Lisa Winberg von Friesen:
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Marine nitrogen fixation (reduction of nitrogen gas to ammonia) was originally thought to take place mainly in tropical oceans but has now been detected even in parts of our northernmost ocean – the Arctic Ocean. Nitrogen is the main nutrient limiting the growth of photosynthesising organisms in the Arctic Ocean, which means that if nitrogen fixation is active in a certain location, the nitrogen it adds can enable more primary production to take place. Climate change is rapidly transforming the Arctic Ocean, where the decreasing sea ice results in more light entering this previously darker ocean. The combination of light and nutrients like nitrogen will influence what role the Arctic Ocean plays in the global climate system now and in the future.
In this PhD thesis, I investigate how much nitrogen fixation takes place in seawater, sea ice and sediment and study which diazotrophs (nitrogen fixing prokaryotes) are present in Atlantic-influenced parts of the Arctic Ocean (Eurasian Arctic Ocean). Different diazotrophs have different metabolisms where some require light and can photosynthesize (cyanobacterial diazotrophs) and others instead need organic matter for energy (non-cyanobacterial diazotrophs). It is in this thesis shown that nitrogen fixation is an active process in the Atlantic-influenced Arctic where it was previously only known from the coastal Pacific-influenced parts of the Arctic. The main diazotrophs likely responsible for nitrogen fixation are different from those in the Pacific Arctic and other oceans by being dominated by non-cyanobacterial groups. Which diazotrophs that are present sharply change when moving from temperate to polar waters and are, once inside the Arctic Ocean, influenced by glacial meltwater and season. Three key groups of non-cyanobacterial diazotrophs are identified in this study, and the nitrogen fixation activity is mainly linked to organic carbon and the availability of nutrients.
Altogether, it is here shown that nitrogen fixation is an overlooked source of nitrogen in the Atlantic-influenced Arctic Ocean, being driven by different diazotrophs than elsewhere. The distribution of diazotrophs and their nitrogen-fixing activity is likely to change and regionally increase with climate change-induced alterations of the Arctic system. The information in this thesis contributes to understanding how the Arctic ecosystem functions in present-day conditions and proposes hypotheses of how nitrogen availability may change in the future.