Rapid climate warming in the Arctic region intensifies permafrost thaw, increases active layer depth in summer and enhances riverbank and coastal erosion. This causes the release of organic matter (OM) into streams and rivers and result in discharge of terrestrial OM into the Arctic Ocean. The focus of our study, particulate OM (POM) consists of a complex mixture of compounds from different sources with different chemical/physical resistance towards decomposition and mineralization. Particulate organic carbon (POC) transported by the Lena River represents a quantitatively important carbon pool exported to the Laptev Sea, but its quantity, quality, transport and transformation features are still not fully understood. With this study, we determined the sources (e.g., permafrost, soil, peat, phytoplankton, vegetation, etc.), quality and age of organic carbon transported by Arctic rivers to understand the effects of climate change on the river watersheds as well as on the Arctic coastal nearshore zone. Our study is embedded into the project ‘Changing Arctic Carbon cycle in the cOastal Ocean Nearshore (CACOON)’, which aims to investigate composition, seasonal effect, and changes during the transport and further fate of organic matter discharged by the Lena River to the Laptev sea (see Strauss et al 2021, this abstract book). To assess these data, we have had an intensive fieldwork in the Russian Arctic in summer 2019. Samples were collected across a ~1500 km transect from the Yakutsk through the centre of the Lena Delta to the Nearshore zone, covering the fresh-salt water transition. We analysed water samples from one to three different water depths to capture stratification in the water column. In a next step, the water was filtered at Samoylov Research Station through precombusted GF/F filters (25 mm diameter). Filters with POM were stored frozen in precombusted glass petri dishes. Later, the filters were analysed at AWI Bremerhaven for total suspended matter, total POC concentration, stable (δ13C) and radiocarbon (Δ14C) isotopes. We found significant qualitative and quantitative differences between the OM composition in the Lena River main channel and its delta. Further, we found suspended matter and POC concentrations decreased during the transit from the river to the Arctic Ocean. We demonstrate that deltaic POC is depleted in 13C relative to fluvial POC, and that its 14C signature suggests a modern composition indicating phytoplankton origin. This observation likely reflects the difference in hydrological conditions between the delta and the river main channel, caused by lower flow velocity and average water depth.