Abstract: In order to analyze the problems and influencing factors of shallow lake ecosystem, and to promote the protection and management of lake ecological environment more scientifically and precisely, Baima Lake was taken as a typical case and an index system was established for the evaluation of ecological security of lakes in plain water network area based on the Diring force-Presure-Status-Influence-Response (DPSIR) model and the comprehensive assessment method of lake ecological security ("4+1" assessment methodology framework). Meanwhile, the ecological security status, spatio-temporal difference and their affecting factors were also investigated, and the results show that the ecological security situation of Baima Lake was improved in the last few years, with the ESI (Ecological Security Index) rose from 66.70 in 2013 to 74.79 in 2018, which was at comprehensive safety degree. However, it was difficult to make quick balance within the ecosystem, and the improvement of water ecological health had an obvious delay. The whole ecological security status was ranked as in the "General State" level with a trend of gradually declining from south (ESI 82.78) to north (ESI 68.36). The correlation analysis results indicate that the main factors affecting the ecological security level were the pollution load from the upper reaches of the basin, the water quality of inflow rivers, the function of water conservation and the ability of pollution interception and purification. While the high interference of human activities were the fundamental reasons which caused the progressive deterioration of ecological security status in Baima Lake. Therefore, the primary task for the protection and management of shallow lakes is to control the external disturbance factors within the carrying capacity of lake basin.

Key words: shallow lake, ecological security, assessment methodology, spatio-temporal difference, affecting factors