Illustrating The Role Of Rivers And Coastal Lakes In Transporting Plastic Particles From Terrestrial Sources To The Marine Environment

Microplastics,as an emerging pollutant in the environment,has attracted increasing attention from the scientific community,environmental policy makers and society.As plastic continues to be used,microplastics have a continuous tendency to accumulate in the aquatic environment.This makes microplastics pollution in aquatic environments around the world increasingly serious.Aquatic organisms are facing increasing survival threats.At present,there are many researches on microplastics in aquatic environment,especially in marine environment.In recent years,researches of microplastics in freshwater systems have also increased.However,there are few reports on how microplastics are transported between the terrestrial and marine environments.Studies have shown that microplastics in the marine environment mainly come from emissions of land-based sources.Therefore,this study focuses on the impact of terrestrial freshwater bodies on microplastics pollution in the marine environment.Specifically,this research:(1)investigate the distribution and source of microplastics in the sediments of Dishui Lake,and determine the role of coastal lakes in the accumulation and reworking of plastic particles from continental environment;(2)determine the distribution of microplastics in surface sediments and seawater of the East China Sea,and explore their sources and the impact of terrestrial rivers on microplastic pollution in the East China Sea.Coastal lakes are close to the ocean and are a transitional aquatic system between the continental and marine environments.We hypothesize that coastal lakes can serve as a buffer zone for plastics from land to the marine environment,and for plastic reworking and redistribution.In this study,Dishui Lake was selected as the research area.Based on the distribution characteristics and sources of microplastics in Dishui Lake,we further tested our hypothesis about plastic buffer zone.Through density flotation,with the help of stereo microscope and Fourier transform infrared spectrometer(FT-IR),this study collected and then detected microplastics from all 14sediment sampling sites in Dishui Lake.The main results of this study are as follows:(1)Microplastics are commonly found in surface sediments around Dishui Lake(Around the Lake Sites,ALS)and the canal leading to the East China Sea(The Connecting Sites,TCS).The overall microplastic pollution level of Dishui Lake is lower than that of other lakes and rivers in China.The average microplastic abundance of the 14 sediment samples of Dishui Lake was 86.5 items/kg dry weight sediments(d.w.s.).The average abundance of microplastics at ALS is lower than the sites at TCS.The average abundance of microplastics at ALS was 46 items/kg d.w.s.,and that of TCS was 230 items/kg d.w.s..The microplastic colors found in this study include white,green,blue,yellow,and orange,with the highest content being white microplastic.The shape of microplastics was also rich,including line,foam,film,pellet,and sheet.The highest content is sheet microplastics.The size of microplastics in the entire study area was generally large,with no microplastics smaller than 100?m.In addition,microplastics with a size larger than 2.5 mm and smaller than 5mm in diameter were most abundant the.Through FT-IR analysis,the identified polymer type of microplastics include polypropylene,polystyrene,polyethylene,rayon,high density polyethylene,low density polyethylene,acrylic,ABS,and polyethylene terephthalate.The highest content was polypropylene.These plastic materials are all commonly used in daily life and industry.(2)The surface sediments at some sampling site of Dishui Lake contain meso-and macroplastic debris.These meso-and macroplastic particles had similar physical characteristics and chemical composition as the microplastics.(3)The distribution and source of microplastics are closely related to the construction progress of Dishui Lake and human activities.The abundance of microplastics in the southern and western regions with a high degree of construction and development is higher than in the north,which is in the early stages of construction.And the north and east of Dishui Lake are mostly farmland and ecological protection areas,while the west and south are mostly residential areas and entertainment places.There are two sources of microplastics in Dishui Lake.On the one hand,a series of improper human activities caused plastics not to be recycled in accordance with regulations,but entered the environment and became microplastic pollution.This is the primary source of microplastics in Dishui Lake.On the other hand,according to the similarity of microplastics with meso-and macroplastic particles,it can be inferred that the secondary source is another important source of microplastics in Dishui Lake.Plastic waste entering the environment,including primary microplastics,gradually degrades into smaller plastics under the action of physical,chemical,and biological processes.(4)The results of this study further show that the coastal lakes could be a reworked and redistribution zone for all three sizes of plastics(microplastic,mesoplastic,macroplastic)originated from land-sources to the ocean,and a buffer zone that could provide smaller size plastic debris for the marine environment.Dishui Lake is one of the representative coastal lakes.The plastic debris from the incoming lakes and rivers could be reworked and fragmented in such lakes to become smaller pieces.Plastic debris that absorb more chemical pollutants then transport with flow to the ocean.For future research,it is important and useful to focus on the sources,sinks,reworking,and redistribution of all three sizes of plastics at catchment scale.Terrestrial rivers are a major source of microplastics of marine environment.Based on the study of microplastics in surface sediment and surface water in the East China Sea,the conclusions are as follows:(1)The abundance of microplastics in the surface seawater of the East China Sea ranged from 24-216 items/m~3 with an average of 78.46items/m~3 s.w..Microplastics in seawater were rich in color,including white,black,red,blue,orange,and green,and the highest content was white microplastics.The shapes of microplastics in seawater included line,pellet,sheet,fragment and film and the highest content is line microplastics.The abundance of microplastics in the surface sediments of the ECS ranges from 0-22.22 items/kg d.w.s.,with an average abundance of 5.51 items/kg d.w.s.The colors of the microplastics in the sediment include white,blue,black,green,and yellow,and the highest content is also white.The shape of the microplastics in the sediment is the same as in seawater,and the highest content is still line microplastics.The size of the microplastics in the surface sediments and seawater of the ECS is relatively small,and the microplastics content in the size of 0.1-2.5mm is high,while the microplastics of the size<0.1mm and 2.5-5mm are low.The types of microplastics found in the East China Sea are very rich.A total of 11 polymer types were found in the sediment,and 16 types were found in seawater.These plastic types are materials that commonly used in industry and daily life.(2)The ECS is connected to the land,and its microplastic pollution is greatly affected by the discharge of land-based pollutants.The sampling area of the East China Sea is divided into two parts.The average abundance of the N area is 6.07 items/kg d.w.s.,which is higher than the S area of 4.67 items/kg d.w.s.The reason why the abundance of microplastics in the N area is higher than that in the S area is that the Yangtze River water is directly discharged into the N area,and the Qiantang River water in the S area is discharged into the Hangzhou Bay and then into the ECS.In addition,we collected the samples in summer,and the southeast wind prevails on the surface of ECS.The southeast wind blows pollutants from rivers into the ocean to the north.Therefore,the microplastics abundance in the N region is higher than that in the S region.There has been less research on the transport of plastic pollutants from land to ocean.The results in this study show that terrestrial rivers are the main sources of marine microplastic pollution.Coastal lakes can serve as a buffer zone for plastic debris,and could rework and redistribute plastic debris.We hope that this article could stimulate more research on the transport of plastic from land to the sea.