Numerical Simulation Of Distribution Characteristics And Transport Process Of Microplastics In The Bohai Sea

The wide application of plastic products has brought great convenience to human life,but plastic waste has exerted an increasingly negative impact on the marine ecological environment.After land-based plastic waste enters the ocean,it gradually fragmented into microplastics with a diameter of less than 5 mm under various physical,chemical and biological processes.It may accumulate or migrate in coastal areas or open oceans,which will obviously affect the stability of the marine ecological environment.At present,microplastics have become a hot spot in international research as a new type of marine pollutant.The Bohai Sea is the largest semi-enclosed inland sea in China,and the water exchange capacity of the three major bays is weak.Understanding the distribution characteristics and transport process of microplastics in the sea area is of great significance for the future protection and treatment of microplastic pollution problems in the sea area.In this paper,using the survey data of microplastics in the Bohai Sea in different seasons from 2016 to 2017,the preliminary analysis of the concentration distribution of microplastics was carried out,several microplastic high-concentration areas that may exist were found and the inventory of microplastics in the Bohai Sea was estimated.Based on the coupled hydrodynamic model and Lagrangian particle tracking model,the transport of microplastics along the coastline of the Bohai Sea and in the Bohai Strait in different seasons was simulated.On this basis,two sets of sensitivity experiments were designed to discuss the transport laws of the microplastics in the Bohai Sea and the factors that may affect their transport processes.(1)The temporal and spatial distribution characteristics of microplastics in the Bohai Sea.According to the field survey data of microplastics(diameter between 0.33 and 5mm),the distribution of microplastics in the surface water of Bohai Sea in different seasons were obtained.The results showed that the spatial difference of microplastic concentration was great,and the highest concentration of microplastics was 0.53±0.45piece/m~3 in spring,0.33±0.36 piece/m~3 in summer,0.30±0.52 piece/m~3 in winter,and the lowest was 0.22±0.21 piece/m~3 in autumn.The comparison with other research results shows that the microplastic pollution in the Bohai Sea is at medium and low level.The vertical distribution model is used to simulate the depth profile distribution of the microplastics in the Bohai Sea.It is found that the larger the wind speed,the more obvious the vertical distribution characteristics of the microplastics,especially when the wind speed is greater than 5 m/s.Affected by wind-induced mixing,floating microplastics easily diffuse from the sea surface layer into the mixed layer,and the actual value of the microplastics is easily underestimated by the sampling method of the surface trawl.Based on the vertical-integrated concentration of microplastics,the inventory of microplastics by the Kriging space interpolation method in the Bohai Sea was estimated to be 4.13×10~100 pieces.Among them,the inventory of microplastics in the center of Bohai Sea was the highest,accounting for 54%,followed by the Liaodong Bay and Bohai Bay,and the inventory of the Laizhou Bay was the lowest,accounting for only 4%.The high concentration area of microplastics were mainly concentrated in the Bohai Strait,the northeast of Liaodong Bay and the mouth of the Bohai Bay.The circulation of the Bohai Sea and the runoff into the sea have certain influence on the concentration distribution of microplastics.(2)Numerical simulation results of the transport process of microplastic tracer particles in the Bohai Sea.Through the coupled hydrodynamic model and Lagrangian particle tracking model,the transport process of microplastic tracer particles at the main estuary along the Bohai Sea and the Bohai Strait in different seasons were simulated.The results show that most of the tracer particles released from the coasts of various bays in different seasons are transported along the shore.The average transport distance and displacement of all tracer particles are 603.4 km and 18.1 km,respectively,and the average transport rate and transport speed are 6.7 km/d and 0.2 km/d,respectively.Among them,the transport process of tracer particles along the Liaodong Bay is more obvious than that of other bays.The transport trend of tracer particles along the Bohai Bay is weak overall,and the transport process of the tracer particles released at the mouth of the Laizhou Bay is more obvious than that in the bay.The transport process of the tracer particles in the Bohai Strait changes significantly with the seasons,and an average of 12.5%of the tracer particles can be transported outside the model domain within a three-month tracking period.(3)Comparative analysis of sensitivity experiments.Through the comparison of sensitivity experiments,it is found that:(a)Under the condition of no wind,the average transport rate and velocity of all tracer particles along the coast of the Bohai Sea are 6.0%and 33.3%lower than those under wind conditions,respectively,and most of the particles move around the point source.The transport trend of the tracer particles in the bohai strait is significantly weaker than that under wind condition.Only 9.4%of the tracer particles are transported outside the model domain during the tracking period,and both are concentrated in winter.It indicates that the wind currents not only changes the transport direction of microplastics,but also accelerates the transport speed of microplastics.(b)The random walk can simulate the influence of turbulent diffusion on the transport process of tracer particles.Without considering the random walk,the average transport rate and velocity of all tracer particle along the coast of the Bohai Sea are 70.1%and125.0%higher than that considering the random walk,respectively,and most of the particles are transported far from the shore.The transport trend of tracer particles in the bohai strait is similar under the two experiments.This shows that the turbulent diffusion has different effects on the transport process of microplastics released from coastal areas or open seas.It inhibits the transport process of microplastics along the coast and makes it show a clear trend of ashore transport,while the transport of microplastics at the strait is less affected.(4)Analysis of the transport law of microplastics in the Bohai Sea.The ability of microplastics along the Bohai Sea to migrate and spread to the center of the Bohai Sea is weak.In summer and winter,the microplastics in the Bohai Strait shows a trend of“north-in and south-out”.The microplastics of the Bohai Sea are transported to the Yellow Sea through the southern part of the strait.Due to the barriers of the Miaodao Islands,only part of the microplastics can be transported to the Yellow Sea.Microplastics from the Yellow Sea cannot penetrate deep into the Bohai Sea.Most of them will accumulate in the northeast of the strait or transport northward along the coast.The microplastics entering the Liaodong Bay may stay in the bay for a long time or migrate to other areas of the Bohai Sea.Microplastic accumulation may occur along the coasts of the three bays of the Bohai Sea and the Bohai Strait,and the microplastic pollution in the corresponding areas needs to be focused.Based on the four-year field survey of microplastics,this paper estimates the inventory of microplastics in the Bohai Sea,and discusses the transport situation and influence mechanism of microplastics along the Bohai Sea and Bohai Strait in different seasons,and has a preliminary understanding of the current status of microplastic pollution in the Bohai Sea.Future research needs to continuously improve on-site survey methods and improve numerical simulation experiments to more accurately assess the dynamic trends of microplastic distribution,transport and fate in the Bohai Sea.