Characteristics Of Phytoplankton Community Structure In Dongting Lake Under Different Landscape Connectivity

River lake system connectivity is not only related to water resources circulation,but also plays a key role in water ecology,water environment and water security.Dongting Lake,as an open lake,is affected by many factors of Yangtze River-river-lake water system and human activities,and its water system connection has changed greatly.The water system and water area of the lake area are gradually shrinking,and the connectivity is declining,which leads to the serious obstruction of water exchange capacity in some water areas.In addition,the impact of industry,agriculture and human life,the water eutrophication intensifies,the aquatic vegetation degenerates,the phytoplankton grows in large numbers,and even the water bloom breaks out.Therefore,it is necessary to study the impact of water system connectivity on phytoplankton community structure in the lake area,so as to provide a basis for ecological regulation to enhance water system connectivity in the lake area by increasing ecological water level.In this paper,the field monitoring of hydrology,water quality and aquatic ecology of Dongting Lake was carried out from 2018 to 2019.The basic characteristics of landscape connectivity and phytoplankton community structure in different time and space of Dongting Lake Basin were analyzed.The lake connectivity was evaluated qualitatively and quantitatively by using the theory of landscape ecology,and the relationship between connectivity and hydrodynamics,water quality and phytoplankton was discussed.The spatial and temporal distribution characteristics of phytoplankton in Dongting Lake under different connectivity,its main influencing factors and connectivity threshold were clarified.The main influencing factors of phytoplankton were analyzed by internal and external factors,and the internal factors of serious bloom area were analyzed to determine its action threshold.The results are as follows:(1)According to the calculation method and evaluation standard of landscape connectivity,the evaluation results of Dongting Lake Basin are poor,general and good,which can be divided into three water areas with significant differences:barrier area of Tongjiang lake,transition area of Tongjiang lake and connection area of Tongjiang Lake;according to the characteristics of hydrodynamic regime of Dongting Lake,the hydrological period of Dongting Lake can be divided into four significant time scales:dry(December to March),rising(April to June),abundant(July to September)and recession(October to November).There are significant spatial and temporal differences in phytoplankton density in Dongting Lake.In spatial distribution,the phytoplankton density of Dongting Lake is Tongjiang Lake barrier area>Tongjiang Lake transition area>Tongjiang Lake connection area;in the time distribution,except the dry season,the phytoplankton density was in the order of wet season>flood season>recession season.(2)According to the quantitative results of landscape connectivity,the relationships between landscape connectivity and hydrodynamic,water quality and phytoplankton in different hydrological periods and different lake areas were obtained.The results are as follows:(1)as far as hydrodynamic velocity is concerned,landscape connectivity has a main influence on the velocity of barrier zone,transition zone and connected zone,and the velocity difference among barrier zone,transition zone and connected zone is significant.When the water disturbance is strong,the influence of connected zone will extend to the transition zone.Therefore,the change of landscape connectivity leads to the change of flow velocity,which changes the characteristics of algae community in the lake area,and the planktonic algae cells in the barrier area and the connection area have the effect of drift;(2)as far as total phosphorus is concerned,there is a significant difference between the barrier zone and the transition zone and the connected zone;there is no significant difference between the transition zone and the connected zone,but there is a significant difference in the connected zone at different stages;(3)compared with the phytoplankton density in the barrier area,the algae density in the transition area and the connecting area is significantly lower than that in the barrier area.(3)According to the results of the impact of landscape connectivity on hydrodynamics,water quality and phytoplankton,as well as the intensive monitoring and analysis of the key areas of water bloom,based on the time coherence(synchronization)analysis of variables and the intra group correlation coefficient(R_K)evaluation,the internal and external factors of algae spatial and temporal distribution were analyzed,and the synchronization intensity of phytoplankton in the lake area was determined,The results show that:(1)transition area and connected area,it can be found that the phytoplankton in the lake area is mainly caused by regional external factors,which shows that the difference of landscape connectivity leads to the difference of algae spatial and temporal distribution;(2)in the transition area and the connected area,the algae succession is mainly controlled by regional external factors,and the landscape connectivity is above average,which leads to faster hydrodynamic force,algae cell drift and higher coherence of algae succession time;(3)in the barrier area,local internal factors are the main factors,the landscape connectivity is poor,and the hydrodynamic force is insufficient to homogenize nutrients and form a unique niche.Orthophosphate affects the activity of alkaline phosphatase in phytoplankton cells,resulting in low coherence of algal succession time.