Characteristics Of Water Environments And Spatial Distribution Of Nitrogen And Phosphorus In Xingyun Lake

With the global changes and the rapid development of local socio-economy,water pollution and eutrophication of nine plateau lakes in Yunnan have become increasingly serious.Nitrogen?N?and phosphorus?P?are the key factors causing the lake eutrophication.As one of the nine plateau lakes in Yunnan,Xingyun Lake is not only the upstream lake of Fuxian Lake,where the water quality is Class I,but also an important controllable water resource and ecological barrier in Jiangchuan District,Yuxi City.The investigation on the spatial distribution pattern of nitrogen and phosphorus nutrients,and the analysis of water quality in Xingyun Lake are conductive to further understand the enrichment of nitrogen and phosphorus in Xingyun Lake and provide scientific basis for lake manegenment and restoration.In this study,the surface and bottom water samples from 120 sample points in Xingyun Lake were collected in April 2019.Simultaneously,the physical parameters,Chlorophyll a?Chl-a?,N and P were also measured.The relationships between N,P,physical variables and Chl-a were exploered based on the correlation analysis.Finally,the water body of Xingyun Lake was divided into two parts through cluster analysis,and their nutrient status were evaluated respectively.The main conclusions are as follows:?1?Xingyun Lake presents an alkaline status according to the spatial distribution of physical parameters.In the vertical direction,no obvious thermal stratification was observed due to the influence of wind disturbance and the strong hydrodynamic force.Water temperature?WT?,turbidity?Turb?,pH and dissolved oxygen?DO?showed reduced trends with the increase of water depth in general,and the electrical conductivity?EC?showed an increased trend with the increase of water depth.Due to the dissimilarity of lake shape,geographical location and wind effect at different sampling sites,all of the physical parameters and Chl-a presented a certain spatial heterogeneity in the horizontal direction.The spatial heterogeneity of WT,Turb,EC,pH and Chl-a in the surface water is higher than that in the bottom layer,and of which,the spatial heterogeneity of Turb and Chl-a presented a significant difference between surface water and bottom water?P<0.05?.?2?Xingyun Lake had a high concentration of nitrogen and phosphorus according to our investigation.The average concentration of total nitrogen?TN?is1.81 mg·L^-1,and the average concentration of total phosphorus?TP?is 0.41 mg·L^-1.Dissolved total nitrogen?DTN?is the main nitrogen form in Xingyun Lake,and also,dissolved total phosphorus?DTP?is the main phosphorus form in this lake.The average concentrations of TN,DTN,particulate nitrogen?PN?,TP and particulate phosphorus?PP?in the surface water were higher than those in the bottom water.The spatial heterogeneity of TN,DTN,TP,DTP and PP in the surface water showed no significant differences with bottom water,but their spatial heterogeneity in the surface water were slightly higher than that in the bottom water.The vertical variation of PN and PP is probably relative to the hydrodynamic force and the light intensity of water surface.Xingyun Lake has encountered a serious external pollution,and the hydraulic disturbance aggravates the pollution degree of the northern lake.Hydrophyte plays an important role in the reduction and control of nitrogen and phosphorus concentration in Xingyun Lake.The TN/TP ratio indicated that the water body of Xingyun Lake is in the state of nitrogen limitation.?3?WT,Turb,pH,EC and DO are all important physical factors in water environments.Correlation analysis indicated that these physical factors have affected the concentration,distribution and form of nitrogen and phosphorus in Xingyun Lake to some extent.The correlation analysis showed that Chl-a was strongly correlated with TN,and TN/TP was significanly positively correlated with TN?P<0.05?,which further showed that nitrogen is a potential limiting factor for the growth of phytoplankton in Xingyun Lake.DTP and EC showed a significant positive correlation?P<0.05?,which could be explained as PO₄^3-contributed a lot to EC,and it also reflected the serious phosphorus pollution in Xingyun Lake.The high concentration of nitrogen and phosphorus nutrients in water promoted the outbreak of algal blooms,and the increase of Chl-a concentration,in turn,affected the concentration,distribution and form of nitrogen and phosphorus in water.?4?TN,TP and Chl-a were selected as the evaluation variables,and the nutrient status of Xingyun Lake was evaluated by the Trophic level index?TLI?.The results showed that the water body of Xingyun Lake was in the state of severe eutrophication at present,and the northern lake presented the most serious degree of eutrophication,which was mainly related to the input of external nutrients and the hydrological conditions.In addition,the eutrophication is still serious in the water area where the impact of external nutrients input is small,which may be related to the inner-pllotion in this lake.Therefore,the internal and external pollution should be jointly controlled in the water management of Xingyun Lake.