Nitrogen Uptake Of Phytoplankton In Taihu Lake And Influencing Factors

Nitrogen is the essential element for phytoplankton.Its concentration and forms affect the biomass and community composition of phytoplankton directly or indirectly.Phytoplankton can uptake dissolved inorganic nitrogen and some small dissolved organic nitrogen,such as urea,dissolved free amino acids?DFAA?,etc.In recent years,Lake Taihu is suffering from frequent occurrence of eutrophication and blooms of cyanobacteria,which has attracted much attention.There is a significant correlation between urea and phytoplankton blooms.In order to enrich theoretical mechanism of nitrogen cycle and eutrophication,the study Characteristics of nitrogen absorption rate of different species of algae was researched,and then the nitrogen uptake rates in different lake areas of Taihu Lake were studied to analyze the temporal and spatial variation of nitrogen uptake in Taihu Lake.finally,the effects of different environmental factors on the physiological characteristics of Microcystis aeruginosa,a typical dominant bloom species in Taihu Lake,were studied through laboratory control experiments.The main conclusions of this article are as follows:The characteristics of nitrogen uptake by Microcystis aeruginosa,Thalassiosira sp.,Tetraselmis sp.,Karlodinium veneficum and Rhodomonas salina were studied from nitrate forms and the difference of algae species.The uptake of three forms of nitrogen:NH₄⁺,NO₃^-and Urea,was studied by using ¹⁵N isotope addition experiments.Phytoplankton preferentially uptakes reduced nitrogen,such as ammonia.The relative preference index of phytoplankton to Urea was 25.18⁷13.42,which shows the specific preference for micro-molecule dissolved organic nitrogen.Different species of algae have different characteristics for uptaking nitrogen.The uptake rate of NH₄⁺and NO₃^-by Thalassiosira sp.was the highest,while the Rhodomonas salina was the lowest.Tetraselmis sp.had the fastest uptake rate of Urea,while the uptake rate of the Karlodinium veneficum was the slowest.Additionally,the nitrogen uptake capacity of the algae was related to the characteristics of cell size and growth cycle of planktonic algae.Different planktonic algae exhibited uptake specificity for different forms of nitrogen,which will play an important role on the nitrogen load of water and the formation of dominant species of planktonic algae.To characterize the nitrogen uptake by phytoplankton and its relationships with phytoplankton community structure and environmental factors,the uptake of ammonium,nitrate and urea by phytoplankton community from different areas of Lake Taihu was measured in four seasons,using the ¹⁵N-tracer technology.The results showed that:the total nitrogen uptake rate of phytoplankton in Taihu Lake was the highest in spring,up to 15.47?mol/?L·h?,and the lowest in winter.Spatially,the total nitrogen uptake rate was the highest in the northwest coastal area of Taihu Lake and Jiapu in winter and spring,and in Meiliang Bay and Zhushan Bay of Taihu Lake in summer and autumn.The nitrogen uptake rate in the east coastal area was the lowest in all seasons.With regards to the forms of nitrogen,planktonic algae in Taihu Lake mainly uptook nitrate in winter and spring,while urea was mainly uptaken in summer and autumn.The uptake rate of ammonia nitrogen is positively correlated with the concentration of nitrate and the ratio of nitrate to ammonia,the uptake rate of nitrate nitrogen is positively correlated with the concentration of dissolved oxygen and nitrate,and the uptake rate of urea nitrogen is significantly positively correlated with the redox potential.Cyanobacteria was positively correlated with urea nitrogen uptake rate,chlorophyta was positively correlated with ammonia nitrogen uptake rate,and cryptophyceae was positively correlated with nitrate nitrogen uptake rate.Microcystis aeruginosa.cultivated in pH of 8 to 9,light intensity of 2000 lx conditions,the temperature of 30?,and the ratio of nitrate nitrogen and ammonia nitrogen is 1:2 had the highest nitrogen uptake rates..Environmental factors affect the nitrogen uptake rate by altering the key enzyme activity and photosynthesis of nitrogen uptake,thus changing the community structure of phytoplankton.