Occurrence and fate of microplastic debris in middle and lower reaches of the Yangtze River - From inland to the sea.

Based on field investigations and modeling estimates, rivers are considered an important route taken by microplastics to reach the sea. However, only a few studies have directly assessed occurrence and fate of microplastics in rivers. In this study, we surveyed the occurrence and fate of microplastic debris in the middle and lower reaches of the Yangtze River, which was estimated to be the largest riverine source of plastic debris to the sea. The abundance of microplastics at 15 sites along the middle and lower reaches of the Yangtze River ranged from 1.95 × 105 to 9.00 × 105 items/km2 with an average of 4.92 × 105 items/km2. The abundance of microplastics was positively correlated with the abundance of mesoplastics. Megacities, large riparian lakes, and seawater encroachment appeared to be main factors affecting the distribution of microplastics in the Yangtze River, while the deposition to benthic and riparian sediments likely led to observed decreases in microplastics in some sections of the river. The results of this study indicate that a considerable amount of microplastics generated in large river catchments are not transported to the sea and past modeling estimates of microplastic flux may contain biases. The flux of microplastics reaching the sea should be calculated based on long-term monitoring at the tidal limit of rivers. More importantly, the fate and effects of microplastics retained in the river system should receive more attention.