Seasonally driven dynamics of microplastic–heavy metal composite pollutants: a systematic review of cross-media processes and fates across the freshwater-to-ocean continuum

Microplastics (MPs) and heavy metals (HMs) frequently co-occur in aquatic environments, forming binary complexes via adsorption. The environmental behavior and associated risks of these MP–HM assemblages are not static; rather, they are dynamically shaped by natural seasonal cycles. This review provides a systematic synthesis of current knowledge on how seasonal variations—governed by key physicochemical drivers such as temperature, hydrology, salinity, and light, alongside biological factors—modulate the fate, transport, and ecological impacts of MP-HM combined pollution across the critical river–estuary–ocean continuum. We examine the seasonally varying dynamics of adsorption–desorption equilibria, cross-interface transport, and transformation processes, which collectively govern pollutant source–sink patterns and risk differentiation along the continuum. The review underscores the limitations of conventional static risk assessment paradigms in capturing such spatiotemporal variability. Finally, we propose a framework for developing dynamic, seasonally informed risk models and adaptive management strategies, thereby facilitating more accurate prediction and effective life-cycle control of composite pollution under evolving environmental conditions.