The Indication Of Suspended Organic Matter ?¹⁵N On The Evolution Trend Of Water Quality In Different Polluted Water Bodies

In order to solve the increasingly serious problem of water pollution,the ecological restoration of lakes has been fully carried out,which is one of the important tasks of the current ecological civilization construction in China.Ecological restoration makes the lake ecosystem evolve,and a simple method is needed to evaluate the restoration effect.Suspended organic matter is mainly composed of plankton,aquatic plants,terrestrial plants and soil organic matter.It is the key carrier of nutrient cycle in water.Its stable nitrogen isotope can reflect the source of nitrogen and the process of nitrogen cycle,and it is an important tool to indicate nitrogen transfer and transformation in water.Therefore,using the change trend of suspended organic matter?¹⁵N to indicate the evolution direction of water pollution can not only make up for the tedious traditional evaluation methods,but also explain the evolution trend of water quality from the perspective of nitrogen fractionation mechanism,providing theoretical basis and support for lake water restoration and protection and ecological safety management.Firstly,taking three lakes of different pollution types in Wuhan as examples,the preliminary indication of suspended organic matter?¹⁵N on the water pollution state of lakes was proposed,and the main influencing factor was sewage input;Then control experiments were carried out to explore the fractionation trend of ¹⁵N in different aquatic organisms without the influence of sewage,and the indicator hypothesis of the suspended organic matter?¹⁵N on the evolution trend of water pollution was put forward;Secondly,the experiment was designed with sewage input and air input as control factors,and the variation trend of suspended organic matter?¹⁵N caused by sewage input and nitrogen fixation was analyzed from the perspective of nitrogen fractionation mechanism;Finally,the plateau lake group and the South Lake were taken as the research objects to analyze the change trend of water quality and suspended organic matter?¹⁵N,and verified or supplemented theoretical assumptions with practical cases.The main conclusions were as follows:?1?There were significant differences in the nutrient concentration,water quality category and comprehensive nutritional status index TLI???of South Lake,East Lake and Neisha Lake in Wuhan.South Lake had the highest nutrient concentration,the most serious water quality and eutrophication,followed by East Lake and Neisha Lake.The suspended organic matter?¹⁵N in South Lake was the highest,which was in the range of sewage?¹⁵N,and the lowest in Neisha Lake.The suspended organic matter?¹⁵N had a very significant positive correlation with the comprehensive trophic state index?P<0.01?.It showed that the input of domestic sewage leaded to the high?¹⁵N value of suspended organic matter.?2?In the experiment of ¹⁵N fractionation trend in different aquatic organisms,the¹⁵N released by sedimentated algal residue was used by the submerged plant tape grass,and its?¹⁵N value increased slowly,only increased by 2.462‰,while the?¹⁵N value of the floating plant duckweed decreased to a total of 6.226‰;The suspended organic matter?¹⁵N decreased significantly with time.In the presence of aquatic plant interference,?¹⁵N was more negative.In the absence of aquatic plant interference,?¹⁵N rebounded to a certain extent and was close to 0‰.?3?In the experiment of analyzing the change trend of?¹⁵N of suspended organic matter caused by sewage input and nitrogen fixation,no matter whether domestic sewage was added or not,the concentration of nitrogen nutrients in the open group was always higher than that in the sealed group,while the suspended organic matter?¹⁵N in the open group was lower than that in the sealed group.The suspended organic matter?¹⁵N of all open groups eventually tended to 0‰,reflecting biological nitrogen fixation.The main nitrogen source of algae came from the air,while the suspended organic matter?¹⁵N of the sealed groups eventually reached 3‰.The nitrogen source of algae came from dissolved nitrogen in the water.When the domestic sewage was continuously added,the suspended organic matter?¹⁵N rapidly increased and remained stable.Once the sewage input was stopped,?¹⁵N rapidly decreased.?4?In the case study of the lake,case 1 was a survey of 16 plateau lakes,of which2 lakes?Muricuo and Sun Lake?were in an eutrophic state,3 lakes?Yangzonghai,Dianchi Lake and Xingyun Lake?were in a slightly eutrophic state,and the remaining11 were in medium nutrition state.The?¹⁵N of suspended organic matter in eutrophic lakes was higher than that in eutrophic lakes.There was a large difference in the?¹⁵N value in the nutritional state gradient,indicating that the nutritional state will affect the fractionation of nitrogen isotopes of suspended organic matter.Case 2 was to track and monitor the South Lake,which was currently under water pollution control.Compared with 2017,the concentration of nutrients in the South Lake water has decreased in 2019,but the water quality category was still worse than Class V,and the eutrophication degree of the water was not stable.The suspended organic matter?¹⁵N in 2019 was slightly reduced but still fluctuated within a higher range of 9.24‰¹5.23‰,which reflected the impact of sewage input and sediment internal release,indicating that the ecosystem was still unstable,and the South Lake treatment work still needed to be vigorously promoted.The above results showed that,during the evolution of lake water quality,suspended organic matter?¹⁵N continued to decrease,indicating that the water quality was improving and the eutrophication degree had a downward trend;If the suspended organic matter?¹⁵N was close to 0‰,it meaned that the water nitrogen mainly came from the atmosphere,and nitrogen-fixing algae dominated.If?¹⁵N tended to a negative value,it meaned that the algae used nitrogen derived from nitrogen fixation;Only when the water ecosystem reached stability,when the lake was no longer disturbed by human beings and had the function of natural regulation and restoration,could the suspended organic matter?¹⁵N?0‰,and gradually became stable.