The Study Of Time-space Distribution And Source-sink Relation Of Microplastics In Coastal Sediment From Beibu Gulf

Currently,tackling plastics pollution has become an important topic for global sustainable development within national community.China recognized a long-term endeavor is essential to tackling plastic pollution and reducing plastic pollution was written in The 14th Five-Year Plan for National Economic and Social Development and the Long-Range Objectives Through the Year 2035.The prevention of plastic(especially microplastic)pollution in ocean is the key point of the mitigation of plastic pollution worldwide.As a result,the issue of tackling microplastic pollution in marine environment has frequently listed in the agenda of the United Nations Environment Assembly recently.Scientific researches on microplastic pollution in marine environment can contribute to the safety of eco-environment.However,there are challenges ahead of researches on microplastic pollution in marine environment globally.Most studies focused only on land-based sources,sea-based sources are also important sources of microplastic pollution,but receive little attention,however,lots of plastic products were used in fishery activities which may lead to plastic pollution in marine environment and studies on fishery-derived microplastic pollution span and depth are necessary.Sediment is considered as the most important source of microplastics in marine.Previously,efforts were devoted to studying microplastic pollution in surface sediment,however,much less of the literature is available on the vertical distribution of microplastics in sediment.Furthermore,studies on the sink of microplastics in coral reef and mangrove is still too limited.The researches on microplastic pollution were supposed to guide the policy making toward preventing plastic pollution,whereas,previous studies seldom make a great contribution to plastic pollution mitigating policies.To make efforts to cope with scientific problems mentioned above,Beibu Gulf,a traditional fishing ground of China,located in in the geographic center of China and the Indo-China Peninsula,were selected as a study area.Focus was on time-space distribution and source-sink relation of microplastics in sediment.The main work and achievement are as follow.(1)This study has surveyed the microplastic pollution level of sediment from the north area of Beibu Gulf(major study area),including 5 inflowing rivers.Results show that all the selected(n=8)polymers were found in the surface sediment.PP(polypropylene)was the most abundant polymer in this area,accounting for 68.7%of the total abundance,following by PE(polyethylene,accounting for 18.5%of the total abundance).The morphology study suggested that fiber was the main type of microplastics,accounting for 69.6%of the total abundance.The average abundance of microplastic from surface sediment is 405±336 particles/kg,which is slightly higher than the intermediate level comparing with surface sediment from other marine environment in global scale.Microplastic pollution level of sediment from different environmental functional districts decreased in the following order:urban river>mariculture area>suburban river>intertidal area>offshore area>port district.(2)A source analysis of microplastics in surface sediment from north area of Beibu Gulf(major study area)was conducted in this study.Based on the usage of different plastic fibers,the pathway of plastic fibers entering environment,and the comparison of plastic fibers from fishing gear,PP fibers and PE fibers in the sediment from the study area were verified that originated from the abrasion of fishing gear during fishery activities.Furthermore,the abundance of PP fibers and PE fibers exhibited a strong correlation(R²=0.8586,p=0.015)with values of fishery yields of different districts,which highlighted the effects of different fishery activities on microplastic contamination in marine sediment,and consisted with the conclusion that PP fibers and PE fibers in the sediment from the study area were mainly generated from the abrasion of fishing gear during fishery activities.(3)This study found that the sediment of coral reef and mangrove which are important eco-systems can also be consider as the sink of microplastics.Generally,coral reef is distributed in marine of high environmental quality.This study found that all the coral reef area(n=3)in Beibu Gulf,including coral reef evolves along the fringe of continent and island with an average abundance of 363±476particles/kg.Microplastic abundance of mangrove sediment is higher than other types of sediments with an average abundance of 655±526 particles/kg.Mangrove sediment is a sink of microplastics with high concentration attributing to the trap effect causing by mangrove.According to the field survey,mangrove especially mangrove in small scale is easy destroyed by anthropogenic activities,as a result,microplastics accumulated in mangrove sediment could be released into the environment,leading to a conversion of sink into source.(4)The result of this study showed that the average abundance of microplastics in sediment core(60 cm depth)is 410 particles/kg,indicating microplastics can be buried in deep sediment.Furthermore,microplastics can be“hidden”in deep sediment with a deep up to 60 cm(average abundance 167particles/kg),updating and modifying the knowledge that microplastics were mainly accumulating in the surface sediment.The abundance distribution and size distribution of microplastics in the sediment core suggested long-term burial of microplastics in deep sediment,as a result,deep sediment is an important source of microplastics in marine environment overlooked by previous studies.The estimated storage of microplastics in deep sediment(185 tons)was 5 times that in surface sediment.The assessment of microplastic storage worldwide might be underestimated because most previous studies only examined surface sediment.(5)The ²¹⁰Pb-based and ¹³⁷Cs-based dating method was used to establish an accurate chronological framework for the sediment core in this study.Results showed that microplastics could be found in sediment layer older than 1933.However,since the 1940s,polyolefin plastics began to be produced worldwide and were not extensively used until the 1970s.Therefore,theoretically,microplastics should not be detected at depths below 22 cm(i.e.,1933).Biodisturbances and the grain size of sediment particles might be responsible for the vertical transport of“fresh microplastics”to“old sediment”.(6)This study tried to find out key factors about migrating plastic pollution in marine environment,especially microplastic pollution derived from fishery activities,based on the analysis of current governing measures against plastic pollution in China and the study results of source-sink relation.To promote scientific study support policy and governing measurements making better for the mitigation of total microplastic pollution in marine environment,this study recommended the development strategy for Beibu Gulf should be“protective exploitation in whole and exploitative protection in part”Some policy advisements were presented,such as a shift from extensive to intensive fishery is need;whole life cycle assessment of plastics should be insisted during policy making;a new waste management system for urban area as well as rural area is essential.Furthermore,this study advised that the Beibu Gulf should initiatively develop national and regional cooperation against marine plastic pollution because of the location advantage which is connecting South East Asia Countries(west side)and Guangdong Province and Hainan Province of China(east side).