Beyond the surface – Microplastic hotspots in the water column of a top plastic-polluted deep lake

Most research on microplastics in lakes has focused on particles floating on the surface, while little is known about microplastic occurrence in subsurface layers, even though these layers comprise most of the lake’s volume. This knowledge gap is concerning because, without a deeper understanding of microplastic vertical occurrence, the full impact of microplastics on lake ecosystems cannot be accurately assessed. To fill this gap, this study investigated the seasonal variation in the concentration and composition (size, shape, and polymer type) of microplastics in different layers of the water column in Lake Lugano (Switzerland and Italy), a deep southern perialpine lake known for high microplastic pollution on the surface. Microplastic samples were collected seasonally from the water column, specifically from four layers representative of the surface (0–0.2 m), subsurface (0–10 m), middle (10–20 m), and bottom (20–80 m) layers of the lake. The highest microplastic concentrations were found in the upper three layers (surface: 18.9 particles m −3 ; subsurface: 29.2 particles m −3 ; middle: 30.9 particles m −3 ), whereas the lowest concentrations were found in the bottom layer (4.3 particles m −3 ). In addition, the layers showed differences in microplastic composition (size and shape) and intra-annual variation, suggesting that the interplay between seasonal environmental changes and hydrodynamic conditions may be a key driver of plastic distribution in deep lakes. The observed high concentration of microplastics between 0 m and 20 m depth, which encompass the euphotic zone, suggests a high risk of interactions between microplastics and freshwater organisms, warranting further investigation.