The Arctic Ocean is undergoing irreversible perturbations as a result of accelerated climate warming. Of major significance is the expanding influence of Atlantic water that expedites sea-ice decline, alters stratification and vertical mixing of the water column and facilitates northward expansion of temperate biota. Our understanding on how these processes will impact biological communities is severely limited. The Fram Strait is the primary entry route for Atlantic water into the Arctic Ocean and exit point for polar water and sea-ice. With the presence of two major current systems combined with horizontal mixing processes, the Fram Strait is characterised by a longitudinal gradient of hydrographic regimes reflective of Arctic, mixed and Atlantic conditions. This provides an invaluable opportunity to study the ecology of microbes over an environmental gradient and under changing conditions. Furthermore, given its high-latitude position, it also facilitates investigations on how dramatic seasonal transformations in conditions, such as sea-ice cover and light availability, influence microbes in the context of water mass history. This thesis provides an ecological characterisation of microbial communities over temporal and spatial scales in the Fram Strait in an effort to address these topics. In Chapter II, we employed metagenomics from short- and long-read sequencing platforms to gain insights into microbial community composition across water masses in the Fram Strait. As that study incorporated the first PacBio HiFi (long-read) metagenomes from the marine environment, it was necessary to perform a methodological comparison. We show that using PacBio HiFi metagenomes, we are able to recover more metagenome-assembled genomes (MAGs) that, on average, are more complete, less fragmented and more frequently contain complete rRNA gene operons compared to using short-read metagenomes. This not only influenced our investigative toolkit throughout the remainder of this thesis but provides valuable data for future considerations on using long-read metagenomics in the study of marine microbial ecology. From the analysis conducted in Chapter II, we observed a flavobacterial clade that is commonly associated with coastal temperate ecosystems, the NS5 Marine Group, to be prominent in high-latitude waters. This motivated us to delve deeper into this group and understand their diversity and function. By combining cultivation, metagenomics, epifluorescence and transmission electron microscopy, we were able to delineate this group into four novel candidate genera and evidence distinctions in function and spatiotemporal dynamics at the species and genus level (Chapter III). In that study, we also presented the first pure isolate and complete genome for a member of the NS5 Marine Group. In Chapter IV, we performed the first high-resolution temporal analysis on microbial taxonomy and function in Arctic polar waters. Using a four-year 16S amplicon dataset and one annual cycle of PacBio HiFi metagenomes, we evidenced that Atlantic water influx and sea-ice cover had a profound impact on the composition and function of microbial communities. Based on their omnipresence irrespective of conditions, we also identified a small fraction of the community that likely represents the resident microbiome of the Fram Strait. Furthermore, we showed that a transition to low-ice and high Atlantic water influx shifted the community to one dominated by heterotrophic clades that are functionally linked to phytoplankton-derived organic matter. Our findings suggest that the continued expansion of Atlantic water into the Arctic Ocean will be reflected in a Biological Atlantification of the microbial community, with populations adapted to Arctic conditions exhibiting reduced ecological niche space. These changes will have implications for the future ecosystem functioning and the carbon cycle. In Chapter V of this thesis, we combined metagenomics and metatranscriptomics with analytical techniques to characterise the carbohydrate fraction of particulate organic matter and carbohydrate utilisation by microbes in the Atlantic waters of the Fram Strait during late summer. A high spatial heterogeneity was observed in both carbohydrates and their utilisation, which indicated patchiness in local productivity and a responsive microbial community. Carbohydrate utilisation was dominated by distinct microbial assemblages across sampling sites and consisted of populations making use of labile (communal) and more complex (specialist) substrates. We therein proposed that local biological and physical processes are important for continuing to shape the availability and utilisation of carbohydrates into the late summer. In an effort to clearly and concisely convey the main findings from this thesis in the context of its original aims, a detailed description on the current and future state of the Fram Strait and Arctic Ocean microbiome is provided in the discussion. In addition, insights and recommendations on how to apply long-read metagenomes to answer questions on microbial ecology is provided, given its fundamental importance for this thesis and its relative infancy in environmental research applications. Lastly, owing to it representing an underlying theme throughout much of the research conducted, a discussion on the ecological niche concept is provided along with a proposal for its redefinition in marine microbial ecology.