| Summary: | Marine microorganisms have markedly great functional and phylogenetic diversity and sustain major elemental cycles, including those of carbon and nitrogen. However, a major challenge in microbial observation is that the spatial scales of microbial biodiversity patterns and microbial activity differentially change within their physical oceanographic environment, which requires sampling across multiple scales. In this thesis, I applied a combination of metabarcoding (16S and 18S rRNA gene sequencing) and stable isotope C and N2 fixation measurements of surface ocean samples (0 - 40m) against the backdrop of chemical (dissolved inorganic nutrients, particulate organic matter) and physical (temperature, salinity, and surface currents) environmental variables in the Atlantic, Indian and the Arctic Ocean. I demonstrate how functional activity can be decoupled from phylogenetic diversity. I show that beta diversity patterns generally reflect ocean provinces and can also be used to refine oceanographic boundaries. In a pan-Arctic study, I show how microbial communities disperse and form regional and within-fjord signals, with different co-occurrence patterns between fjords with and without marine-terminating glaciers. The presented calculations of a productivity-specific length scale can help identify sample patchiness and scale sample diversity in relation to marine ecosystem structure. In order to harmonize research in meta-analyses and across global scales, we provided perspectives on best practices in method documentation. In conclusion, my work helps to better understand pelagic microbial ecosystems, taking into account the patchiness and ecosystem boundaries and their impact on productivity and food web interactions that are typically overlooked in marine microbial ecology. The presented approaches will support mapping microbiomes to relevant oceanographic scales and have potential implications for researching, observing, and monitoring marine ecosystem structures.
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