Fluxes of carbon and nutrients to the Iceland Sea surface layer and inferred primary productivity and stoichiometry

Abstract

Abstract. This study evaluates long-term mean fluxes of carbon and nutrients to the upper 100 m of the Iceland Sea. The study utilises hydro-chemical data from the Iceland Sea time series station (68.00 • N, 12.67 • W), for the years between 1993 and 2006. By comparing data of dissolved inorganic carbon (DIC) and nutrients in the surface layer (upper 100 m), and a sub-surface layer (100-200 m), we calculate monthly deficits in the surface, and use these to deduce the long-term mean surface layer fluxes that affect the deficits: vertical mixing, horizontal advection, air-sea exchange, and biological activity. The deficits show a clear seasonality with a minimum in winter, when the mixed layer is at the deepest, and a maximum in early autumn, when biological uptake has removed much of the nutrients. The annual vertical fluxes of DIC and nitrate amounts to 2.9 ± 0.5 and 0.45 ± 0.09 mol m −2 yr −1 , respectively, and the annual airsea uptake of atmospheric CO 2 is 4.4 ± 1.1 mol C m −2 yr −1 . The biologically driven changes in DIC during the year relates to net community production (NCP), and the net annual NCP corresponds to export production, and is here calculated as 7.3 ± 1.0 mol C m −2 yr −1 . The typical, median C : N ratio during the period of net community uptake is 9.0, and clearly higher than the Redfield ratio, but is varying during the season.

Links between surface productivity and deep ocean particle flux at the Porcupine Abyssal Plain sustained observatory

“…The MLD-integrated seasonal new production (from February to August) was 0.37 ± 0.14 mol N m −2 . This is within the range of new production estimates, using different approaches, from 0.23-1.1 mol N m −2 from the Northeast Atlantic Ocean and Icelandic Sea (Fernández I et al, 2005;Hartman et al, 2010;Jeansson et al, 2015).…”

Contrasting drivers and trends of ocean acidification in the subarctic Atlantic

“…Here, this criterion is adopted for both the IS-TS and IRM-TS to define the surface layer. The gradients of T, S and nutrients show a clear attenuation at 300 metres 39 , which allows us to define the subsurface layer from the MLD to a depth of 300 m. Between 300 and 600 m, there is a transition zone with a small vertical gradient 31 , which is considered here as an interme- www.nature.com/scientificreports/ diate layer above the very homogeneous deep layer (600-1900 m). To make a comparative study, the same criteria have been maintained for the IRM-TS, although its oceanographic conditions are different.…”

“…Following the methodology applied at the IS-TS by Jeansson et al 39 , profiles were interpolated for the chosen depths: (i) every 10 m in the upper 300 m, (ii) every 50 m between 300 and 500 m, and (iii) every 100 m from 500 to the bottom. For determining the mixed layer depth (MLD), Jeansson et al 39 selected the density difference criterion of 0.05 kg/m 3 after testing several criteria based on differences in temperature and density. This criterion is consistent with early studies 31,64 .…”

Links between surface productivity and deep ocean particle flux at the Porcupine Abyssal Plain (PAP) sustained observatory

“…Studies have shown an increasing C : N of sinking material due to preferential remineralization of nutrients (Schneider et al, 2003;Lee and Cronin, 1984), and the potential feedback of a depth-dependent C : N ratio may influence atmospheric CO 2 concentrations by about 20 ppm (Schneider et al, 2004). However, the deep ocean remineralization ratio of C : N has been shown to be close to the Redfield ratio (Anderson and Sarmiento, 1994), and if the C overconsumption is mainly during summer (Koeve, 2004;Jiang et al, 2013) and remineralized above the depth of the winter mixed layer, it could be questioned whether the "extra-Redfield" C is sequestered in the deeper ocean, and can therefore influence the oceanic C-budget on longer timescales (Koeve, 2006). However, the deep ocean remineralization rates of Anderson and Sarmiento (1994) did not include the Atlantic Ocean.…”