1 Microplastic particles in shoreline sediments of the Southern Caspian Sea Mohammad Javad Nematollahi 1,2* , Farid Moore 1 , Behnam Keshavarzi 1 , Rolf David Vogt 2 , Hassan Nasrollahzadeh Saravi 3 1 Department of Earth Sciences, College of Sciences, Shiraz University, Shiraz, Iran 2 Department of Chemistry, Faculty of Mathematics and Natural Sciences, University of Oslo (UiO), Oslo, Norway 3 Caspian Sea Ecology Research Center (CSERC), Iranian Fisheries Science Research Institute (IFSRI), Agricultural Research, Education and Extension Organization (AREEO), Sari, Iran * Corresponding author: m.nematollahi@shirazu.ac.ir, mjnematollahi.shirazu@gmail.com Abstract The occurrence of microplastic (MP) particles in shoreline sediments from the coastal region of the southern Caspian Sea, Mazandaran province, Iran, was investigated. A total of 312 MP items were visually detected in 17 sediment samples. The mean abundance of MP particles was 18 items kg-1. The MP particles were mainly fibers, and black color dominated. The dominant size class of MP particles was between 250 and 500 μm. Polyethylene terephthalate (PET), polystyrene (PS), and nylon (NYL) were the major polymers and/or copolymers comprising MP particles in both sediment samples. The number of MP particles in sediment samples showed a general decrease from west to east in the study area. This may be reflecting the spreading of MP loading from the outlets of Sefidrud, Tonekabon, Chalus, the major rivers entering the Caspian Sea just west of the study area, and the overall decrease in the spatial distribution of touristic and fishery activity. Keywords: Microplastic (MP), shoreline sediment, Caspian Sea, Mazandaran 1. INTRODUCTION Recent studies document the presence of MP particles in all marine environmental compartments. Sediments are a good sink and source of contaminants, thus they are frequently used to evaluate the state of the environment in terms of levels of contaminants in the sediments (Moore et al., 2015). Research focus on MP has recently shifted from documenting the omnipresence of MP particles in a wide range of different environmental compartments to paying more attention to their sources, transport mechanisms, distribution within water and sediment, and the fate of MP particles in the environment (Browne et al., 2010, 2011; Woodall et al., 2014; Eriksen et al., 2014; Digka et al., 2018; Karthik et al. 2018; Lehner et al., 2019; Yao et al., 2019; Mu et al., 2019; Sathish et al., 2019; Wang et al. 2019). A large number of rivers and numerous discharge pipes from wastewater treatment plants entering the Caspian Sea may be considered as main point sources of MP particles to the sea environment. Besides, there is a wide range of shipping and fishing activities in the area that comprise significant diffuse sources of MP particles to the Caspian Sea. There has been a couple of studies (Mataji et al., 2019; Mehdinia et al., 2020) of the presence of MP particles along shore sediments of the southwestern Caspian Sea. Nevertheless, there has been a lack of enough study on transport and the distribution of MP particles along the coast of Mazandaran province. Hence, this research is carried out within shore sediments of the southern regions of the Caspian Sea to provide an insight into the distribution of the MP particles, as well as identification of their size, shape, color, and chemical composition, to assess main sources and the biogeochemical fate of the MP particles in this rather unique brackish sea environment. 2. MATERIALS AND METHODS 2.1. STUDY AREA