Bacterial diversity and function across time and space in the Arctic Ocean, including the Polar Night, remain virtually unknown. In the FRAM Observatory, we study microbial composition and genetic potential in ice-covered and ice-free regions of the Fram Strait, the major gateway between the Arctic and Atlantic Oceans. A continuous amplicon time-series, derived from moored autonomous samplers, revealed marked taxonomic and functional seasonality among bacterial communities in the ice-free West Spitsbergen Current, with distinct succession of taxonomic modules. PacBio long-read metagenomes showed peaks of proteorhodopsin- and DMSP-utilizing genes in late summer, whereas winter mixing of the water column covaried with ammonia- and nitrite-metabolizing bacterial genes. In the ice-covered East Greenland Current, taxonomic and functional diversity varied less with seasons, with prominent influence of ice cover and polar water masses. For instance, high-ice conditions coincided with higher number of peptidoglycan-utilizing genes. Continuous observations were contextualized with five-year amplicon data from summertime samples collected across Fram Strait, integrating seasonal and interannual patterns of bacterial community dynamics. This fundamental baseline information helps understanding ecological and biogeochemical processes in a marine region severely affected by climate change.