Permafrost is considered to be a resting titan, exceptionally vulnerable to the impact of ongoing climate change. Warming temperatures intensify permafrost thawing, leading to the release of previously stored and frozen organic matter (OM) back into the active biogeochemical cycle. Once released from permafrost, OM eventually appears in lakes, streams and rivers, which carry their waters into larger water bodies. In this study, I aimed to investigate particulate and dissolved organic carbon (POC and DOC) discharged by the one of largest Arctic rivers—the Lena River—into the Arctic Ocean. While writing this work, I was deeply curious and focused on the major question: what is the composition and fate of organic matter that the Lena River transports across the continent, through its delta, to the Arctic coast? This thesis was written to resolve this research curiosity as a part of the German–British scientific research project “Changing Arctic Carbon cycle in the COastal Ocean Near-shore (CACOON)”. The main body of this thesis comprises three manuscripts, each focusing on specific research themes. The first manuscript, a scientific publication titled “Particulate organic matter in the Lena River and its delta: from the permafrost catchment to the Arctic Ocean,” was published in the journal Biogeosciences (DOI: https://doi.org/10.5194/bg-20-1423-2023). Here, the dynamics of POC were studied in the Lena River delta and compared with those in the river’s main stem using C isotopic composition to define the sources of organic matter. This study reveals that total suspended matter and POC concentrations decreased by 70 % during transit from the main stem to the delta and Arctic Ocean. Dual-carbon (D14C and d13C) isotope mixing model analyses revealed a dominant phytoplankton contribution to deltaic POC and also demonstrated an additional input of permafrost-derived OM. These results highlight the importance of deltaic and estuarine processes in shaping OM dynamics in Arctic nearshore zones. The second manuscript “Dissolved and Particulate Organic Carbon Characteristics in Summer and Winter Waters of the Lena Delta” was submitted to Permafrost and Periglacial Processes journal. It aimed to identify sources and seasonality of organic C at the Lena delta. C isotopes (∆14C and d13C) were measured in DOC and POC along a 140 km transect of the Lena delta. The study showed that DOC concentrations in the Lena delta were unexpectedly higher in winter than in summer. Isotopic analyses revealed that winter DOC showed a greater contribution of older C compared to summer DOC. These findings provide valuable insights into carbon dynamics in the largest Arctic delta, contributing to the understanding of how climate warming affects the critical interface between land and ocean. Finally, the third manuscript “Exploring the mysteries of GDGTs in the Arctic: Insights from the Lena Delta and Laptev Sea Nearshore” is a draft focused on glycerol dialkyl glycerol tetraethers (GDGTs) —biomarkers used as proxies for environmental conditions. Isoprenoidal (isoGDGT), hydroxylated (OH-GDGT), branched (brGDGT), and H-shaped brGDGTs (H-brGDGTs) were extracted from POC to analyse their distribution in the Lena delta in winter and summer and along a transect to the Laptev Sea (only in summer). The result showed that IsoGDGT and OH-GDGT reflected marine influence - salinity and water temperatures. BrGDGTs, largely soil-derived, were more concentrated in winter, indicating higher terrestrial input, whereas summer levels reflected dilution by vegetation and aquatic microorganisms. H-brGDGTs, associated with peat, showed enhanced deposition near the coast, likely due to flocculation in saline waters. Additionally, this study highlighted the unique geochemical signature of Arctic river systems such as brGDGT signature with the dominance of 6-methyl isomers (IIIa’, IIa’) and the predominance of 5-methyl isomers (IIIa, IIa). These findings reveal how temperature, salinity, and organic matter sources shape the distribution of GDGTs in Arctic riverine and coastal systems. Supporting the findings of my research, I added three published and co-authored manuscripts to the appendix of this thesis. First, the paper “Organic matter characteristics of a rapidly eroding permafrost cliff in NE Siberia” by Haugk et al. (2022) demonstrates how permafrost thaw and coastal erosion mobilize significant quantities of organic carbon and nitrogen, driving changes in the biogeochemistry of Arctic nearshore waters. Second, “Degrading permafrost river catchments and their impact on the Arctic Ocean nearshore processes” by Mann et al. (2022) explores the transformations and ecological impacts of permafrost-derived organic matter as it is transported from land to the ocean, highlighting its role in shaping nearshore ecosystem dynamics. Third, “Seasonal nitrogen fluxes of the Lena River Delta” by Sanders et al. (2022) expands on these insights by examining the broader implications of these processes for Arctic carbon fluxes and nutrient cycling, emphasizing the importance of nearshore zones as hotspots of change under ongoing climate warming. Overall, this thesis provides comprehensive and multifaceted research on dissolved and particulate organic carbon in the Lena delta, including their origin, transformation during transport from the source to the Arctic nearshore zone, and seasonal changes.