Estimation Of Atmospheric Nitrogen Deposition Flux And Its Ecological Effect In Lake Taihu

As eutrophication intensifies in lakes worldwide,the governance of lakes faces significant challenges.Lake Taihu,a typical eutrophic lake,has undergone decades of treatment with effective control of point source pollution.However,cyanobacterial blooms still occur frequently,presenting a bottleneck in lake treatment.Nevertheless,atmospheric nitrogen deposition,as a potential nutrient input pathway to lakes,has not been given enough attention.Due to variations in observation methods and duration,the contribution of nitrogen deposition to the eutrophication of Lake Taihu has varied greatly among different studies.This study aims to accurately quantify the contribution of nitrogen deposition to Lake Taihu,as well as identify the long-term trends,driving factors,and ecological effects of nitrogen deposition.These results will provide a scientific basis for the eutrophication governance of Lake Taihu.The main conclusion of this study are as follows:（1）In Lake Taihu,the annual TN dry deposition flux is 30.24 kg ha^-1 yr^-1 from 2010 to2022,in which NH₄⁺is the main component（82.28%）and gaseous deposition is the main form（61.62%）.The nitrogen load from dry deposition into the lake is 7070 t,with gaseous N load contributing 4357 t and particulate N load contributing 2713 t.Taking wet deposition into account,the direct atmospheric nitrogen input to the lake surface was 9812 t,which accounts for around 26%of the external nitrogen input load and provides a new primary productivity of28.57 g C m^-2 yr^-1 to Lake Taihu.However,to avoid exceeding the critical load of Lake Taihu,atmospheric TN deposition needs to be reduced by 88%.Nitrogen deposition in Lake Taihu is typically higher in spring and summer than in autumn and winter.TN and NH₄⁺deposition exhibit three peaks in a year（March,July,and November）,which corresponds with the fertilization time of rice-wheat rotation systems in the Lake Taihu region.（2）From 2010 to 2021,there was a significant downward trend in atmospheric nitrogen deposition in Lake Taihu（r²=0.77,p<0.01）,with the largest decrease observed in NH₄⁺（55.45%）.As a result,there was a significant decline in the NH₄⁺/NO₃^-ratio.However,the proportion of NH₄⁺was still higher than that of NO₃^-,indicating that agricultural sources such as fertilizer application and the number of livestock remained the dominant contributors to nitrogen deposition in Lake Taihu.Nevertheless,there was a gradual trend towards industrial sources.There was a significant positive correlation between fertilizer application rate and livestock number and nitrogen deposition（r²=0.64,r²=0.73,p<0.01）,inferred that agriculture was the main driving factor for the decrease of N deposition.In addition,wind speed and temperature are the main meteorological factors affecting the deposition process.（3）Nitrogen deposition drives phytoplankton community succession by affecting the nitrogen concentration in the lake.A decrease in nitrogen deposition may promote the dominance of cyanobacteria while inhibiting the growth of diatoms,and further enhance the dominance of Dolichospermum spp.,a type of cyanobacteria with N₂-fixing ability,leading to a transformation from a non-N₂-fixing cyanobacteria community to an N₂-fixing one.In Lake Taihu,environmental variables accounted for 44.1%of the evolution of the phytoplankton community,with temperature being the most important driving factor（12.01%）.The influence of nitrogen deposition on phytoplankton community succession was second only to that of climate variables,and TN bulk deposition and dry deposition together explained 15.17%of the changes observed.（4）To effectively manage the eutrophication of Lake Taihu,urgent actions are required to control non-point source pollution.For crop cultivation,it is necessary to determine the optimal amount and timing of nitrogen application,adopt new nitrogen fertilizer and adjust planting structure to improve nutrient utilization efficiency and reduce nitrogen emission.For livestock and poultry breeding,it is essential to optimize the farm layout and integrate crop cultivation with livestock and poultry farming to promote the resource utilization of manure and reduce nitrogen loss.These measures can significantly reduce nitrogen input into lakes through atmospheric deposition and prevent the occurrence of cyanobacterial blooms.