| Summary: | Landfast ice is a motionless continuous with the shore sea ice over. It forms seasonally in the majority of the Arctic coastal areas. Although it comprises a only small fraction of winter Arctic sea ice extent, it plays a significant role in the global climate system and is particularly important for coastal ecosystems and human activity. Along with the ongoing changes in the Arctic sea ice cover, the reduction of fast ice season and extent were reported in the majority of the Arctic marginal seas. A detail understanding of the mechanisms controlling fast ice development on a regional scale is important to predict future changes in fast ice cover and coastal environment. The main goal of this thesis is to investigate the variability of the fast ice extent in the Laptev and East Siberian seas and to find the mechanisms responsible for this variability. Using operational sea ice charts produced at the Arctic and Antarctic Research Institute (Russia) we analyzed seasonal and interannual variability of fast ice extent in the southeastern Laptev Sea between 1999 and 2013 and in the East Siberian Seas between 1999 and 2015. We characterized seasonal fast ice development in these regions by identifying key events in the course of fast ice growth and decay. Analyzing the timing of the fast ice key events, we found a decrease in duration of fast ice season in both regions with a rate of 2.8~d/y in the Laptev Sea and 1.5~d/y in the East Siberian Sea. This changes are caused by both a later beginning and earlier end of fast ice season, which can be partially explained by long-term trends in the onset of freezeup and melt. The winter fast ice extent did not show any changes during the investigation period, however previous studies report on the reduction in winter fast area \citep{Yu2013}. A time series of Synthetic Aperture Radar (SAR) imagery was used to investigate small-scale processes contributing to the advance of fast ice edge to its winter location in the southeastern
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