Marine diatoms are the dominant primary producers in coastal and shelf regions, and contribute to about 20% of the annual photosynthesis on Earth. Diatoms also exert a major control on the marine silicon (Si) cycle through the formation of biogenic silica (bSiO2). Continental shelves account for half of the total marine area in the Arctic, yet our knowledge of the cycling of Si for this critically climate-impacted region is limited. The overall objective of this thesis was to improve our understanding of marine bSiO2 dynamics and Si cycling in marine Arctic and Subarctic ecosystems using novel techniques. Phytoplankton and nutrient observations, including dissolved and particulate silica concentrations, are presented from a period of ten years within five biological ‘hotspots’ in the Bering and Chukchi Seas. The first measurements of bSiO2 production and dissolution rates are also presented from a period of four years at the same sites. Results from this work show that (i) although interannual variability is high, diatoms are responsible for most of the high primary productivity in the Bering and Chukchi Seas, (ii) bSiO2 is primarily re-dissolved within the euphotic zone rather than exported, and (iii) phytoplankton phenology and marine Si cycling are affected by short-term climatic changes in this region. We also present the first measurements of bSiO2 production rates along a transect from the Canadian Arctic Archipelago (CAA), through Baffin Bay and into the Labrador Sea. We show that diatoms are both abundant and productive throughout these regions in summer, despite widespread Si limitation in the low-nutrient surface waters. Finally, we also investigated the natural variations in the Si isotopic composition of silicic acid (30Si(OH)4). On a transect through the Bering and Chukchi Seas, Canada Basin and CAA, and finally to Baffin Bay and the Labrador Sea, we found that δ30Si(OH)4 signals reflect water mass composition, the dissolution of bSiO2 throughout the water column, and the biological utilization of Si in surface waters. Ultimately, this work provides insight into the processes controlling marine Si cycling within the Arctic and its links to the global marine Si cycle and other biogeochemical cycles. / Graduate / 2020-03-13