Study On Distribution Law And Influencing Factors Of Microplastics In Lake Ulansuhai During Freeze-thaw Process

As a new kind of emerging contaminants,Microplastics(<5mm)has attracted countless attention at home and abroad in recent years because of its nature as difficult degradation,small particles and wide distribution.The research on microplastics pollution mainly focuses on the ocean,water body and sediment of freshwater system.On the contrary,the data about microplastics pollution in lake ice sheets during the ice-sealing period is still scarce,the distribution regular pattern of microplastics during the freezing and thawing process of lake water is still unclear,and whether the concentration of nutrients,which is an important indicator of lake water quality will affect the distribution of it still is unknown.By studying the occurrence characteristics and influencing factors of microplastics in lakes during the ice-sealing period,the distribution law of it in lake and its ice sheet during the thawing process and the main factors affecting its distribution would be found,which can play a complementary role for the shortage of microplastics pollution data in freshwater lakes in cold areas and which is also of great significance for comprehensive treatment of it in the future.Therefore,Lake Ulansuhai during the ice-covered period was taken as the research object in this paper,and samples of different ice layers of ice sheets and different layers of lakes after the ice sheets melted were collected.The abundance distribution,color,shape and species of microplastics in the ice sheets and water bodies of the lake was identified and analyzed by means of microscope observation,Fourier transform infrared spectroscopy and correlation analysis.And the influencing factors of vertical distribution of microplastics in the ice sheets and lake were explored by correlation analysis.The main results are as follows:(1)The abundance of microplastics in the surface water of Lake Ulansuhai ranges from 4.8-19 n·L-1,and the amount of the organic pollutant in the middle layer ranges from 4.5516.25 n·L-1.The average abundance of microplastics in the surface layer is about 1.42 times higher that in the middle layer.The shape of microplastics in surface water was mainly fiber(46.6%),and it was mainly fragment(53.2%)in middle layer.The polymer composition of it was mainly polyethylene(PE),followed by polystyrene(PS)and polybutylene terephthalate(PET).The content of PE and PS in surface water were generally higher than those in middle layer,while the abundance of PET in middle layer was higher.The results show that the freezing and thawing process of the lake can affect the abundance and horizontal distribution of microplastics in Lake Ulansuhai.The abundance of it in the lake before freezing and after ice sheet melting is quite different,and the distribution pattern is opposite.The density is the main factor affecting the settlement of microplastics in the vertical direction.(2)The average abundance of microplastics in the ice sheet of Lake Ulansuhai is 56.75141 n·L-1,which is about 10-100 times of that in the surface water of the lake,and it reflect that the ice sheet is a main temporary storage.Its abundance gradually decreases from upstream to downstream in the horizontal direction,and the amount distribution of it in surface ice and bottom ice is larger than that of middle and near bottom layer in the vertical direction.Fibrous and black microplastics(58.6%)are the most common types of it in the ice sheet,and the plastic particles whichsize<0.5 mm(27.4%)are the main ones.The results of Fourier transform infrared spectroscopy showed that three types of synthetic polymers were found in the ice sheet,namely polyethylene(PE),polystyrene(PS)and polyethylene terephthalate(PET),among which PE(68.1%)and PS(24.6%)were the dominant ones,followed by PET(7.3%).(3)The nutrients of the lake have no obvious changes in the horizontal distribution after the ice sheet melts,and it is distributed evenly in the vertical direction.The analysis of the vertical distribution of nutrients in the ice sheet showed that the vertical distribution of salinity in the ice sheet showed a trend that the content of it in surface and bottom layers were higher than the middle and near bottom layers.The vertical distribution of Chlorophyll a concentration was disorganized,and the total phosphorus,total solubility Phosphorus and total nitrogen show vertical distribution characteristics similar to the trait of salinity,while the data of COD shows a trend of aggravation with the increase of ice thickness.The statistical analysis results show that there is no correlation between the content of various nutrients in the lake with the amount of microplastics.The vertical distribution of the salinity in the ice sheet and the abundance of the microplastics has a significant correlation relationship,of which indicating that the salinity and the content of microplastics can influence mutually,and there is no significant correlation between other nutrients and the abundance of microplastics.