Microplastic Contamination in Freshwater and Coastal Ecosystems — Spatial Distribution, Polymer Characterisation, and Source Attribution Across Northwestern Europe

Microplastic contamination of freshwater and coastal ecosystems has emerged as a critical environmental challenge, with particles detected in virtually every aquatic matrix sampled globally — from Arctic sea ice to remote mountain lakes — yet the quantitative spatial distribution, polymer composition, and source attribution of microplastic loads across connected freshwater-coastal continua remain incompletely characterised in European contexts. This study presents a systematic multi-site sampling campaign across 84 aquatic monitoring locations in Sweden, the Netherlands, France, and Poland, spanning six water body typologies — urban rivers, peri-urban rivers, rural streams, coastal zones, stratified lakes, and groundwater — sampled seasonally over a two-year period (2022–2023). A total of 6,482 individual microplastic particles were characterised by attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) for polymer type identification. Urban rivers showed the highest mean concentrations (1,842 particles/m³) and coastal zones the second highest (2,284 particles/m³), driven by wastewater effluent discharge and urban stormwater runoff. Positive matrix factorisation source attribution identified wastewater effluent (34.2%) and urban runoff (22.8%) as the dominant freshwater microplastic sources. Seasonal variation was significant, with summer concentrations 38–52 percent higher than winter values, consistent with increased recreational plastic use and reduced dilution under lower summer discharge.