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This study documented seasonal levels of microplastics (MPs) and biomarkers (condition index, neurotoxicity, energy, oxidative stress) in mussels (Mytilus galloprovincialis), and water physico-chemical parameters in the Douro estuary (NE Atlantic coast), and estimated the human risk of MP intake (HRI) through mussels. Mussel stress was determined through the Integrated Biomarker Response (IBR). HRI was estimated from mussel MP concentrations and consumer habits. MPs were mainly micro-fibres (72 %) with varied chemical composition. Seasonal MP means (±SEM) in mussels ranged from 0.111 ± 0.044 (spring) to 0.312 ± 0.092 MPs/g (summer). Seasonal variations of mussel stress (IBR: 1.4 spring to 9.7 summer) and MP concentrations were not related. MeO-BDEs, PBDEs, temperature, salinity and other factors likely contributed to mussel stress variation. HRI ranged from 2438 to 2650 MPs/year. Compared to the literature, MP contamination in mussels is low, as well as the human risk of MP intake through their consumption. • Microplastic (MP) concentrations and biomarkers were studied in marine mussels. • Seasonal means (0.111 ± 0.044 to 0.312 ± 0.092 MPs/g) indicate low MP contamination. • Neurotoxicity and oxidative and energy stresses were observed (mainly in summer). • Seasonal variation trends of MP concentrations and stress levels were not related. • Human risk of MP intake through the consumption of Douro estuary mussels is low. [ABSTRACT FROM AUTHOR]