Microplastics in Aquatic Ecosystems: Sources, Pathways, Biological Impacts, and Mitigation Strategies — A Comprehensive Review

Microplastics (MPs), defined as plastic particles less than 5 mm in diameter, have emerged as ubiquitous and persistent contaminants across global aquatic ecosystems. Their widespread distribution, bioavailability, and capacity to adsorb toxic substances pose significant threats to aquatic organisms across all trophic levels. This review synthesizes current knowledge on the sources, transport pathways, ecological distribution, and biological impacts of microplastics on aquatic fauna, highlighting physiological, behavioral, and ecosystem-level consequences. Originating from both primary sourcessuch as microbeads in cosmetics and industrial abrasives and secondary sources resulting from the environmental degradation of larger plastic debris, microplastics are now detected across oceans, rivers, lakes, polar ice cores, and urban freshwater systems. Their persistence and ubiquitous distribution pose a grave threat to aquatic biodiversity and water quality. A broad range of marine and freshwater organisms, including fish, shellfish, plankton, and cetaceans, ingest these particles, resulting in physical blockages, reduced energy intake, oxidative stress, endocrine disruption, and toxic exposure to co-contaminants such as heavy metals and persistent organic pollutants (POPs). These toxins bioaccumulate through food chains, potentially reaching human consumers via seafood and drinking water. Microplastics also settle into sediments, disrupting benthic habitats and altering microbial communities essential to nutrient cycling. Environmental monitoring highlights the role of mechanical degradation and its interaction with physico-chemical stressors in freshwater systems. Evidence further indicates that MP exposure impairs reproduction, growth, and behavior in aquatic species while facilitating the dispersal of invasive microbes and antibiotic-resistant pathogens. Their small size renders detection and removal technically challenging, necessitating advanced analytical technologies and standardized monitoring protocols. Solutions demand integrated waste management, reductions in primary plastic production, biodegradable material substitution, and enforceable global policy frameworks. The impact of microplastics on aquatic ecosystems is multifaceted and far-reaching, demanding urgent, coordinated interdisciplinary action to protect biodiversity, ecosystem services, and public health from long-term harm.