The spatial distribution and abundance of microplastics in lake waters and ice during ice-free and ice-covered periods

Understanding the spatial distribution and characteristics of microplastics (MPs) in lake waters is essential to assessing and addressing lacustrine MP pollution. This study investigated how lake ice affects the abundance, spatial distribution, and characteristics (size, shape) of MPs in Lake Ulansuhai by analyzing samples collected at ten sites uniformly distributed throughout the lake during ice-free and ice-covered periods. The abundance of MPs ranged between 204 ± 28 and 1224 ± 185 n·L in lake waters during the ice-free period, and from 34 ± 8 to 216 ± 21 n·L and 269 ± 84 to 915 ± 117 n·L in water and ice during the ice-covered period, respectively. During the ice-covered period, MPs were 2.74-8.14 times higher in the ice than in water beneath the ice. Ice formation decreased MP abundance in lake waters, in part, by incorporating a relatively high percentage of MPs into the ice mass during freezing and by inhibiting atmospheric MPs from reaching the lake waters. The abundance of MPs in the water during the ice-free period was 4.50-11.30 times greater than during the ice-covered period. Seasonal variations in MP shape also occurred; the proportion of fibrous MPs in water decreased during the ice-covered period. Variations in MP abundance were partly due to differences in sedimentation rates; the settling of fibrous MPs is slower, making it easier for them to be captured during the formation of surface ice. Spatially, MPs were uniformly distributed during the ice-free period, but exhibited a spatially distinct pattern during ice-covered periods, when MPs in lake waters were higher in the northeast and lower in the southwest portions of the lake. During the ice-free period, small MPs (0.05-0.5 mm) were more likely to move with currents in the lake, whereas water velocities were reduced by ice formation, allowing small MPs to accumulate near the lake inlet.