Spatio-temporal Distribution Of Nitrogen And Phosphorus Nutrients And Composition In Xiaowan Reservoir Of Lancang River

Dams disturb the original material and energy cycle of rivers,while the effect of reservoir regulation on the biogenic elements such as nitrogen and phosphorus nutrients attracted more attention.In this study,the Xiaowan Reservoir with annual adjustment capacity in the lower reaches of the Lancang River was taken as a case to identify the composition distribution of nitrogen and phosphorus nutrients and investigate the influence of the power station on the spatial and temporal distribution of different forms of nitrogen and phosphorus nutrients in water.Our results could provide scientific references for river health management and ecological environment security regulation after hydropower development.In 2017,surface water at four gauging sections of the Xiaowan Reservoir on the Lancang River(the Xiaowan mainstream reservoir tail,the Xiaowan tributary reservoir tail,upper and lower stream of the Xiaowan dam)for a period of five months(March,June,July,March and November of 2017)was sampled,and then several nitrogen-phosphorus nutrient compositions were tested in the laboratory according to the corresponding national standard testing methods,temporal and spatial changes of nitrogen and phosphorus nutrients near the Xiaowan Reservoir in the Lancang River were explored to reveal the influence of the Xiaowan Hydropower Station on the nutrient transport of the river and the reservoir area.Our results are considered as following.(1)From the perspective of the composition of nitrogenous nutrients in rivers,the proportion of organic nitrogen in nitrogen composition in dry season(63.80%)is higher than inorganic nitrogen(36.2%),which is the main form of nitrogen in the water body;The proportion of ammonia nitrogen in wet season(37.54%)is much larger than that in dry season(17.19%),indicating that the mainstream of Lancang River in flood season is greatly affected by human sewage.On the spatial scale,the main form nitrogen in the water during both wet and dry periods showed a trend that the upstream section was larger than the downstream section.(2)On the time scale,the total nitrogen total phosphorus has obvious seasonal variation characteristics,and the total nitrogen concentration shows a tendency that the dry season(1.402mg/L)is higher than the wet season(0.625mg/L).Total phosphorus concentration also shows a tendency that the dry season(0.238mg/L)is higher than the wet season(0.074mg/L).The nitrate nitrogen concentration is consistent with the total nitrogen phosphorus that the dry season(0.228mg/L)is higher than the wet season(0.100mg/L).The concentration of ammonia nitrogen has no obvious rules.On the spatial scale,there is no obvious change trend of total nitrogen on the main stream,and the change trend of total phosphorus concentration from high to low is: the Xiaowan mainstream reservoir tail,upper of Xiaowan dam,downstream of Xiaowan dam.Ammonia nitrogen and nitrate nitrogen have no significant changes on spatial scale.(3)Studies on the relationship between major water quality indicators and nutrients show that the main water environmental factors that play an important role in the study of nitrogen and phosphorus nutrients in the study area are turbidity,dissolved oxygen,temperature and electrical conductivity.To a certain extent,it affects the content and temporal and spatial distribution of TN and TP in the water of the study area.Relative to the section near the Xiaowan Dam,the Xiaowan trunk stream tail section and the Xiaowan tributary reservoir tail section are less affected by the water environment factor.(4)The eutrophication analysis of the reservoir shows that the nutrient structure of the section near the dam is mainly phosphorus,and the tributary section is dominated by nitrogen.The comprehensive nutrition status index shows that the section near the dam is basically maintained in a state of lean nutrients to medium nutrition.The risk of eutrophication is low.