Distribution Characteristics And Sedimentation Of Microplastics In Jiaozhou Bay

Microplastics are plastic particles with a length or diameter of less than 5 mm.These particles are emerging contaminants that have received increasing attention.Given that microplastics are difficult to degrade and easy to spread by the wind and ocean current,it can persist in the environment.The global marine ecosystem is facing severe challenges.Research on microplastics has mostly focused on ocean and offshore areas.However,the bay environment,which is closer to the area of human activities,is fragile,and research on the environmental behavior of microplastics is insufficient.This research selected the Jiaozhou Bay in Qingdao,China as the research area,analyzed the differences among the three different sampling tools of seawater microplastics,and investigated the distribution characteristics and the correlation of the distribution of microplastics in seawater and sediments.The variation law of the vertical distribution of microplastics in the sediment was analyzed.Furthermore,the factors affecting the distribution of microplastics in the Jiaozhou Bay were discussed.In this study,seawater was collected by direct filtration(0.45μm),sieve(20μm),and surface trawling net(150μm).By comparing the distribution characteristics of microplastics,such as abundance,polymer type,size,shape,and color,the correlations and differences among different methods were discussed.Different sampling methods may affect the abundance and characteristics of microplastics.In the seawater collected using filters with pore sizes of 0.45,20,and 150μm,the average abundances of microplastics in 2018 and 2019 were 2960.0±825.3,27.0±11.5 and 0.2±0.1,and 2000±740.7,40.7±18.4 and 0.2±0.1 items/m~3,respectively.The average sizes of microplastics in seawater collected using the three methods in 2018 and 2019 were 1190±982,1742±1199 and 2414±1710,and830±707,1106±870 and 1289±973μm,respectively.The results of the two studies showed that the larger the diameter of the sampling tool,the larger the size of the microplastics and the lower the abundance with two orders of magnitude difference.Large-aperture sampling tools may fail to collect small or fibrous microplastics.However,more samples could be collected in a short period of time.These three sampling methods were complementary,which can be applied to the sampling work in different sea areas and sea conditions to satisfy different research purposes.According to the comparison and analysis of the distribution results of microplastics,the distribution characteristics of microplastics showed no significant change in a short period of time.Polyethylene terephthalate(PET),polyethylene(PE),and polypropylene(PP)were detected in seawater for three consecutive years.PET and polyvinyl chloride(PVC)were consistently detected in the sediment.The abundance and polymer composition of microplastics in the seawater and sediment showed a certain correlation(p<0.05).The high-density polymer type,such as PET and PVC,accounted for a larger proportion of the sediment than that in the seawater,indicating that high-density polymers were likely to settle.The proportion of<0.5 mm microplastics in the sediment was larger than that in seawater in the same year,which indicated that the microplastics were constantly broken and minimized during migration or deposition process.To determine the contamination of microplastics in different periods in Jiaozhou Bay and to investigate the deposition law,we collected columnar sediment samples in the bay.The microplastics in 45 cm-deep sediment,which was approximately 70years old,were analyzed.The vertical distribution of microplastics in the sediment can clearly show the accumulation of microplastics with time.The abundance of microplastics in surface sediments(0–5 cm)was high,and almost no microplastics were detected in the bottom-most sediments(40–45 cm).The abundance of microplastics decreased with increased sediment depth but also varied with some special factors during the sedimentation period,such as the completeness of solid-waste treatment equipment and the amount of plastic used.This trend indicated that the abundance of microplastics was significantly affected by human activities and the output and waste of plastics.According to the results of this study,at least 3.71tons of microplastics had accumulated in the Jiaozhou Bay.Water and sediments were collected at six estuaries around the bay in the same period in 2017.The distribution characteristics of microplastics at the estuaries were analyzed and compared with the microplastic distribution in the bay.Microplastics at the estuaries had higher abundance and were longer than microplastics in the bay,and a clear correlation between polymer types was observed(in water,r=0.828,p<0.05;in sediment,r=0.759,p<0.05).In addition,in Jiaozhou bay with residual currents,microplastics were highly abundant,and more low-density(PE and PP)and small(<2 mm)microplastics were found.In areas with sedimentation tendency or rapid sedimentation rate,the microplastics appeared in deep sediments.The deposition dynamics varied in each area.Thus,the vertical changes of the microplastics were not synchronized.The influence of dynamics should be considered when studying the distribution characteristics of microplastics.In summary,microplastics were common in the Jiaozhou Bay,and the distribution of microplastics in the bay was affected by several factors.A comprehensive study of microplastics in Jiaozhou Bay and the surrounding estuaries was conducted,and the distribution characteristics and influencing factors of microplastics in the seawater and sediments of the Jiaozhou Bay were discussed.This research could provide theoretical support for the research of microplastics in typical bays and basic data for the study of the distribution mechanism of microplastics in the marine environment.