Research On Main Control Factors Of Eutrophication And Ecosystem Transition Threshold Based On Regime Shifts Theory In Dongting Lake

Dongting Lake,as one of the two biggest lakes still linked with the Yangtze River,plays a decisive role in the social and economic development of the middle and lower Yangtze.On the one hand,the construction of water conservancy facilities and reclamation of land from the lake reduced the water environmental capacity;on the other hand,the development of industry and agriculture in the basin and the urbanization process led to a sharp increase in the discharge of sewage,leading to a series of water environmental problems such as the increasing eutrophication degree of Dongting Lake water body these years,frequent“water-bloom”phenomenon and degradation of aquatic vegetation.How to identify the cause of water eutrophication effectively and put forward the control threshold which is suitable for Dongting Lake has become the key point of water environment protection in Dongting Lake.Considering the significant spatio-temporal heterogeneity of the water environment characteristics of Dongting Lake,the eutrophication characteristics,influencing factors and their action thresholds of different hydrological periods and lakes have not been clarified.In this paper,the hydrology,water quality and water ecology field monitoring data of Dongting Lake from 2018 to 2019 are used to systematically analyze the eutrophication characteristics and its main influencing factors under different time and space of Dongting Lake.By using the Regime shift theory and frequency distribution method,the existence of multiple steady-state in Dongting Lake ecosystem was verified,and the spatial and temporal distribution characteristics of"macrophyte-algae"steady-state in Dongting Lake and its main influencing factors were clarified.The threshold of"macrophyte-algae"steady-state transformation in Dongting Lake was analyzed by selecting the critical period of submerged plant growth and the key lake area of steady-state transformation.The main results of the study include:?1?The connectivity index of the East Dongting Lake water system is significantly lower than inflow rivers of Donting Lake,the West Dongting Lake and the South Dongting Lake.Combined with the hydrodynamic characteristics,the East Dongting Lake can be divided into one category,and the remaining four lake areas can be divided into the second category;According to the characteristics of the hydrological situation,Dongting Lake can be divided into four hydrological periods with significant differences in time,dry period?December-March?,rising period?April-June?,high water period?July-September?and withdrawal period?October-November?;The eutrophication degree of the water body of Dongting Lake is shown in the dry season>dewatering period>high water period>rising period;there are obvious differences in the characteristics of the eutrophication degree in different regions,the inflows of Yangtze River is more eutrophicated in high water and withdrawal period than in dry and rising period;the remaining four lake areas is more eutrophicated in dry and withdrawal period than in rising and high water period.In terms of spatial distribution,total nitrogen?TN?,total phosphorus?TP?and transparency?SD?are consistent with Comprehensive trophic level index?TLI?during the rising and high-watering periods,the permanganate index(COD_Mn)is consistent with the TLI during all hydrological periods,and chlorophyll-a?Chl-a?is consistent with the TLI consistency is poor.The velocity of water and the comprehensive nutritional status index have a good negative correlation between East Dongting Lake,South Dongting Lake and West Dongting Lake,and the correlation between the inflows of Dongting lake is poor.?2?According to the frequency distribution method,the spatial existence of the multi-stable state of the Dongting Lake water ecosystem was verified,and the“space-to-time”method was used to find out the stable distribution characteristics of“submerge-algae”in Dongting Lake.The specific distribution results are as follows:?1?In dry period,the inflow rivers from Yangtze and South Dongting Lake are basically in the transition state,and the inflows of Hunan and West Dongting Lake are in the submerge-dominated clear water stable states.?2?In rising and high water period,the inflow rivers of Dongting Lake are in a submerge-dominated stable state,the west and the South Dongting Lake are in the transition stable state,the East Dongting Lake is in the transition state and the algal-dominated stable state.?3?In the dewatering period,the inflow rivers from Yangtze and South Dongting Lake are basically in the transition state,the inflow rivers of Hunan and the South Dongting Lake are in the submerge-dominated stable state,the East Dongting Lake is in the turbid state.?3?The results showed that total phosphorus was the main factor affecting the biomass of phytoplankton and submerged plants in East Dongting Lake.The total phosphorus threshold of the water ecosystem in East Dongting Lake was 0.07mg/L from the submerged-dominated stable state to algae-dominated stable state,and0.046mg/L from the algae-dominated stable state to the submerged-dominated stable state.By controlling the total phosphorus concentration in the water below 0.07mg/L,the ecosystem can be maintained in the submerged-dominated stable state;by reducing the total phosphorus concentration to 0.046mg/L,the aquatic vegetation can be restored and the ecosystem can be transformed from the algae-dominated stable state to the submerged-dominated stable state.Although the eutrophication level of remaining areas is relatively high,the water ecosystem is generally health due to the hydrodynamic conditions.