The influence of iron and light on net community production in the Subantarctic and Polar Frontal Zones

Abstract

The roles of iron and light in controlling biomass and primary productivity are clearly established in the Southern Ocean. However, their influence on net community production (NCP) and carbon export remains to be quantified. To improve our understanding of NCP and carbon export production in the Subantarctic Zone (SAZ) and the northern reaches of the Polar Frontal Zone (PFZ), we conducted continuous onboard determinations of NCP as part of the Sub-Antarctic Sensitivity to Environmental Change (SAZ-Sense) study, which occurred in January–February 2007. Biological O<sub>2</sub> supersaturation was derived from measuring O<sub>2</sub>/Ar ratios by equilibrator inlet mass spectrometry. Based on these continuous measurements, NCP during the austral summer 2007 in the Australian SAZ was approximately 43 mmol O<sub>2</sub> m<sup>−2</sup> d<sup>−1</sup>. NCP showed significant spatial variability, with larger values near the Subtropical front, and a general southward decrease. For shallower mixed layers (<50 m), dissolved Fe concentrations and Fe sufficiency, estimated from variable fluorescence, correlated strongly with NCP. The strong correlation between NCP and dissolved Fe may be difficult to interpret because of the correlation of dissolved Fe to MLD and because the concentration of iron may not be a good indicator of its availability. At stations with deeper mixed layers, NCP was consistently low, regardless of iron sufficiency, consistent with light availability also being an important control of NCP. Our new observations provide independent evidence for the critical roles of iron and light in mediating carbon export from the Southern Ocean mixed layer

Evaluating Southern Ocean biological production in two ocean biogeochemical models on daily to seasonal timescales using satellite chlorophyll and O<sub>2</sub> / Ar observations

“…• S since a large number of O 2 /Ar measurements have been collected from this area (Huang et al, 2012;Reuer et al, 2007;Cassar et al, 2011). While satellite-derived Chl is a well-known and widely used property, O 2 /Ar based NCP will require a more detailed introduction.…”

“…We use these sampling clusters in our analysis as natural areas to compare and contrast different parts of the Southern Ocean. A more detailed description of the sampling strategies, measurement methods, and data sources can be found in Reuer et al (2007) and Cassar et al (2009Cassar et al ( , 2011.…”

Ecosystem metabolism in salt marsh tidal creeks and ponds : applying triple oxygen isotopes and other gas tracers to novel environments

“…, Emerson et al 1991. Extending the equations by Cassar et al [2011] such that argon is not assumed to be at saturation and neglecting horizontal and vertical mixing, the time evolution of oxygen relative to argon in the mixed layer is controlled by production and gas exchange:…”

Neural network-based estimates of Southern Ocean net community production from in situ O<sub>2</sub> / Ar and satellite observation: a methodological study

“…In addition, NCP is highly variable in space and time and cannot be derived from linear interpolation between in situ measurements. Field experiments also reveal that the plankton ecosystem and CO 2 flux variability are not dominated by just one single mechanism but by a confluence of several processes that shift in relative importance over time and space (Banse, 1996;Abbott et al, 2000Abbott et al, , 2001Cassar et al, 2011;Tortell et al, 2012), which are difficult to capture in biogeochemical models.…”

“…The O 2 / Ar method measures biological O 2 supersaturation in the mixed layer (Craig and Hayward, 1987) and yields NCP estimates over the O 2 residence timescale (one to two weeks) (Reuer et al, 2007;Cassar et al, 2007Cassar et al, , 2009Cassar et al, , 2011. On this timescale, the NCP derived from this method is tightly linked to the export of organic carbon from the mixed layer at steady state, under the assumptions that both vertical mixing of O 2 -depleted waters from below and accumulation of POC and DOC in the mixed layer are negligible (Cassar et al, 2009(Cassar et al, , 2011Jonsson et al, 2013). Although we use NCP and carbon export production interchangeably in this study, it should be noted that under some circumstances, the assumption of steady state is violated (Hamme et al, 2012;Jonsson et al, 2013).…”