The Crtical Factors Of Water Quality Affecting Phytoplankton Community In Lake Erhai

Recently,Many lakes are facing eutrophication,even outbreak of blooms.Erhai lake is in the early stage of eutrophication,and water quality is between ? and?.Since 1996,it has several algae cyanobacteria blooms.Although a lot of researches in Erhai lake eutrophication has been done,the effect of nitrogen and phosphorus on phytoplankton community has not been fully elucidated by using fieldsurveys and in-situ space simulation.By means of in-situ simulation research and comparative analysis,this study is to compare nitrogen and phosphorus in Erhai lake in the role of phytoplankton community structure,to analyze the key factor of water quailty influencing phytoplankto n community,and to provide the theoretical basis for Erhai lake Ecological restoration.In this study,Erhai lake as the object,and carried out two aspects of research:(1)co nduct a annual survey of water quality and plankton of the four lakes in Erhai lake basin(Haixihai lake,Cibihu lake,Xihu lake and Erhai lake),analysis the relationship between phytoplankton community and water quality,finding the key factor of water quality in dif ferent seasons affecting phytoplankton community.At the same time,analysis the usage rate of the four lakes watershed land by remote sensing data,collecting data of nitrogen and phosphorus pollution,and finding the reasons leading to the differences of water qual ity in the four lakes.(2)making an in-situ simulation experiment,adding nitrogen and ph osphorus regularly to simulate the slow input of nitrogen and phosphorus to Erhai lake,getting to know the influence of nitrogen and phosphorus on Erhai lake phytoplankton co mmunity.The survey of water quality and plankton in four lakes shows the nutritional status of four lakes in different levels.Both Haixihai lake and Cibihu lake are mesotrophic throughout the year(TLI index at 30-40).Erhai lake is in the eutrophic level(TLI index at the average of 53 annually).Xihu lake is also in the eutrophic level(TLI index up to 64 in October).The density of Chla in Cibihu lake and Haixihai lake is relatively low,and the maximum is only 6.23 ?g/L.The density of Chla in Erhai lake and Xihu lake is relatively high,and the concentration in Xihu lake reaches to 92.53?g/L in October,and in Erhai lake between 10.28 ?g/L and 27.11 ?g/L in October.TP in July and October is higher than other months in the four lakes of Erhai basin,which may be due to infusion of external pollution in rainy season.TP in July and October in Xihu lake,respectively,reaches to 0.145 mg/L and 0.080 mg/L.TP in Cibihu lake is low throughout the year,and the largest is only 0.018 mg/L.The site of Erhai lake in central district near the east coast in October,TP is about 0.060 mg/L,and it's worthy of attention.TN in Haixihai lake changes hardly,in January,April and October at about 0.50 mg/L,in July reaching 0.72 mg/L.TN in Cibihu lake is slightly higher in January and April.TN in Xihu lake ist he most highest among the four lakes,especially in January up to 2.46 mg/L,in July and October at about 1.5 mg/L,and water quality is very poor.For phytoplankton,the cell number of phytoplankton in Erhai lake and Xihu lake is significantly more than that in Haixihai lake and Cibihu lake.The maximum number of algae cell in Erhai lake is in July(9.8×106cell/L),followed by October(2.8×106cell/L).Cibihu lake is similar toErhai lake,but the total number of cells are less.The cell number of phytoplankton in October(6.01 ×106cell/L)in Xihu lake is obviously higher than other months.The phytoplankton cells in Haixi lake are consistent in the four months survey,which are between 1.02×105 cell/L to 2.15 × 105cell/L.From the land usage of various small watershed of Erhai lake basin,it finds that TN in the four lakse is signally related(p<0.05)to farmland-use area among the utilization of farmland,residential areas,forests,lakes,grassland and bare land,as well as TP and Chla.By analyzing the pollution load of nitrogen and phosphorus on Erhai lake basin,it f inds that farmland fertilizer,sewage and livestock breeding account for 93%and 94%of total nitrogen,total phosphorus emissions in Erhai lake basin.Through statistical analysis and calculation,the main pollution source in the Erhai lake basin is the livestock farmingFor enclosure experiment,it has five groups in the two-factor experiment in nitroge n and phosphorus:the ammonia nitrogen,the nitrate nitrogen,the phosphorus,the mixed and the blank.The result shows that the density of Chla is the most highest in the high le vel of phosphorus group which reaches to 20?g/L in the final,and it's obviously differe nt(p<0.05)from the blank group.There is no significant difference(p>0.05)between th e blank and the other groups.Therefore,we speculate that phosphorus is the nutrition dri ving factor of Erhai Lake.In the next step,the experiment has seven groups:the nitrogen,the mixed,the low phosphorus,the phosphorus in low,the phosphorus in high,high pho sphorus and the blank.Also,the result shows that Chla in each group is inconsistent.And the variation of nitrogen is not obvious(p>0.05),which Chla is up to 22.53 ?g/L in the largest and similar to the blank group.The mixed reaches 30.54?g/L in the last sampling,and it is significantly different(p<0.05)from the blank group.The nitrogen in the high p hosphorus group grows at the soonest,which is up to 41.05?g/L finally and extremely di fferent(p<0.01)from the blank group,so it has the characteristic of algal bloom.According to the results of this study,we can draw the following conclusions:1,The nutrition status of the four lakes are different,Xihu lake and Erhai lake reached the lever of eutrophication,while Haixihai lake and Cibihu lake reached level of Medium eutrophication.2,In the Erhai lake basin,the driving factors of water quality which influences the density of chla were diverse with the seasona.Phosphorus is the main driving factor in summer and winter,phosphorus and ammonia nitrogen is the main driving factor in fall,nitrogen is the main driving factors in spring.Which indicated that we should take different strategies in different seasons.Meanwhile zooplankton was becoming smaller,causing the less predation on the algae.3,In summer and fall when blooms occurred easity,higher permanganate index is more suitable for the growth of Microcystis,higher nitrate nitrogen is more suitable for the growth of Porphyridium and Dinobryon,while higher ammonia nitrogen is more suitable for the growth of Anabaena.4,For the four lakes,the pressure of human disturbance was the key factor to their differences of the water ecological characteristics.Especially Livestock fanning is the main part of the various kinds of pollution sources.5,In enclosure experiment,When the amounts of KH2PO4 into the enclosure reached to high group,the density of chla grew sharply.6,Barricading research shows that higher phosphorus is suitable for many algals such as Anabaena circinalis,and higher nitrogen is suitable for Anabaena spiroides,while higher phosphorus and nitrogen concentration is suitable for some algal such as Navicula sp.and Selenastrum bibraianum.