Response Characteristics Of Phytoplankton Community During Regime Shift In Caohai Lake,Guizhou Province

Caohai Lake is the largest natural lake in Guizhou Province,its main protection objects are plateau wetland ecosystem and 246 rare birds represented by Grus nigricollis.The Lake has attracted much attention because of its important ecological value.In recent years,Caohai Lake has gradually shown signs of regime shift from grass-type clear water state dominated by submerged plants to algae-type turbid water state dominated by phytoplankton,and its ecological function has decreased.Based on the monitoring data in Caohai Lake ecosystem from 2020 to2022,combined with historical documents and data,this paper collated the data of Caohai Lake ecosystem since 1982,and analyzed the variations of water quality and phytoplankton in Caohai Lake ecosystem from long time series.The dynamic characteristics of phytoplankton community during the regime shift of Caohai Lake aquatic ecosystem（2020-2022）were analyzed,and the important influencing factors affecting the abundance of phytoplankton were found out.The process and causes of Caohai Lake ecosystem disaster were analyzed to provide more theoretical basis for the regime shift process of shallow lake ecosystem.The main results were as follows:（1）The submerged plants in Caohai Lake were seriously declining,and the dominant species tend to develop pollution-resistant species.From 1983 to 2019,submerged macrophytes were widely distributed in Caohai Lake,with 8 main communities.In 2020,there were basically no submerged plants in the central area of Caohai Lake,and there were three main communities.In 2021,the submerged plants in Caohai Lake almost completely subsided,and there was only one species of macroalgae in the main community.In 2022,the submerged macrophytes in the Caohai Lake began to recover,with three main communities,but the biomass of submerged macrophytes in the Lake was still far from that in 2020 or even before.（2）Before the regime shift（2010-2020）,that was,before the decline of submerged plants,the abundance of phytoplankton cells in Caohai Lake was relatively low（1.68×10⁶cell/L）.After the regime shift（2021-2022）,that was,after the decline of submerged plants,the phytoplankton in the Caohai Lake area proliferated,and the abundance of phytoplankton cells was relatively high（9.22×10⁶cell/L）.The cell abundance of phytoplankton in summer and autumn of 2021 and 2022 were significantly higher than that in summer and antumn of 2020（p<0.05）,and the mass reproduction of phytoplankton began in 2021.After the disappearance of submerged plants,the proportion of cyanobacterial phytoplankton increased.（3）The disappearance of submerged plants makes the water quality of Caohai Lake deteriorate rapidly,and 2020 was the node of ecological disaster.Nutrients were not the main driver of the disaster.Before the regime shift（1982-2020）,the Caohai Lake water body was in a clear water state（Secchi depth[SD]>0.80 m）;after the regime shift（2021-2022）,the Caohai Lake water body was in a turbid state（SD<0.80 m）.Before the regime shift（2011-2020）,the comprehensive nutrition state of Caohai Lake water body was basically in the middle nutrition state,and after the regime shift（2021-2022）,the comprehensive nutrition state of Caohai Lake water body had been in the mild eutrophication state.（4）The main factors affecting the proliferation of phytoplankton in Caohai Lake were total nitrogen（TN）concentration,permanganate index(COD_Mn)and water terperature（WT）.In this study,it was found that the process of Caohai Lake aquatic ecosystem disaster was submerged plant extinction-water quality deterioration-phytoplankton mass reproduction.After the disappearance of submerged plants in Caohai Lake ecosystem,the subsequent dominant position of phytoplankton was the result of vegetation disappearance rather than the reason.（5）From 1983 to 2020,the Caohai Lake aquatic ecosystem was obviously in the clear water state dominated by submerged plants;from2021 to 2022,the Caohai Lake aquatic ecosystem was obviously in a phytoplankton-dominated algae-type turbid water state.The demise of submerged macrophytes in Caohai Lake has led to the rapid deterioration of the aquatic ecological environment in Caohai Lake.After the large area of submerged plants began to disappear,the ecological disaster occurred in the Caohai Lake.The large area of aquatic plants disappeared,especially the large area of submerged plants,which could be used as an early disaster signal for the disaster of the Lake.（6）It is recommended to include more monitoring indicators that are likely to cause ecosystem disasters in lake monitoring and management,such as fish and water level monitoring;in addition,the nutrient content of Caohai Lake water body was high at present.In the future,the restoration of Caohai Lake should focus on reducing the nutrient content of water body.Secondly,wave elimination measures should be taken to increase the transparency of water body and help the reconstruction of submerged plants in Caohai ecosystem.