Spatial Distribution Of Nitrogen And Phosphorus Nutrients In Lake Taihu And Its Influencing Factors

In recent years,the import of exogenous sewage into Taihu Lake has been obviously controlled,but the eutrophication problem in Taihu Lake is still relatively serious.The influence of endogenous release of shallow lake bottom mud will become more obvious with the aggravation of eutrophication and cyanobacteria bloom.By combining field sampling and laboratory simulation experiments,this study analyzed the differences in the content and composition of nitrogen(N)and phosphorus(P),the composition of sediment microbial community,and the distribution of sedime-water exchange characteristics of each environmental medium in Taihu Lake(Meiliangwan section),weak Shuihua area(lake center section)and non-Shuihua area(East Taihu section).The purpose of this study is to explore the N and P cycle pattern of water environment in Taihu Lake under the influence of cyanobacteria water depletion and the key ways to strengthen the N and P cycle.The main findings are as follows:(1)The contents of chlorophyll a,suspended particulate matter,total nitrogen(TN)and total phosphorus(TP)in the water body of Meiliang Bay were significantly higher than those in the East Taihu Lake and the central lake area,and the proportion of total particulate nitrogen(TPN/TN)and total particulate phosphorus(TPP/TP)in the upper water body of Meiliang Bay were significantly higher than those in the East Taihu Lake and the central lake area.(2)The contents of organophosphorus(OP)in suspended particulate matter and OP and Fe-bound phosphorus(Fe P)in sediments in Meiliang Bay were higher than those in the eastern Taihu Lake and the center of the lake.The OP in suspended particulate matter in Meiliang Bay accounted for 69% while the inorganic phosphorus(IP)in sediments accounted for 63%,indicating that the OP form in suspended particulate matter changed into IP form during the migration process to sediments.(3)In the three lakes,the ammonia nitrogen content(NH4+-N,0.277 ～1.594mg/L)and phosphate content(PO43-,1.2391mg/L ～ 1.7853mg/L)in the sediment pore water of Meiliang Bay were the highest.The diffusion fluxes of NH4+-N and PO43-from pore water to overlying water(0.72mg/(m2·d)and 0.4mg/(m2·d))were also higher than those in non-aquatic areas.Nitrogen and phosphorus in sediment pore water in aquatic areas provided nutrients for cyanobacteria bloom growth.(4)The relative abundance of sulfate reducing bacteria(e.g.Desulfobacca)in surface sediments of Meiliang Bay is higher than in other lakes,and the relative abundance of iron-reducing bacteria(e.g.Crenothrix)and ammonia-oxidizing bacteria(e.g.Nitropira)in East Taihu Lake is higher than in other lakes.(5)The simulation experiment showed that the decomposition of algal bodies was basically completed within the 10 th to 20 th day,and a large amount of NH4+-N,PO43-,S2-and Fe2+ would be released into the water body,resulting in significant Fe P accumulation in the sediment.It can be concluded that cyanobacteria water depletion process can accelerate the cycle of various forms of N and P in water,suspended particulate matter,sediment and pore water in the water bloom area,especially by driving the anoxic environment of sediments and the abundance changes of sulfur reducing bacteria,iron reducing bacteria and ammonia oxidizing bacteria.Thus,the key pathways such as OP accumulation,Fe P formation and Fe P reduction dissolution,NH4+-N oxidation in sediments were strengthened.The research results can provide scientific reference and theoretical basis for lake eutrophication control and cyanobacteria bloom prevention and control.