Drivers of Atmosphere-Ocean CO2 Flux in Northern Norwegian Fjords

Nerea J Aalto,Hans C Bernstein,Karley Campbell,Hans C Eilertsen

doi:10.3389/fmars.2021.692093

Nerea J Aalto, Hans C Bernstein + Show 2 more

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https://doi.org/10.3389/fmars.2021.692093

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Abstract

High-latitude fjords and continental shelves are shown to be sinks for atmospheric CO2, yet large spatial-temporal variability and poor regional coverage of sea-air CO2 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 CO2 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 CO2 flux and partial pressure of CO2 in surface water via correlation to physical oceanographic and biological measurements. The results show that the study region acts as an overall atmospheric CO2 sink throughout the year, largely due to the strong undersaturation of CO2 relative to atmospheric concentrations. Wind speed exerted the strongest influence on the instantaneous rate of sea-air CO2 exchange, while exhibiting high variability. We concluded that the northernmost fjords (Altafjord and Porsangerfjord) showed stronger potential for instantaneous CO2 uptake due to higher wind speeds. We also found that fixation of CO2 was likely a significant factor controlling ΔpCO2 from April to June, which followed phenology of spring phytoplankton blooms at each location. Decreased ΔpCO2 and the resulting sea-air CO2 flux was observed in autumn due to a combined reduction of the mixed layer with entrain of high CO2 subsurface water, damped biological activity and higher surface water temperatures. This study provides the first measurements of atmospheric CO2 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.

Highlights

High-latitude fjords and continental shelf regions are sinks for atmospheric carbon dioxide (CO2) due to prominent undersaturation in surface water partial pressure with respect to atmosphere, there exists large spatial-temporal variability as a result of heterogeneity in biogeochemical cycles and seasonal abiotic and biological processes (Takahashi et al, 2002; Bates, 2006; Chen et al, 2013; Yasunaka et al, 2016; Jones et al, 2020)
The highest pCO2 from fjord stations were observed in May, except at MS in Malangen Fjord in April (−160 ± 2 pCO2) and from Finnfjord Indre at ST22 in April −194 ± 7 pCO2, whereas the smallest pCO2 at all stations occurred in December when range between stations was −49 ± 1 to –13 ± 0.4 (Figure 1)
From our assessment we find that wind speed is the physical factor which has the greatest effect on the variability in CO2 flux between stations, followed by the magnitude of atmospheric CO2 flux

Summary

Introduction

High-latitude fjords and continental shelf regions are sinks for atmospheric carbon dioxide (CO2) due to prominent undersaturation in surface water partial pressure (pCO2) with respect to atmosphere, there exists large spatial-temporal variability as a result of heterogeneity in biogeochemical cycles and seasonal abiotic and biological processes (Takahashi et al, 2002; Bates, 2006; Chen et al, 2013; Yasunaka et al, 2016; Jones et al, 2020). The primary cause of undersaturation is complex but may be attributed to several combined processes, including: The Marine CO2 Sink in Northern Norway (i) intense summer drawdown by phytoplankton primary production (PP) and subsequent vertical export of organic matter to the benthos, (ii) horizontal export of CO2 as dissolved inorganic carbon with local ocean circulation patterns, and (iii) atmospheric cooling of surface waters in winter that increase CO2 solubility and associated disequilibrium of the water with the atmospheric CO2 (Tsunogai et al, 1999; Thomas et al, 2004; Bates, 2006). Wind speed has a critical role controlling instantaneous sea-air exchanges of CO2 because it is used as a function of gas transfer velocity and can cause considerable temporal and spatial variability (Sejr et al, 2011; Chen et al, 2013; Wanninkhof, 2014; Ericson et al, 2018)

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