Temporal And Spatial Characteristics Of Nutrient Metabolism In Lake Sediment And The Influencing Factors

Eutrophication is a severe ecological and environmental problem in freshwater lakes and rivers around the world.In case when the external pollution of the lake is controlled,the nutrients in the sediment could released into the water bodyies under certain conditions,and the internal pollution cannot be ignored.Current study was focused on the internal pollution factors in Lake Taihu,located in eatern China.Sediment pore-water and overlying water samples were collected seasonally from the different sampling points of Lake Taihu（at 16 sampling sites）for a period of one year.Through sediment core flow-through incubations,exploring the temporal and spatial characteristics of the migration and transformation of carbon,nitrogen,and phosphorus at the sediment-water interface,analyzing the characteristics and related influencing factors,and estimating the annual internal source load of carbon,nitrogen,and phosphorus in the sediments of Lake Taihu.The temporal and spatial characteristics of nutrient migration at the sediment-water interface show that the annual average value of dissolved organic carbon（DOC）release flux was 653.31±1248.58μmol/（m²·h）,which is generally endogenous release,and the release flux was highest in spring and lowest in winter.The annual average values of NH₄⁺,NO₃^-and NO₂^-fluxes were 48.30±103.55μmol/（m²·h）,66.84±149.13μmol/（m²·h）,-0.17±3.82μmol/（m²·h）,and the overall performance was also Endogenous release,and the release flux was highest in spring and lower in winter.The annual average value of PO₄^3-release flux was 2.11±9.31μmol/（m²·h）,which was dominated by endogenous absorption in spring and autumn,while performance in summer and winter for endogenous release.The high values of carbon,nitrogen,and phosphorus release mainly occured in the more polluted areas such as Zhushan Bay,Meiliang Bay,the Western Coast,and East Taihu Lake.The relationship between environmental factors and the basic properties of overlying water and sediments and the migration of carbon,nitrogen,and phosphorus indicates that oxygen consumption,temperature,and DO at sediments were the key factors affecting the migration of carbon,nitrogen,and phosphorus at the sediment-water interface.In addition,DOC was also affected by the composition of organic matter.NH₄⁺,NO₃^-,NO₂^-were affected by p H and ORP,and PO₄^3-was affected by aquatic biological disturbance.The temporal and spatial characteristics of nutrient conversion at the sediment-water interface show that the annual average values of CO₂ and CH₄ fluxes were 95.18±129.33μmol/（m²·h）and 5.38±14.45μmol/（m²·h）,respectively,with the highest release in spring,the lowest in winter.The annual average values of N₂O flux,denitrification rate,anammox rate,and DNRA rate were 0.01±0.13μmol/（m²·h）,28.63±58.07μmol/（m²·h）,0.36±0.50,respectivelyμmol/（m²·h）and 1.39±3.17μmol/（m₂·h）,which were the highest in spring and lower in autumn.The annual average BAP content in surface sediments was 179.98±174.70mg/kg,and the seasonal difference was not evident.The conversion rate of carbon,nitrogen,and phosphorus at the sediment-water interface shows a decreasing trend in space from north to south and west to east.The relationship between environmental factors and the basic properties of overlying water and sediments and the transformation of carbon,nitrogen,and phosphorus indicates that the concentration of nitrogen and phosphorus in the overlying water and the oxygen consumption of sediments were the main factors affecting the transformation of carbon,nitrogen,and phosphorus at the sediment-water interface.In addition,CO₂ and CH₄were also affected by p H,water temperature,DO,etc.,N₂O,denitrification,anammox,DNRA rate were affected by organic matter composition,and BAP was affected by algae activities.The estimation of the endogenous load combined with the results of the migration and transformation of nutrients at the sediment-water interface shows that the annual carbon（DOC）release of Taihu Lake was 16.06×10⁴ t,and the removal（CO₂,CH₄）was 2.47×10⁴.The release of nitrogen（NH₄⁺+NO₃^-+NO₂^-）was 3.30×10⁴ t,removal amount(N₂O+D_n+D_w+A₂₈)was0.83×10⁴ t.The release of phosphorus(PO₄^3-)was 0.13×10⁴ t.The release of nitrogen and phosphorus account for about 60%and 40%of the external input,respectively.The annual carbon,nitrogen,and phosphorus release characteristics of Lake Taihu all show significant endogenous release characteristics.The research results could deepen the understanding of the migration and transformation mechanisms of carbon,nitrogen,and phosphorus in shallow lakes,and provided a reference for lake endogenous control.