Microplastics in the Baltic Sea region lakes—standardized insights reveal urban shoreline as key driver

Microplastics (MPs) are ubiquitous in aquatic environments and closely tied to human activities, yet linking measured concentrations to urban pressures remains challenging. Methodological inconsistencies inflate variance, obscure spatial patterns, and hinder comparability. We therefore implemented fully harmonized sampling and FTIR-verified analysis across ten lakes in Latvia, Lithuania, and Poland, with all field and laboratory work performed by the same team. In parallel, we calculated an urbanization index to relate MP concentrations to shoreline development and urban influences. This design enabled comparisons of MPs in surface waters and sediments with ranges of 0.67-7.68 particles L⁻¹ (water) and 0.09-3.90 particles g⁻¹ dw (sediments). Polyethylene and polypropylene predominated, with fibres and fragments as the most common shapes; particles < 5 mm, and black, blue, or white, were most frequent. Shoreline urbanization, quantified by the shoreline urbanization index, was the only significant predictor of MPs in surface waters, explaining ~ 74% of variance, while nutrient concentrations, lake type, proximity to point sources, and season showed no consistent effect. Seasonal shifts in particle shape and type were observed but not in total abundance. MP concentrations were broadly similar among countries, with modest, statistically inconclusive differences in particle size and colour, indicating limited regional variation once methodological noise is removed. Our findings highlight that harmonized protocols sharpen the detection of environmental drivers and may overturn previously reported geographic differences driven by inconsistent methods. This first fully standardized, multi-lake, cross-country MP survey provides a robust framework for future monitoring and mitigation in inland waters.