Nitrogen Source And Its Regulation Mechanism In The Estuary Of Taihu Lakeside City

The exogenous nitrogen(N)input and endogenous N release are regarded as the main factors for eutrophication and habitat degradation in lake estuaries,which will not only worsen the quality of people's lives,but also seriously restrict the sustainable development of urban economy.Lakeside urban estuary is an important part of the urban ecosystem,connecting rivers and lake.Recently,the estuarine habitat has been deeply marked by anthropogenic activities due to rapid urbanization and economy.It is of great theoretical and practical significance to identify the spatial and temporal characteristics of N distribution in the lakeside urban estuaries,clarify the key factors affecting N accumulation,quantify the input contributions of each source,and predict the future evolution trend of N in the lakeside urban estuaries.In this study,29 typical lakeside urban estuaries in the northern Taihu Lake were selected as the research objects.Water samples,surface sediment samples,and sediment core samples were collected to analyze the variation of N forms and distribution patterns.Visualization of the temporal-spatial distribution of N in different estuaries was presented with the Arc GIS platform.The multivariate statistical analysis,single factor index,composite pollution index,and stable isotope model were used to evaluate the risk of N enrichment and water quality,identify the crucial factors of N enrichment,quantify the contribution of each N source.In order to provide theoretical support for optimal regulation and management of estuarine N contents,different N management scenarios were set,combined with the random forest model and multiple regression analysis to simulate and predict the future N enrichment in the lakeside urban estuaries.The main results are as follows:(1)A significantly spatial-temporal distribution of N concentrations was observed in the lakeside urban estuaries,and anthropogenic activities were the dominant driving factors.On the time scale,the N concentration in the water in rainy season was lower than that in dry season,while a contrary phenomenon was observed in the sediment.On the spatial scale,the N concentration in the river side of Lihu Lake was significantly higher than that in the lake side,while the trend of N concentration in the Gonghu Bay was opposite.The single factor index analysis shows that the N content in 88.72%of the sampling sites exceeded the standard values.The results of comprehensive nutrient status index(TLI),water quality index(WQI)and modified Nemero pollution index(NPI)reflect that the estuarine water was mildly to moderately eutrophication,and the water was moderately polluted.The N content in sediment is 3.21-times higher than the standard values,reaching the level of heavy pollution.Based on the multivariate statistical analysis,land use pattern,dam construction,estuarine shape and monsoon variation are the key factors of estuarine N enrichment and water quality degradation.SIAR model shows that sewage/manure,cropland fertilization,endogenous N release and atmospheric N deposition were the main nitrate(NO₃^--N)sources in estuarine water,contributing42.68%,23.79%,23.71%and 9.80%,respectively.(2)The estuarine N load has a sensitive response to the change of constructed land and farmland.The area of farmland decreased year by year while that of constructed land increased rapidly,and became the dominant land use type during1985?2018.Urbanization and policy adjustment were the main forces driving changes of land use types.The constructed land and farmland were the dominant contributor to the total nitrogen(TN)losses in runoff,accounting for 86%?97%.Compared with1985,the production of TN from LUCC was 1.83 times in 2018,while a decreasing trend of TN losses was observed during 1985?2018.The annual variation of TN losses was in good consistency with the TN concentrations in rivers and lake,indicating that the change of LUCC and the N reduction measures are the main controlling factors and impacting on N concentration in waterbody.At different scales,LUCC may have a great influence on N migration of regional water environment.The small-scale N load in the water body was not only affected by LUCC but also controlled by the coupling of multiple factors,e.g.dam/gate construction,hydrodynamic conditions,environmental management,and estuary shape.(3)The annual average TN deposition was 6154 kg/km²,of which TN dry(TN_dry)has become the major component.In terms of chemical composition,nitrogen oxides(NO_x)deposition was dominant,accounting for approximately 66.10%.The seasonal fluctuations of TN deposition were observed,with a higher N deposition flux occurring in spring and summer.Spatially,the distribution of TN_dry deposition flux declined gradually from the southeast to the northwest,while the TN_wet deposition flux increased.Meteorological conditions are the main factors affecting TNwet deposition flux.Ammonia(NH₃)emissions from agriculture and NO_x emissions from urban factories and automobiles exhaust are the major sources of TN_dry deposition.The TN deposition annually contributed 14,400 t to Taihu Lake,which accounted for14.36%?17.30%of the total inputs.The TN deposition flux has an increasing trend compared to previous years,while the contribution of TN deposition to the total inputs has been decreasing in recent years.(4)The release rate of sediment N and chemical components was of the released N were significantly affected by the disturbance level and temperature variation.Based on the analysis of variation(ANOVA),disturbance level(F=142.49,p<0.01)had a much greater impact on the release rate than temperature variation(F=6.39,p=0.02).In a static release,the main form of released nutrients was inorganic dissolved N(65%),whereas undissolved organic N(69%?85%)was the main form in a dynamic release.Whatever in the dredged or un-dredged region,the cumulative nutrient concentrations released from the sediment aggravated the levels of pollution risk in the overlying water from moderate to heavy pollution(NPI=7.79).Dredging practice only removes the internal loading of surface sediment.A higher potential risk of nutrient release existed when the particulate matter with high activation ability deposits on the bottom sediment.Aquatic plants have the function of water quality purification and sediment disturbance inhibition,which is the key factor for water quality improvement in the river region.Under the static and dynamic releases,the TN released rate between the sediment-water layer were 56.67 and 180.67 mg/(m²·d),respectively.Approximately 297.56 t N were released from sediment to estuarine water annually which far exceeded the TN losses load from fertilization during sowing time.This suggests that if the sediment was constantly disturbed under unstable hydrodynamic conditions,more attention should be given to the effects of endogenous pollution on the estuarine aquatic habitat in future environmental restoration.(5)The estuarine N load was mainly imported from exogenous sources(71%?80%),whereas the endogenous contribution rate increased year by year.During the period of 2000?2019,the total N inputs in the estuaries of the study area show a decreasing trend.Among them,domestic sewage and farmland runoff were the main contributors to exogenous N input.Although a variety of measures have been taken to control the N source and intersect the pollution,the N concentration in the discharged wastewater has been reduced to some extent,which is still higher than the TN target concentration in Taihu Lake.The effect of increasing total water volume and accelerating water circulation makes TN load decrease limited.The regulation and control measures have different effects among estuaries due to the different contributions of endogenous and exogenous pollution sources in North Taihu Lake.In the estuary near the residential areas and wetland parks,controlling exogenous N input is the key to estuary ecological restoration.While in the fishing port and trumpet-shaped estuary,the most effective measure is to reduce sediment disturbance.Both endogenous control and exogenous treatment are important for N management and habitat restoration in the estuaries.Rational planning of LUCC,construction of riparian buffer strips;improvement of rainwater and sewage diversion system;enhancing the sewage treatment capacity;improving the large-scale culture of livestock,and rational N application are the crucial measures to reduce exogenous N input in the estuaries.Substrate improvement,wave-reduction measures,and aquatic macrophyte community building are long-term mechanisms to reduce endogenous N release from the sediments.