| Summary: | The Arctic Ocean is a unique marine environment with respect to irradiation periodicity and intensity, temperature, ice formation and strong stratification. Changes due to global warming might be more pronounced in this system compared to other oceans, as the melting of both sea ice and permafrost will accompany high nutrient input to the Arctic Ocean simultaneous with increasing light penetration of the water column. Microbial communities play an important role in carbon cycling in the ocean, as they constitute both the main primary producers (phytoplankton) and the main consumers of dissolved organic carbon (heterotrophic bacteria) which make dissolved organic carbon available for higher trophic levels. Increasing atmospheric carbon dioxide (CO2) might also have a positive effect on the production of dissolved organic carbon in the oceans. This may in particular impact heterotrophic bacteria and, thereby, the trophic state of the ocean. In this thesis, in situ investigations of the microbial community and the trophic state of a representative Arctic marine system are presented (Paper I) as well as nutrient manipulation experiments performed in mesocosms in the Arctic (Papers II, III and IV) and in the laboratory (Paper V). In the in situ study performed in Fram Strait (Paper I), the microbial community demonstrated net-autotrophy although the microbial biomass was dominated by heterotrophs. In three nutrient manipulation experiments we studied the effects of (i) increased organic carbon load on mineral nutrient competition between bacteria and phytoplankton (Paper II), (ii) increased organic carbon load on the bacterial community structure (Paper III), (iii) increased partial pressure of CO2 (pCO2) on bacterial community shifts in response to increasing organic carbon load (Paper IV) and (iv) carbon complexity and viral lysis on the bacterial community structure and diversity (Paper V). All experiments showed that the bacterial community was affected by increasing nutrient loads. During the mesocosm experiment ...
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