Organic Carbon And Nitrogen Isotopes Environmental Relevance Of Lake In Cold And Arid Region During The Frozen Period

Lakes in cold and arid regions are an important part and indispensable part of the natural water cycle.The unique ice age and arid climate conditions make it unique among lakes in my country.The icing conditions have caused drastic changes in the water environment,which has posed a challenge to the problem of lake eutrophication.The speed of lake eutrophication has been greatly accelerated.One of the reasons is that excessive organic pollution has been accepted.Organic matter consumes a lot of oxygen during the mineralization and decomposition and releases nutrients such as C and N.Therefore,it promotes eutrophication.Organic pollution has become a problem that cannot be ignored in the water environment.Among them,the study of particulate organic matter is of great significance to the trace of lake pollution sources,biogeochemical processes and the structure of the food web,which can provide a scientific theoretical basis for the control and management of lake eutrophication.But so far,the research of our country's scholars is mainly concentrated in the southern region,and there are few reports on the lakes in the cold and dry areas.Nanhai Lake is located in Baotou City,Inner Mongolia.It is a typical urban lake in the cold and arid area of the Inner Mongolia Plateau.The Nanhai Lake is taken as the research area.In January 2019,the physical and chemical indicators of water bodies and the?¹³C,?¹⁵N,TOC and TN indicators of suspended matter and columnar sediments were tested,and the distribution characteristics of particulate organic carbon and nitrogen isotopes were analyzed.Determine the range of end-element isotope characteristic values,use the IsoSource model to trace the source of particulate organic matter and calculate the contribution rate.Summarizing the changes of the water environment,the lake was evaluated for pollution and the impact and connection of the water environment changes in the cold and arid regions on isotopes were explored through environmental correlation analysis,in order to prevent future changes in the water environment of the lake and the eutrophic pollution control research provides data reference.The main conclusions are as follows:?1?The range of suspended organic matter?¹³C during the ice-sealing period was-31.94‰^-27.87‰,and the range of?¹⁵N was 15.16‰¹8.66‰.The contents of suspended organic?¹³C,?¹⁵N,and TOC in the intake area were significantly different from those in the other three areas?P<0.05?,indicating that the composition of suspended organic matter in this area was different from the other three areas.The phytoplankton abundance is the highest in the water quality protection area,and the suspended organic matter is mainly dominated by phytoplankton.The absorption and utilization of carbon sources by the phytoplankton leads to a negative?¹³C value in this area.The?¹³C value of surface sediments varies from-24.60‰to-18.27‰,and the?¹⁵N value varies from 10.63‰to 13.66‰.The surface sediments?¹³C,?¹⁵N,TOC and TN of the lake core island area are the highest values in each region,and the sample is dark in color with strong irritating odor.This is due to excessive organic matter,resulting in incomplete degradation.The reed area has larger sediment size and smaller surface area,and the TOC and TN contents are lower than those in other areas.In the vertical direction,?¹⁵N decreases with increasing depth,and?13C fluctuates within a certain range in most samples.The contents of TOC and TN generally show a tendency of high surface layer and low layer.There is no significant correlation between the sediment?¹³C and TOC and C/N.The diagenesis has no significant impact on the sediment.?2?Five potential sources of aquatic algae,SOM-C3,SOM-C4,aquatic plants and C3plants were selected as terminal materials.Using the IsoSource model to calculate the contribution ratio of each pollution source,the internal source of algae is the main source of suspended organic matter during the ice period,followed by terrestrial soil organic matter?SOM-C3 and SOM-C4?.Soil organic matter?SOM-C3 and SOM-C4?and aquatic plants are the main sources of sediment.?3?The pollution assessment showed that all the water bodies were in the state of heavy eutrophication during the ice period,and except for the lake heart island area in summer,the other three areas were in the state of heavy eutrophication.The surface sediments in the Huxin Island area are already under organic pollution.The lake core island area,water inlet area and water quality protection area are all in the state of organic nitrogen pollution,among which the lake core island area has the highest pollution degree.At low temperature,the suspended matter?¹⁵N is significantly negatively correlated with temperature and nitrate nitrogen,and positively correlated with nitrite nitrogen.In addition,the suspended matter?¹³C was significantly positively correlated with salinity.The analysis also found that chlorophyll was significantly negatively correlated with suspended matter C/N,which was related to the phytoplankton community structure during the ice period.The organic evaluation index and sediment?¹³C are significantly positively correlated,and organic carbon determines the degree of organic pollution of the sediment.