| Atmospheric nitrogen deposition is one of the important ways for land-based pollutants to input into water body.Excessive nitrogen deposition will have a significant impact on the health and function of aquatic ecosystems,resulting in eutrophication,decrease of dissolved oxygen and biodiversity.In this study,the Xichuan area of Danjiangkou Reservoir,the water source of the middle route of South-to-North Water transfer Project,was selected as the study area,By field investigation and experimental analysis,the nitrogen deposition flux,water quality and community structure of phytoplankton in the four seasons were studied,this paper indicated,the temporal and spatial characteristics of nitrogen deposition in the reservoir area and its influence on water quality,and revealed phytoplankton community structure and its driving factors in the reservoir area.The effect of nitrogen deposition on phytoplankton community structure was clarified,which provided theoretical basis and data support for exploring targeted ways to control nitrogen pollution in the reservoir area.The main conclusions are as follows:(1)The atmospheric nitrogen deposition in Xichuan reservoir area is 56.28kg/(hm~2·a),including 35.44kg/(hm~2·a)of dry deposition and 20.84kg/(hm~2·a)of the wet deposition.The dry deposition component was mainly organic nitrogen,which is higher in spring and autumn than in summer and winter,while the wet deposition is mainly ammonia nitrogen,which is higher in summer and autumn than in winter and spring.The wet sedimentation of total nitrogen,ammonia nitrogen and organic nitrogen were positively correlated with rainfall(P<0.05).The load of total nitrogen deposition into the reservoir is 2274t/a,accounting for 24.52%of the total annual river nitrogen input.(2)Based on the the correlation between nitrogen deposition and water total nitrogen in the reservoir area,the random forest regression method,the forest regression model of water total nitrogen and total nitrogen deposition in the reservoir area,and the regression model between six nitrogen deposition monitoring points and the corresponding total nitrogen were established.The model errors are all less than0.25mg/L.By using this method,it is convenient to predict and track the changes of nitrogen deposition or water quality in different seasons,especially for the rapid prediction of water quality changes under extreme environmental conditions,which provides some basic theories and scientific research methods for the supervision of water ecological environment in the reservoir area.(3)A total of 230 genera and 399 species of phytoplankton were detected in Xichuan reservoir area,which were mainly composed of Chlorophyta,bacillariophyta and chrysophyta.The dominant species are Dinobryon,Discostella,Aulacoseira,Mallomonas,Follicularia and Monactinus.The diversity and richness of phytoplankton community are the highest in winter and lower in summer and autumn.Water temperature,p H,TN,ammonia nitrogen and Chl-a are the main factors,which drived the change of phytoplankton community structure.Notably,the contribution rates of TN and ammonia nitrogen to phytoplankton community in summer and winter are 57.3%and 75.2%,respectively.(4)Based on the in-situ observation of daily nitrogen deposition in the reservoir area,the experiments were conducted by application of five concentrations nitrogen to culture solution for simulation of nitrogen deposition.The results showed that the biomass of phytoplankton increased gradually with the culture time,and the biomass of phytoplankton was N1>N2>N0.5>Blank group(CK)>N3>N5 under different conditions.Under the background of simulated nitrogen deposition,the diversity of phytoplankton community decreased gradually,while its richness decreased at first and then increased with the culture time.(5)Under the condition of different nitrogen concentrations,the dominant species of phytoplankton community were Cyanobium and Synechococcus of cyanobacteria,but the dominant species were different in different culture periods.In the early stage,the abundance of Cyanobium was the highest in the phytoplankton community.In the middle stage,the abundance of Synechococcus was the highest under low nitrogen condition.In the later stage of culture,Synechococcus was dominant under the low nitrogen condition,while Cyanobium was dominant under the high nitrogen condition.(6)Under the condition of single nitrogen addition of nitrate nitrogen,ammonia nitrogen and organic nitrogen,the diversity of phytoplankton in each treatment group decreased at first and then increased with the increase of nitrogen addition.The effects of different forms of nitrogen on the richness of phytoplankton community were quite different.Under the condition of the addition with nitrate and ammonia,the richness of phytoplankton decreased at first and then increased with the increase of nitrogen addition.On the other hand,the richness of phytoplankton increased at first and then decreased with the addition of organic nitrogen.Under the condition of low concentration of nitrogen,Cyclotella was the dominant species in phytoplankton communities in nitrate,ammonia and organic nitrogen treatments.With the increase of nitrogen addition,the dominant species in nitrate nitrogen group and ammonia nitrogen group were Cyclotella and Cyanobium respectively,and the dominant species in organic nitrogen group were Phormidium and Mychonastes... |
