Spatiotemporal Patterns of Microplastic Distribution in Surface Water and Nearshore Sediments of a Large Tropical Lake: Evidence From Lake Toba, Indonesia

Microplastic (MP) pollution is increasingly documented in freshwater ecosystems, yet lake-scale evidence from large tropical lakes remains limited. We assessed MPs in surface water and nearshore sediments at 21 stations across Lake Toba, Indonesia, during the dry and rainy seasons. Mean MP abundance was higher in the rainy season than in the dry season in both surface water (1.461 ± 0.836 vs. 0.288 ± 0.110 particles m-3) and sediment (3.14 ± 0.59 vs. 1.26 ± 0.31 particles g-1 dry weight). Fragments and particles measuring 1000-2000 μm were dominant, while black and blue particles jointly represented 67%-68% of the observed MPs. Surface-water abundance was highest in the northern sector, whereas nearshore sediment abundance was higher in the southern sector. These contrasting patterns are consistent with spatial redistribution and retention within the lake, although transport pathways and sediment-focusing processes were not measured directly. Aquaculture, fishing, tourism, and domestic activities are therefore discussed as potential rather than confirmed sources. This study provides the first seasonally resolved, lake-wide baseline integrating surface water and nearshore sediments in Lake Toba and identifies priorities for source-specific monitoring and hydrodynamic investigation.