| Summary: | The pronounced warming at high latitude alters a range of physical conditions i.e. the magnitude of runoff, sea-ice extent and strength of stratification and thus affect the biological systems. As microorganisms form the living base of the pelagic food web and are the major drives of biogeochemical processing it is critical to understand their response to these changes. This Ph.D. project focuses on the smallest (<2μm) and most abundant microorganisms, heterotrophic bacterioplankton (bacteria and Archaea) and autotrophic picophytoplankton, and the factors regulating their abundance, diversity and activity in the Arctic-Subarctic Atlantic Ocean. The study covers hydrographic regimes off and around Iceland, Norway (including Svalbard) and East Greenland (60-83°N), and combines field observations and experiments during different seasons. The main aim is to elucidate the three following topics: 1) Challenges phytoplankton face related to high seasonality and low light conditions 2) Bioavailability of dissolved organic matter (DOM) to bacterial communities and their response to an increase in terrestrial loading 3) Importance of top-down control by heterotrophic nanoflagellates (HNF) on both pico-sized phytoplankton and bacteria. My study underpins that picophytoplankton are important contributors to primary production, especially during the winter-spring transition (Paper I and III) and autumn (Paper V). They boosted the growth of heterotrophic microorganisms before the onset of the diatom spring bloom in the Subarctic Atlantic (Paper I) and dominated the phytoplankton biomass in the high turbid parts of a NE Greenland fjord influenced by glacial meltwater (Paper V). Picophytoplankton were better adapted to low light conditions and demonstrated higher growth rates, than larger phytoplankton (Paper I, II, III, V). In the Polar-influenced water near Greenland, Synechococcus were negligible, while in the Atlantic influenced waters picoeukaryotes and Synechococcus were often equally abundant and the latter dominated ...
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