Aquatic vegetation is an important part of lake ecosystems and plays a vital role in improving water quality and maintaining biodiversity. At present, China's lakes are facing eutrophication and the degradation of aquatic vegetation. The monitoring of temporal and spatial variations in aquatic vegetation and elucidating the main influencing factors are of great significance for protecting aquatic vegetation and restoring eutrophic lake ecosystems. Therefore, we introduced the Vegetation Present Frequency (VPF) method to extract data on aquatic vegetation and combined this with meteorological factors and human activities to analyze the temporal and spatial in Lake Hongze based on MODIS data from 2007 to 2017. The VPF of aquatic vegetation in Lake Hongze showed clear seasonal and interannual variations. The VPF was significantly higher in spring and summer than in autumn and winter (P<0.05, one way-ANOVA). The maximum VPF of 0.43 occurred in June but the minimum VPF of 0.21 was recorded in January. The VPF from April to October, during the growing season of aquatic vegetation, was significantly higher than in other months. The annual mean VPF of the northern lake area (Z1) decreased significantly (R2=0.56, P<0.01), ranging from the highest value of 0.50 in 2008 to the lowest value of 0.27 in 2016 (a decrease of 45.8%), indicating a significant loss of aquatic vegetation. Spatially, the VPF of Lake Hongze decreases from the coastal zone to the open water, and the VPF values of the northern (Z1) and western sub-lakes (Z2) are higher than that of other lakes segments (Z3-Z5). The interannual variation in VPF for the entire lake was not significantly affected by annual mean temperature, annual precipitation, annual mean wind speed, or annual sunshine duration (P>0.05), indicating that meteorological factors have little influence on interannual variation of aquatic vegetation in this lake. However, total suspended matter concentration was significantly negatively correlated with VPF in Z1 area (R2=0.48, P<0.01), with strong sand-mining activities occurring in this area. These results indicate that the increase of total suspended matter concentrations caused by sand mining is an important driving factor in the decline of aquatic vegetation in the Z1 segment.
Keywords: Lake Hongze; MODIS images; aquatic vegetation; meteorological factors; sand mining activity.