| Summary: | The European Arctic is characterized by large surface areas of coastal seas and long coastlines where important ecosystem processes are regulated by marine microbiomes that contribute to global carbon cycling via primary productivity and atmospheric CO 2 drawdown. In addition to biogeochemical cycling, these complex microbial ecosystems also support major marine food webs and lend themselves to marine bioprospecting for novel biotechnologies. However, they are not well understood and remain unpredictable. For example, northern coastal Norway is periodically affected by unwanted ecosystem functions such as sporadic harmful algae blooms that are detrimental to local aquaculture industry. These cold adapted marine microbiomes perform ecological processes that are often driven by phytoplankton and their associated heterotrophic communities which undergo strong seasonal variation within complex and variable oceanographical and biogeochemical conditions. Climate change driven temperature increases with resulting direct and undirect changes on environmental conditions have been reported to alter microbial community structures of surface microbiomes. Hence, these important microbial ecosystems are in transition, and it is difficult to predict the trajectory of how specific ecosystem functions may be changing. This thesis is centered around cold-water communities of phytoplankton and bacterioplankton with the aim to enhance our contemporary understanding on surface microbial ecosystems and their function with respect to carbon cycling, community dynamics and community selection. Paper I and Paper II took place in northern Norwegian fjords and costal systems where spatial variation and magnitude of atmospheric CO 2 uptake were investigated and the main physical and biological factors driving surface partial pressure of CO 2 with respect to atmospheric CO 2 were assessed in four fjords and a coastal bay through seasonal changes. The results indicate that northern Norwegian fjords free of sea-ice act as a sink for ...
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