Weathering processes of microplastics in the sea ice and ocean environment: an 18-month-long mesocosm study

The persistence and impact of microplastics (MPs) in marine ecosystems are of increasing concern. In recent years, sea ice has been recognized as an effective temporary reservoir for MPs in polar oceans. While many studies have reported the presence of MPs, the weathering processes of MPs and their interactions with other contaminants remain largely unknown, especially in polar oceans with a dynamic sea ice environment. This study conducted an extensive 18-month-long, outdoor mesocosm experiment to investigate the weathering processes of MPs in the marine cryosphere that involved two complete cycles of sea ice growth and melt under natural ambient conditions. Microplastics of different size, color and chemical composition were used in the study to examine the weathering processes and the interactions between microplastics with other trace metal species (e.g., mercury). By simulating a polar marine cryosphere dominated by a seasonal sea ice cover at a mesocosm scale, the study captures complex interactions between MPs and the surrounding environment. Key findings reveal that prolonged exposure to natural weathering processes, primarily due to photodegradation, biofilm formation, and seasonal sea ice dynamics, significantly alters MP surface properties and Hg accumulation efficiency in ways not observed in laboratory scale experiments. Photodegradation of MPs occurred, but to a much lesser degree than reported from laboratory studies; instead, seasonal changes in temperature, and sea ice and biofilm dynamics had a much more prominent role in the weathering and Hg accumulation. The unique natural weathering patterns found in the mesocosm study indicate that such a process cannot be easily reproduced in lab-based, short-term, single-factor studies. The study also reveals the role of weathered MPs as a medium for toxic substances such as Hg, which might present additional ecological risks when ingested by marine biota. This work promotes the understanding of the long-term fate of MPs in marine ecosystems and informs mitigation strategies, advocating for policies grounded in realistic environmental conditions. By bridging the gap between laboratory experimentation and real-world complexity, the study shows the value of the employed mesocosm approach for future research on plastic pollution, especially on its implications in the polar marine cryosphere under a rapidly changing climate.