Abstract:

Nansi Lake is the largest lake in Shandong province and one of the largest freshwater lakes in China.In recent years,Nansi Lake has been threatened by the serious flooding of Potamogeton crispus L. From April to May, P.crispus grows vigorously in the lake area,becoming the only dominant species in the submerged plant area.In June, P.crispus declines rapidly,which not only causes severe ecological problems such as water pollution and waterway blocking,but also becomes seasonal threats to the water quality of the South-to-North Water Transfer Project.Based on the characteristics and phenological information shown by the NDVI time-series curves of different typical landscape extracted from MODIS MOD13Q1 16d that are combined with Landsat data,the community of P.crispus in Nansi Lake was extracted and visualized as the distribution maps.The results showed that from the beginning of 21st century,the area of P.crispus in Nansi Lake increased from 25.22 km ² in 1980s to 171.46 km ²in 2018,and that the distribution range of P.crispus had been expanded from the lakeside to the centre of the lake from past 30 years,with the ratio increasing from 3.61% to 24.55%.On inter-annual scales, P.crispus developed from an ordinary dominant species into an absolute dominant species in the lake.Additionally,the Pearson Correlation and the Spearman Rank Correlation Analysis were used to reveal the correlations between the area of P.crispus and lake eutrophication level or water quality.Results revealed that there was no correlation between the area of P.crispus and water quality,but the correlations were significantly positive between the area of P.crispus and the other three related factors.Specifically,the correlation coefficients are higher than 0.8,which means a significantly positive correlation.The current situation of the P.crispus in Nansi Lake ask a higher requirements for the management in Nansi Lake.Departments should strengthen the control of P.crispus in order to prevent further extension in Nansi Lake.

Key words: Nansi Lake, Potamogeton crispus L., NDVI Time-series, phenological information, MODIS, Landsat, remote sensing of environment