Riverine microplastics (MPs), synthetic polymer particles ≤5 mm, are a global environmental problem. Rivers receive MPs from different sources, transport MPs to the oceans, and can also act as sinks for accumulating MPs. The sources and fate of MPs in river catchments are not fully identified and understood. To advance the understanding of the fate of small (25-300 µm) MPs in urban agglomerations, a one-year monitoring in the Seine river (France) was carried out. We analyzed MP concentrations in the water upstream and downstream of the Greater Paris area in four sampling campaigns from July 2022 - July 2023. The monitoring covered seven sites along the Seine river and tributaries and different water flow conditions. Water samples up to one m3 were taken with the help of a novel in-situ pump and cascade filtration system (UFO®, Aalborg University, DK). MPs were extracted with oxidation and density separation and further analyzed via µ-FT-IR spectrometry. Preliminary, we found concentrations of polypropylene (PP) and polyethylene (PE) were most abundant across all samples (~59 % and ~25 %, respectively). In total, we detected 16 different polymer types, including polystyrene (PS), polyvinyl chloride (PVC), polyester (including polyethylene therephtalate, PET), styrene butadiene rubber (SBR), polyamide (PA), and polyurethane (PU). The variety of polymer types is larger downstream (16 polymer types) compared to upstream (9 polymer types). Concentrations range between ~16 4900 # particles/m3 and vary temporarily and spatially along the river catchment, which suggests potentially complex MP transport and fate dynamics. Very likely, parameters such as surface runoff, combined sewer system overflow (CSO), and hydrodynamics influence MP concentrations. To explain the observations and identify influences on the MP levels observed, we apply the field data to hydrodynamic modeling. We strongly recommend more frequent, intense, and continuous MP monitoring in river catchments to understand the local dynamics, identify sources and sinks (hotspots), and strategically develop mitigation measures.