Controlling Factors And Influence Of Denitrification On Cyanobacterial Bloom Dynamics In Lake Taihu

Currently,lake eutrophication and cyanobacterial blooms have become an ecological and environmental problem of global concern.Whether algale growth is limited by nitrogen or phosphorus depends on the differences in the biogeochemical cycling of nitrogen and phosphorus in aquatic ecosystems.Shallow lakes may be more susceptible to the release of phosphorus stored in sediments for cyanobacterial growth than deeper lakes,while nitrogen can be removed from the water column through denitrification,which is important for the control of lake eutrophication and cyanobacterial blooms.In this study,we took large-scale shallow lake Taihu as the research object,investigated the spatial and temporal differences of water quality conditions in different lake regions,revealed the denitrification potential and the influencing factors,and clarified the interrelationship between denitrification and cyanobacterial blooms.The results are significant for insight into the nitrogen cycle of Lake Taihu,revealing the mechanism of cyanobacterial blooms,and controlling cyanobacterial blooms in similar shallow lakes such as Lake Taihu.The main research results are as follows:（1）Water qualityof Meiliang Bay,Zhushan Bay,Gonghu Bay,Dapukou and Xukou Bay in Lake Taihu was monitored quarterly in 2021 to determine the seasonal change patterns.The results showed that the water quality in different regions of Lake Taihu differed significantly,and the pollution status of the algae-dominant areas was significantly higher than that of the macrophyte-dominant areas.Massive cyanobacterial accumulation was observed in Zhushan Bay in May,with Chla concentration up to 223.24μg·L^-1.The overall situation of cyanobacterial blooms was the most serious in August,with Chla concentration exceeding 50μg·L^-1 in both Zhushan Bay and Gonghu Bay.The nitrogen concentration in all regions of Lake Taihu showed a trend of higher in winter and spring,lower in summer and autumn,andthe alage-dominant areas were significantly larger than the macrophyte-dominant areas.The spatial variation of phosphorus concentration was similar to that of nitrogen,with the highest TP concentration in Zhushan Bay,followed by Dapukou,and the lowest in Xukou Bay in May.However,it showed an opposite trend to nitrogen in terms of time,with higher in summer and autumn compared to lower in winter and spring.（2）Sediment and water samples were collected from Zhushan Bay in June 2022,and the denitrification potential of different lake regions in Lake Taihu was determined by isotope mud method.The results showed that denitrification potential may be influenced by TN,DTN,NO₃^--N and Chla of the overlying water.The higher pollution level in the northwestern part of Lake Taihu was favorable for denitrification.（3）Sediment and water samples were collected from Zhushan Bay in April 2022,and the temperature and NO₃^--N concentration thresholds of denitrification potential in Lake Taihu were determined by isotope slurry method.Denitrification showed a significant linear relationship with temperature between 10-25°C（R²=0.99）,and reached the maximum value in Lake Taihu at 25°C.The nitrate concentration threshold at the maximum denitrification rate was 4 mg·L^-1,while the NO₃^--N concentration in Lake Taihu Lake was much lower than 4mg·L^-1,and its actual denitrification rate may not reach the potential value.（4）By collecting cyanobacteria,sediment and water samples from Zhushan Bay of Lake Taihu,indoor incubation experiments were set up as lake water only（A0）,lake water with algae in light-proof culture（A1）,sediment column without algae in light-proof culture（A2）,sediment column with algae in light light-proof culture（A3）,lake water with algae in light culture（A4）and sediment column with algae in light culture（A5）to investigate the process of nitrogen transformation during different periods of cyanobacterial growth and decay.The results showed that cyanobacteria would uptake a large amount of dissolved nitrogen in the water column,and finally the DTN concentration decreased by 46.4%and NO₃^--N concentration decreased by61.7%in A4,and the DTN concentration decreased by 60.7%and NO₃^--N concentration decreased by 80.6%in A5.The decay of cyanobacteria decreased the DO concentration and substantially increased the DTN concentration in the form of NH₄⁺-N.The final NH₄⁺-N concentrations were（1.49±0.21）mg·L^-1 in A1 and（2.09±0.12）mg·L^-1 in A3.The NO₃^--N concentration in A1 was essentially constant at the end of the experiment,and the NO₃^--N concentrations in A2 and A3 decreased by 40.8%and 56.6%,respectively.Cyanobacterial growth processes would inhibit denitrification efficiency by reducing the NO₃^--N concentration in the water column;The decay of cyanobacterial blooms would promote denitrification of sediment,but the decrease of dissolved oxygen concentration caused by blooms decline may inhibit nitrification.Therefore,it is necessary to improve the lake environment to inhibit the massive growth of cyanobacteria and prevent them from competing with denitrification for NO₃^--N resulting in lower denitrification rates;It also need to prevents the accumulation of large amount of cyanobacteria during blooms to construct a coupled nitrification-denitrification denitrification process,thus accelerating the self-purification efficiency of nitrogen in lakes.（5）Effect of denitrification on cyanobacterial blooms in lakes was investigated based on the historical water quality monitoring data of North Lake Taihu from 2017 to 2021,combined with the incubation experiments of cyanobacteria.The monitoring data showed that algal biomass（Chla as a proxy）peaked mainly in summer and autumn.The seasonal variation of TN concentration had completely opposite trend in comparison to that of algal biomass,which peaked in winter and spring.Nitrate is the main component of dissolved inorganic nitrogen,and the nitrate concentration was close to zero in summer and autumn.The Chla concentration showed a highly significant negative correlation with NO₃^--N concentration（p<0.01）and a highly significant correlation with TP concentration（p<0.01）.Cyanobacterial blooms in Lake Taihu are limited by nitrogen in summer.The experimental results showed that cyanobacteria did not grow when the temperature was below 20°C.This study indicated that the advance in lake temperature warming due to climate change would result in earlier growth of cyanobacteria,which leads to large amounts of N to be assimilated by algae before denitrification,further affecting the dynamics of cyanobacterial blooms.