Seasonal disparities in vertical distributions of microplastics and driving factors in a deep reservoir

Microplastic (MP) accumulation in freshwater reservoirs is a growing concern. However, research investigating the MP occurrence in large deep reservoirs, particularly with an emphasis on their vertical transport, remains limited. This study explored the MP pollution in a typical deep reservoir in southwest China (Pubugou Reservoir), with a particular focus on the MP vertical profiles. Results showed that fibers and fragments were the predominant shapes. MP concentrations in surface waters ranged within 1.98-6.13 items L (N/L), with low-density (< 1.0 g/cm) polymer types (mainly PE and PP) constituting a greater proportion than high-density types. Vertically, the MP concentration generally increased from the water surface to the bottom, with a maximum of 9.77 N/L in summer and 9.81 N/L in winter. MP characteristics differed most at 5-10 m depth, with summer exhibiting higher low-density fractions and smaller sizes than winter. Additionally, the daily MP deposition to sediments was estimated at 2.61 (range: 1.42-5.05) tons. We found that the presence of thermal stratification had a retardation effect on the vertical transport of MPs, resulting in their accumulation at the thermocline interface and within the metalimnion in summer. This retardation effect was more pronounced for low-density MPs compared to high-density counterparts, regardless of MP shape. The reshaped distribution pattern of MPs due to thermal stratification indicated seasonally varying MP exposure levels and associated risks within the vertical profiles of deep reservoirs.