Microplastic research initially focused on marine environments leaving freshwater ecosystems mainly unexplored, despite their hypothesized importance as suppliers of microplastics to the ocean. In addition, relevant approaches for unravelling microplastic sources and transport pathways at the catchment level are still missing to enable efficient actions towards mitigation and remediation. Here, we investigate microplastic stocks and fluxes in a Nordic lake ecosystem, Lake Mjøsa. We utilised a multidisciplinary approach for identifying sources of microplastic pollution and draw a plastic budget for the catchment of L. Mjøsa. The microplastics spatial and vertical distribution in lake sediments is compared to socio-economic modelling of plastic fluxes as well as spatial datasets on land use and potential plastic sources in the catchment and sub-catchments. We quantified the different plastic morphologies within sediment cores from 20 sites in the lake and explain their spatial distribution with an innovative spatially comprehensive dataset. We estimate that 32.5 tonnes (5.8-117.1 t) of plastics are released into the lake annually of which 90% is transported downstream and only 10% is deposited in the sediment. Synthetic textiles and packaging are likely the main sources of sediment contamination in L. Mjøsa probably through agricultural sludge application and urban waste mismanagement and littering. Most of the spatial variability of sediment microplastic is explained by the distance to the nearest town. The 10 most polluted sites, so called urban sites, contain polymers of higher diversity than natural sites and show increasing microplastic concentrations, diversity and richness over 1980-2018. This interdisciplinary approach to understand microplastics in Lake Mjøsa utilising catchment processes takes microplastic research further than simply presence, absence and abundance. By corroborating sediment inventories with top-down estimates of plastics emissions, modelled on the socio-economic activities in the catchment we have been able to provide a microplastic budget for the whole lake catchment.