Assessment of secondary microplastics trapped in mangrove ecosystem of a highly populated tropical megacity, India

• Mumbai mangrove sediments contain 6730±2063 MPs particles/kg dry weight. • Sediment samples are dominated by silt, followed by sand and clay. • Microplastic load decreases with increasing sediment depth in the mangrove ecosystem. • Fibers (56.1%), particles <100 µm (38.5%), and polyethylene dominate microplastic types. This study quantified microplastics (MPs) in six zones of the Mumbai mangrove ecosystem through random sampling at 30 sites using the density separation method for extraction and analysis. A total of 2,035 particles were identified based on their shape, color, and size using a stereomicroscope. The results revealed a high abundance of MPs in mangrove sediments, averaging 6,730.2 ± 2,063.9 particles/kg dry weight. The highest average was recorded in the Versova region (7,885.7 particles/kg d.w.), while the lowest was in Sewri (5,785.7 particles/kg d.w.). Fibers (56.1%), particles < 100 µm (38.5%), and translucent/transparent items (30.8%) were the predominant types. Micro-Fourier transform infrared spectroscopy analysis indicated that the most common plastic polymer was polyethylene (36.91%), followed by polyester (21.33%) and polyamide (13.47%). This study provides critical estimates of MPs abundance in Mumbai's mangrove sediments, highlighting the urgent need for management plans and policies to address microplastic production and release into coastal waters, thereby protecting marine organisms.