Microplastic contamination in the Karnaphuli River, Bangladesh: a PRISMA-guided critical appraisal of evidence reliability, source-attribution uncertainty, and receptor pathways.

Microplastic (MP) contamination in tropical river systems is increasingly reported, yet the reliability and comparability of available evidence remain uncertain. This review critically appraises MP contamination in the Karnaphuli River system, Bangladesh, using a PRISMA-guided systematic literature review and an expanded Source-Pathway-Receptor framework. Fifteen peer-reviewed studies were synthesized to evaluate environmental occurrence, methodological quality, reporting comparability, source-attribution certainty, physicochemical characteristics, ecological receptors, and human exposure plausibility. Because studies differed in sampling design, particle-size thresholds, analytical methods, QA/QC reporting, and measurement units, statistical meta-analysis was not appropriate. Available evidence reports MPs in water, sediment, fish, and fishery products, with fibres, fragments, and films frequently dominant. Industrial effluents, municipal wastewater, textile discharge, urban runoff, port activities, and unmanaged plastic waste are plausible contributors, but source attribution remains mainly inferential. Fish-ingestion evidence supports biological uptake and possible trophic transfer, although Karnaphuli-specific freshwater fish data remain limited. Aquatic plants, biofilms, plankton, and benthic organisms are important but understudied receptors. Human dietary exposure through fish and dried fish is plausible but locally unquantified. Overall, the Karnaphuli River should be treated as a high-concern MP system requiring standardized monitoring, source-specific sampling, polymer confirmation, QA/QC transparency, and receptor-based studies.