Tracing environmental variability in the changing Arctic Ocean with optical measurements of dissolved organic matter

The Arctic Ocean plays an important role on the global hydrological and carbon cycles. It contributes 5a 14% to the global balance of CO2 sinks and sources. Carbon is also cycled in the Arctic Ocean through the primary producers, with high primary production observed in the marginal ice zones, ice-free zones and melt ponds, with increased biogenic carbon export to the deep layers. Although being the smallest ocean basin, the Arctic Ocean receives 11% of the global riverine runoff. Along with the freshwater, high loads of organic carbon are introduced in the Arctic Ocean. Most of it is observed in the fraction of dissolved organic matter (DOM). With the ongoing global warming, glacier melt and permafrost thaw are observed and pointed as the main drivers for increasing the freshwater discharge into the Arctic basin. Along side, permafrost thaw coupled with increased coastal erosion lead to an increase in mobilization of carbon from permafrost, which could have critical implications for microbial processes, primary production, terrestrial carbon fluxes to the shelf seas and, thus, carbon cycling in the Arctic. This thesis is focusing on tracing the mixing of DOM along the Siberian shelves and developing potential applications of DOM as an environmental tracer. Four main objectives have been pursed: (1) to quantify, characterize and assess the distribution and transformation of DOM across the river-shelf transition and provide insights into the fate of Arctic riverine DOM; (2) to assess the potential of DOM, especially its fluorescent fraction (FDOM), as a tracer of freshwater in the surface layers in the Arctic Ocean; (3) to characterize the non-water absorption in the surface central and eastern Arctic Ocean and further test whether bio-optical properties (such as absorption and reflectance) can reproduce hydrographical variability; (4) to evaluate the performance of ocean color algorithms frequently applied for studies in the Arctic Ocean using novel data from a central-eastern Arctic expedition.