Spatial Heterogeneity of Microplastic Contamination in Coastal Surface Waters: A Case Study of 21 Stations in the Southern Caspian Sea, Iran.

Abstract

Microplastic (MP) pollution poses significant threats to coastal ecosystems, yet data on its distribution in the southern Caspian Sea remain limited. This study provides one of the first comprehensive regional assessments of MP contamination in coastal surface waters along the Iranian Caspian coastline. Surface-water samples were collected from Gilan, Mazandaran, and Golestan provinces at three distances from the shoreline. MPs were extracted using density separation and oxidative digestion, characterized by stereomicroscopy and polymer-confirmed by FTIR-ATR. MPs were detected at all 21 sampling stations, with concentrations ranging from 0.05 to 1.30 MPs/L and a mean abundance of 0.24 0.05 MPs/L. MP abundance decreased significantly with increasing distance from shore (p < 0.001), indicating dominant land-based pollution sources. Mazandaran province exhibited the highest contamination levels, associated with intensive tourism and urbanization. Land-use analysis revealed that areas experiencing combined anthropogenic pressures had significantly greater MP abundance than single-activity areas. Fibers dominated particle morphology (73.4%), while polyethylene and polypropylene were the most abundant polymers, reflecting contributions from consumer plastics, packaging materials, and fishing-related activities. Multivariate analysis identified distinct pollution signatures associated with tourism, urbanization, and fisheries. These findings establish a critical baseline for future monitoring. Improved coastal waste management, tourism controls, and sustainable fishing practices are essential to mitigate MP pollution in this ecologically vulnerable inland sea.