Remote Sensing Monitoring Of Aquatic Vegetation Groups In Typical Shallow Lakes In The Middle Reaches Of The Yangtze River Based On Phenological Characteristics

As the primary producers of lake ecosystem,aquatic vegetation plays an important role in the construction,ecological balance and restoration of the aquatic ecosystem.In recent years,the lake ecosystem has been severely damaged in the middle reaches of the Yangtze River due to multiple human activities and global climate change.A large area of aquatic plants has decreased.To make matters worse,some aquatic plants have disappeared from the lake.Satellite remote sensing techniques have incomparable advantages over field investigation because it can obtain monitoring data quickly,in a large range and periodically.Therefore,in this study,we constructed a remote sensing classification algorithm for aquatic vegetation groups based on phenological characteristics of five small and medium sized shallow lakes in the middle reaches of the Yangtze River using moderate resolution Landsat satellite images from 1990 to 2020.What's more,the temporal and spatial characteristics of emergent plants,floating-leaved plants and floating plants and their evolution mechanisms were analyzed on an interannual scale.The results are as follows:(1)Observing the spectral characteristics of emergent plants,floating-leaved plants and floating plants,it was found that it was difficult to accurately classify them with simple spectral information.However,with the help of their differences in phenological characteristics,vegetation classification can be achieved.That was because the NDVI value of emergent plants was greater than 0 throughout the year and the growing season was longer.The growing season of floating-leaved plants was also relatively long,which could last until November,while the growing season of floating plants was shorter,which could last until October.The classification accuracy of aquatic plant groups based on phenological characteristics was 83.33%,and the Kappa coefficient was 0.7687.(2)Emergent plants were mainly distributed in shallow water areas.Floating-leaved plants were distributed around the emergent plants.Floating plants were mainly distributed near floating-leaved plants,but also tend to floated towards the center of the lake.The area distribution characteristics of aquatic plants could be divided into three periods.From 1990 to 1999,aquatic plants grew vigorously,with abundant species and large space area.At this stage,the aquatic plants in Longgan Lake were mainly floating-leaved plants and floating plants,while the aquatic plants in other lakes were mainly floating-leaved plants and upwelling plants.From 2000 to 201 0,a large number of floating plants were developed in Honghu Lake,Liangzi Lake,Changhu Lake and Futou Lake,and the aquatic plants were mainly floating plants and floating-leaved plants.In Longgan Lake,a large number of emergent plants developed and emergent plants were the main aquatic plants in this stage.From 2011 to 2020.the area of aquatic plants in all lakes showed a decreasing trend.In Honghu,Liangzi,Changhu and Futou Lakes,the area of floating plants decreased,and the floating-leaved plants increased,and the aquatic plants were dominated by emergent plants and floating-leaved plants again,but the scale of aquatic plants could not be compared with that in the 1990s.In Longgan Lake,the emergent plants have been greatly reduced,and the aquatic plants were dominated by floating-leaved and floating plants.Aquatic plants respond differently to different extreme climatic events.After the flood,the area of all groups of aquatic plants decreased greatly.Floating plants recovered first after the flood,and then floating-leaved plants and emergent plants gradually recovered.During the severe drought in spring,a large number of floating leaf plants and emergent plants developed,which could change the direction of the structure evolution of aquatic plants.(3)Temperature and annual rainfall had no significant effect on aquatic plants.There was a significant negative correlation between water level in spring and summer and floating-leaved plants and emergent plants.High water level in spring would inhibit seed germination,and high water level in summer would cause a large number of aquatic plants to die.With the change of water depth,emergent plants were mainly distributed in the coastal shallow water.The adaptable water depth of floating-leaved plants and floating plants was about 3 meters.Floating plants can also be observed at depth of 4 meters because they can float with the wind.Human activities such as aquaculture significantly affected the biomass and community structure of aquatic plants.