Temporal Variability in Surface Water pCO₂ in Adventfjorden (West Spitsbergen) With Emphasis on Physical and Biogeochemical Drivers

Abstract

Seasonal and interannual variability in surface water partial pressure of CO 2 (pCO 2 ) and air-sea CO 2 fluxes from a West Spitsbergen fjord (IsA Station, Adventfjorden) are presented, and the associated driving forces are evaluated. Marine CO 2 system data together with temperature, salinity, and nutrients, were collected at the IsA Station between March 2015 and June 2017. The surface waters were undersaturated in pCO 2 with respect to atmospheric pCO 2 all year round. The effects of biological activity (primary production/respiration) followed by thermal forcing on pCO 2 were the most important drivers on a seasonal scale. The ocean was a sink for atmospheric CO 2 with annual air-sea CO 2 fluxes of À36 ± 2 and À31 ± 2 g C·m À2 ·year À1 for 2015-2016 and 2016-2017, respectively, as estimated from the month of April. Waters of an Arctic origin dominated in 2015 and were replaced in 2016 by waters of a transformed Atlantic source. The CO 2 uptake rates over the period of Arctic origin waters were significantly higher (2 mmol C·m À2 ·day À1 ) than the rates of the Atlantic origin waters of the following year.In waters north of Svalbard, the effect of sea ice processes on surface water pCO 2 , mainly through the dissolution of calcium carbonate (CaCO 3 ) in the surface waters, dominates changes (i.e., undersaturation) in surface water pCO 2 on a 6 months' time scale (January to June, Fransson et al., 2017). Further south, in Isfjorden, the largest fjord along the West Spitsbergen coast, the amount of sea ice has decreased substantially over the last decade, both in terms of maximum sea ice cover and days of fast ice (Isaksen et al., 2016; ERICSON ET AL.4888

Seasonal dynamics of the marine CO2 system in Adventfjorden, a west Spitsbergen fjord

“…A seasonal difference in salinity of about 4 was observed at the IsA Station by Ericson et al (2018). The correspondent change in Ω Ar because of the purely thermodynamic relationship between Ω Ar and salinity is very small.…”

“…This water has rather low concentrations of TA and DIC. Measured and calculated values of 294 ± 3 and 339 ± 7 µmol kg −1 for TA and DIC, respectively, were observed in the Adventdalen (Advent valley) riverbed in 2015 (Ericson et al 2018). During the study period, sea ice was scarce in Isfjorden and typically non-existent at the IsA Station, where the water temperature remained above freezing.…”

“…which is based on a mixing line made between land runoff (DIC = 339 µmol kg −1 , S = 0; Ericson et al 2018) and wintertime TAW (DIC = 2152 µmol kg −1 , S = 34.8 as estimated from mean January to March data). This mixing line lies very close to the corresponding mixing line between ArW (i.e., DIC = 2138 µmol kg −1 , S = 34.6, also estimated for mean January to March data) and land runoff; the slopes differ with 0.2 %.…”

“…Onarheim et al (2014) showed, using the ERA-Interim reanalysis product from the European Centre for Medium-Range Weather Forecasts, that the warming north of Svalbard was especially pronounced in the winter season where the mean air temperature has increased by nearly 7°C since 1979. Meanwhile, the AW of the northward flowing WSC has warmed by 0.3°C per decade, which has resulted in reduced sea-ice cover north The question is how the west Spitsbergen fjords will respond to these changes, both in terms of their potential to take up carbon dioxide (CO 2 ) from the atmosphere, which is largely related to the biological production that drives the surface water pCO 2 variability (e.g., Fransson et al 2016;Ericson et al 2018), and in terms of their geochemical resistance to ongoing ocean acidification (e.g., Fransson et al 2015). Ocean acidification refers to the shift in the marine CO 2 system towards a lower pH and less carbonate ions (CO 3 2− ), which results from the uptake of anthropogenic CO 2 (e.g., Doney et al 2009)…”

Seasonal dynamics of carbonate chemistry, nutrients and CO2 uptake in a sub-Arctic fjord

“…This biological productivity is fuelled by oceanic, atmospheric, and terrestrial nutrient inputs that drive biogeochemical exchanges between the coastal and open ocean and enhanced burial of organic and inorganic carbon (Thomas et al, 2004;Chen and Borges, 2009). Observations in high latitude fjord and coastal regions have shown these regions to be predominantly sinks for atmospheric CO 2 , despite large regional variability (Omar et al, 2005;Else et al, 2008b;Signorini et al, 2013;Fransson et al, 2014;Evans et al, 2015;Omar et al, 2016;Yasunaka et al, 2016;Ericson et al, 2018). Mid-and low-latitude coastal regions were thought to be predominantly oceanic CO 2 sources, owing to higher water temperatures and organic matter inputs (Borges et al, 2005;Cai et al, 2006;Chavez et al, 2007;Chen and Borges, 2009).…”