Source tracking microplastics pollution from Qarasu basin rivers to Gorgan Bay, Caspian Sea

Abstract Concerns about the negative effects of MPs on human health have led to increasing attention to the occurrence of MPs in the aquatic environment. Recent studies have focus from the marine environment to inland waters, particularly the spatio-temporal distribution of MPs in rivers for residential and agricultural area. Qarasu river basin, watershed to Gorgan Bay, is the site of many permanent rivers and the levels of MP pollution in those rivers are unknown. This research was conducted in three different type of land use: forest, residential and agricultural along 8 different rivers. A total of 9 fish species were identified and the presence of MPs in 87% of fish was confirmed. The prevalence of MP among species was Liza sp.> C.gibelio> G.holbrooki> R.rutilus> N.melanostomus> C.carpio = V.vimba> R.frisii> Barbus sp. The highest frequency of MPs was observed in Qarasu river (1880 ± 251 n/kg) and Mohammadabad river (184 ± 1340 n/kg) in agricultural land use, the lowest frequency was related to Baghu and Ziarat rivers with 660 ± 77 and 600 ±91 n / kg was found in forest land use respectively. The highest type, color and size of MP sediments were fragment, black and 1-2 mm, respectively. The highest type, color and size of MP in fishes were fiber, black and 0.5-0.1mm respectively. Overally according to the source tracking result, agricultural land use was the main source of MP pollution of Gorgan Bay. The Bay is strongly affected by the incoming rivers, the Qarasu River Basin as the most important river supplying water.