Drivers of Atmosphere-Ocean CO2 Flux in Northern Norwegian Fjords

High-latitude fjords and continental shelves are shown to be sinks for atmospheric CO 2 , yet large spatial-temporal variability and poor regional coverage of sea-air CO 2 flux data, especially from fjord systems, makes it difficult to scale our knowledge on how they contribute to atmospheric carbon regulation. The magnitude and seasonal variability of atmosphere-sea CO 2 flux was investigated in high-latitude northern Norwegian coastal areas over 2018 and 2019, including four fjords and one coastal bay. The aim was to assess the physical and biogeochemical factors controlling CO 2 flux and partial pressure of CO 2 in surface water via correlation to physical oceanographic and biological measurements. The results show that the study region acts as an overall atmospheric CO 2 sink throughout the year, largely due to the strong undersaturation of CO 2 relative to atmospheric concentrations. Wind speed exerted the strongest influence on the instantaneous rate of sea-air CO 2 exchange, while exhibiting high variability. We concluded that the northernmost fjords (Altafjord and Porsangerfjord) showed stronger potential for instantaneous CO 2 uptake due to higher wind speeds. We also found that fixation of CO 2 was likely a significant factor controlling ΔpCO 2 from April to June, which followed phenology of spring phytoplankton blooms at each location. Decreased ΔpCO 2 and the resulting sea-air CO 2 flux was observed in autumn due to a combined reduction of the mixed layer with entrain of high CO 2 subsurface water, damped biological activity and higher surface water temperatures. This study provides the first measurements of atmospheric CO 2 flux in these fjord systems and therefore an important new baseline for gaining a better understanding on how the northern Norwegian coast and characteristic fjord systems participate in atmosphere carbon regulation.