Microplastics in tropical Malaysian rivers: a review on spatial abundance, analytical methods, and ecological risk...

Riverine systems in Southeast Asia are vital pathways for microplastic (MP) flux, but a unified understanding of their spatial patterns, land-use influences, and transport dynamics remains limited. As a primary contributor to global riverine plastic emissions, Malaysia serves as a critical model for understanding microplastic dynamics in tropical monsoonal climates. Microplastics are emerging contaminants that can be harmful to tropical ecosystems. This systematic review synthesizes existing data on MP abundance, morphology, and transport within Malaysian river basins, providing a comprehensive analysis of how local land use and tropical climate drivers control the environmental fate of MPs and evaluating the current state of MP pollution and the evolution of analytical methods. Our findings reveal a broad concentration gradient, ranging from 4.2 × 10^-6 items in the waters of the Cherating River to a peak of 2012 items/L in the industrial reaches of the Langat River. Morphological characterization shows a distinct land-use signature, fiber dominance (> 90%) in the rural domestic Kelantan basin, contrasting with fragment-heavy profiles in the urban industrial Langat and Melaka basins. Methodologically, the field has transitioned from visual microscopy to high-resolution spectroscopic validation (µFTIR and Py-GC/MS), significantly lowering detection limits to 1-10 µm. A conceptual model of tropical transport dynamics is proposed, highlighting the role of monsoonal flushing in mobilizing plastic pulses from terrestrial sinks to coastal shorelines. Comparing Malaysian basins with other high-pressure systems in tropical countries reveals that Malaysia is a globally significant contributor to microplastics. By situating these findings within the global context, this review provides an important framework for understanding microplastic transport in rapidly urbanizing tropical regions and offers essential data for international marine conservation and global climate resilience strategies.