| Summary: | Marine microbes are crucial for the marine food web and carbon cycle. Algae are the main source of organic matter in the oceans with algae blooms triggering reoccurring bacterial succession patterns. Bacteria can recycle nutrients from organic matter coming from land or algae, fueling regenerated primary production. Terrestrial freshwater inputs can have substantial impacts on Arctic marine microbes via import of nutrients, organic matter, and sediments, and via changing the marine hydrography. The aims of this PhD thesis are i) to summarize the current knowledge about microbial ecology in the Arctic seasonal ice zone, ii) to study the effects of terrestrial inputs from rivers and glaciers on the microbial food web over different seasons, and iii) to dive into algae-bacteria interactions with a focus on the importance of regenerated production. We provide evidence that subglacial upwelling is an important process in a tidewater glacier-influenced fjord on Svalbard in spring, a previously understudied season. Subglacial upwelling lead to increased surface water nutrients, a stratified surface layer, and brackish sea ice. At the glacier front, microbial communities were significantly different compared to a sea-ice edge reference site. Phytoplankton primary production was two orders of magnitude higher at the glacier site compared to the reference site, due to upwelling related nutrient inputs and potentially also stratification and a thinner snow cover. During the spring freshet and in summer we investigated the impacts of river runoff on bacterial and archaeal communities with detailed considerations of environmental drivers. We found significant differences between bacterial communities during the spring freshet and late summer, mainly controlled by the concentrations and properties of dissolved organic matter, nutrient concentrations, and fjord hydrography. We recreated major algal spring bloom dynamics in an experimental study with algae-bacteria co-cultures. I used the experimental data to extend a commonly ...
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