuhan decreased by a total area of 8640.456 ha.The overall shrinking rate was 9.05%,and the overall change intensity index was-0.30.The reduction of lake areas in the downtown(16),the suburban(18)and the countryside(15)were 1169.592 ha,3421.45 ha and 3522.414 ha,the overall shrinkage rates were 26.97%,24.59% and 4.70% and the overall change intensity index was-0.8989,-0.8196 and-0.1560,respectively.The lake areas in the downtown decreased continuously from 1987 to 2017,while it increased until 1996 and then decreased in the suburban and the countryside.The lake shrinking center of Wuhan gradually transferred from the downtown to the suburban regions.(2)The water quality of lakes in Wuhan varied between seasons and regions.According to the Single Factor Evaluation Method,total phosphorus concentrations(TP)and Chemical Oxygen Demand(COD)in autumn were higher than those in spring,while total nitrogen concentrations were much higher in spring.Both species richness and abundance of phytoplankton in autumn were higher than those in spring.The proportion of TP,TN and COD which concentrations were lower than national standard,the ratio of comprehensive pollution index(P ? 0.4),and the variations of individual number,species composition and diversity of phytoplankton,comprehensively reflect the eutrophication level and the degree of water pollution.All of those indicators were highest in the lakes of downtown,and then of suburban and countryside.(3)The increase of population and the expansion of urban built-up area resulted in reclaiming land from lakes and shrinking the lake areas.Climatic factor(precipitation and temperature)showed weak effects on the variations in lake areas.The closer to the center of the built-up area,the more disturbed of the lakes.The native species of aquatic plants(especially submerged plants)in lakes significantly decreased nitrogen and phosphorus content and COD in water,while the presence of invasive species aggravated the water pollution. |
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