Seasonal nutrient depletion and carbon export in the Subantarctic and Polar Frontal zones of the Southern Ocean south of Australia

Abstract. The Subantarctic and Polar Frontal zones (SAZ and PFZ) represent a large portion of the total area of the Southern Ocean and serve as a strong sink for atmospheric CO2. These regions are central to hypotheses linking particle fluxes and climate change, yet multi-year records of modern flux and the organisms that control it are, for obvious reasons, rare. In this study, we examine two sediment trap records of the flux of diatoms and bulk components collected by two bottom-tethered sediment traps deployed at mesopelagic depths (~ 1 km) in the SAZ (two-year record) and in the PFZ (six-year record) along the 140° E meridian. These traps provide a direct measure of transfer below winter mixed layer depths, i.e. at depths where effective sequestration from the atmosphere occurs, in contrast to study of processes in the surface ocean. Total mass fluxes were about two-fold higher in the PFZ (24 ± 13 g m−2 yr−1) than in the SAZ (14 ± 2 g m−2 yr−1). Bulk chemical composition of the particle fluxes mirrored the composition of the distinct plankton communities of the surface layer, being dominated by carbonate in the SAZ and by biogenic silica in the PFZ. POC export was similar for the annual average at both sites (1.0 ± 0.1 and 0.8 ± 0.4 g m−2 yr−1, for the PFZ and SAZ, respectively), indicating that the particles in the SAZ were relatively POC rich. Seasonality in the particle export was more pronounced in the PFZ. Peak fluxes occurred during summer in the PFZ and during spring in the SAZ. The strong summer pulses in the PFZ are responsible for a large fraction of the variability in carbon sequestration from the atmosphere in this region. The latitudinal variation of the total diatom flux was found to be in line with the biogenic silica export with an annual flux of 31 ± 5.5 × 108 valves m−2 yr−1 at the PFZ compared to 0.5 ± 0.4 × 108 m−2 yr−1 of the SAZ. Fragilariopsis kerguelensis dominated the annual diatom export at both sites (43 at the SAZ and 59% in the PFZ). POC fluxes displayed a strong positive correlation with the relative contribution of a group of weakly-silicified and bloom-forming species in the PFZ. Several lines of evidence suggest that the development of these species during the growth season facilitates the formation of aggregates and carbon export. Our results confirm previous work suggesting that F. kerguelensis is a major aspect of the decoupling of the carbon and silicon cycles in the high-nutrient low-chlorophyll waters of the Southern Ocean.

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

Biogeosciences ◽

10.5194/bg-8-227-2011 ◽

2011 ◽

Vol 8 (2) ◽

pp. 227-237 ◽

Cited By ~ 57

Author(s):

N. Cassar ◽

P. J. DiFiore ◽

B. A. Barnett ◽

M. L. Bender ◽

A. R. Bowie ◽

...

Keyword(s):

Southern Ocean ◽

Light Availability ◽

Austral Summer ◽

Ocean Mixed Layer ◽

Carbon Export ◽

Net Community Production ◽

Independent Evidence ◽

Frontal Zones ◽

Mixed Layers ◽

Community Production

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 O2 supersaturation was derived from measuring O2/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 O2 m−2 d−1. 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.

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Latitudinal and temporal distributions of diatom populations in the pelagic waters of the Subantarctic and Polar Frontal zones of the Southern Ocean and their role in the biological pump

Biogeosciences ◽

10.5194/bg-12-5309-2015 ◽

2015 ◽

Vol 12 (18) ◽

pp. 5309-5337 ◽

Cited By ~ 23

Author(s):

A. S. Rigual-Hernández ◽

T. W. Trull ◽

S. G. Bray ◽

A. Cortina ◽

L. K. Armand

Keyword(s):

Southern Ocean ◽

Biogenic Silica ◽

Large Fraction ◽

Sediment Traps ◽

Strong Positive Correlation ◽

Carbon Export ◽

Relative Contribution ◽

Frontal Zones ◽

Particle Fluxes ◽

Particle Export

Abstract. The Subantarctic and Polar Frontal zones (SAZ and PFZ) represent a large portion of the total area of the Southern Ocean and serve as a strong sink for atmospheric CO2. These regions are central to hypotheses linking particle fluxes and climate change, yet multi-year records of modern flux and the organisms that control it are, for obvious reasons, rare. In this study, we examine two sediment trap records of the flux of diatoms and bulk components collected by two bottom-tethered sediment traps deployed at mesopelagic depths (~ 1 km) in the SAZ (2-year record; July 1999–October 2001) and in the PFZ (6-year record; September 1997–February 1998, July 1999–August 2000, November 2002–October 2004 and December 2005–October 2007) along the 140° E meridian. These traps provide a direct measure of transfer below winter mixed layer depths, i.e. at depths where effective sequestration from the atmosphere occurs, in contrast to study of processes in the surface ocean. Total mass fluxes were about twofold higher in the PFZ (24 ± 13 g m−2 yr−1) than in the SAZ (14 ± 2 g m−2 yr−1). Bulk chemical composition of the particle fluxes mirrored the composition of the distinct plankton communities of the surface layer, being dominated by carbonate in the SAZ and by biogenic silica in the PFZ. Particulate organic carbon (POC) export was similar for the annual average at both sites (1.0 ± 0.1 and 0.8 ± 0.4 g m−2 yr−1 for the PFZ and SAZ, respectively), indicating that the particles in the SAZ were relatively POC rich. Seasonality in the particle export was more pronounced in the PFZ. Peak fluxes occurred during summer in the PFZ and during spring in the SAZ. The strong summer pulses in the PFZ are responsible for a large fraction of the variability in carbon sequestration from the atmosphere in this region. The latitudinal variation of the total diatom flux was found to be in line with the biogenic silica export with an annual flux of 31 ± 5.5 × 108 valves m−2 yr−1 at the PFZ compared to 0.5 ± 0.4 × 108 m−2 yr−1 at the SAZ. Fragilariopsis kerguelensis dominated the annual diatom export at both sites (43 % at the SAZ and 59 % in the PFZ). POC fluxes displayed a strong positive correlation with the relative contribution of a group of weakly silicified and bloom-forming species in the PFZ. Several lines of evidence suggests that the development of these species during the growth season facilitates the formation of aggregates and carbon export. Our results confirm previous work suggesting that F. kerguelensis plays a major role in the decoupling of the carbon and silicon cycles in the high-nutrient low-chlorophyll waters of the Southern Ocean.

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

Biogeosciences Discussions ◽

10.5194/bgd-7-5649-2010 ◽

2010 ◽

Vol 7 (4) ◽

pp. 5649-5674 ◽

Cited By ~ 3

Author(s):

N. Cassar ◽

P. J. DiFiore ◽

B. A. Barnett ◽

M. L. Bender ◽

A. R. Bowie ◽

...

Keyword(s):

Southern Ocean ◽

Primary Productivity ◽

Light Availability ◽

Austral Summer ◽

Carbon Export ◽

Net Community Production ◽

Independent Evidence ◽

Frontal Zones ◽

Mixed Layers ◽

Community Production

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 O2 supersaturation was derived from measuring O2/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 O2 m−2 d−1. Both gross primary productivity (estimated from the oxygen triple isotope anomaly) and 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. 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.

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Moored sediment trap measurements of carbon export in the Subantarctic and Polar Frontal zones of the Southern Ocean, south of Australia

Journal of Geophysical Research Atmospheres ◽

10.1029/2000jc000308 ◽

2001 ◽

Vol 106 (C12) ◽

pp. 31489-31509 ◽

Cited By ~ 124

Author(s):

T. W. Trull ◽

S. G. Bray ◽

S. J. Manganini ◽

S. Honjo ◽

R. François

Keyword(s):

Southern Ocean ◽

Sediment Trap ◽

Carbon Export ◽

Frontal Zones

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Carbon export associated with free-drifting icebergs in the Southern Ocean

Deep Sea Research Part II Topical Studies in Oceanography ◽

10.1016/j.dsr2.2010.11.027 ◽

2011 ◽

Vol 58 (11-12) ◽

pp. 1485-1496 ◽

Cited By ~ 27

Author(s):

K.L. Smith ◽

A.D. Sherman ◽

T.J. Shaw ◽

A.E. Murray ◽

M. Vernet ◽

...

Keyword(s):

Southern Ocean ◽

Carbon Export

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Phytoplankton biomass and community composition in the frontal zones of Southern Ocean

Deep Sea Research Part II Topical Studies in Oceanography ◽

10.1016/j.dsr2.2020.104799 ◽

2020 ◽

Vol 178 ◽

pp. 104799 ◽

Cited By ~ 1

Author(s):

R.K. Mishra ◽

R.K. Naik ◽

V. Venkataramana ◽

Babula Jena ◽

N. AnilKumar ◽

...

Keyword(s):

Southern Ocean ◽

Community Composition ◽

Phytoplankton Biomass ◽

Frontal Zones

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

Biogeosciences ◽

10.5194/bg-11-3279-2014 ◽

2014 ◽

Vol 11 (12) ◽

pp. 3279-3297 ◽

Cited By ~ 7

Author(s):

C.-H. Chang ◽

N. C. Johnson ◽

N. Cassar

Keyword(s):

Neural Network ◽

Organic Carbon ◽

Southern Ocean ◽

Mixed Layer ◽

Sea Surface ◽

Carbon Export ◽

Net Community Production ◽

Basin Scale ◽

Community Production

Abstract. Southern Ocean organic carbon export plays an important role in the global carbon cycle, yet its basin-scale climatology and variability are uncertain due to limited coverage of in situ observations. In this study, a neural network approach based on the self-organizing map (SOM) is adopted to construct weekly gridded (1° × 1°) maps of organic carbon export for the Southern Ocean from 1998 to 2009. The SOM is trained with in situ measurements of O2 / Ar-derived net community production (NCP) that are tightly linked to the carbon export in the mixed layer on timescales of one to two weeks and with six potential NCP predictors: photosynthetically available radiation (PAR), particulate organic carbon (POC), chlorophyll (Chl), sea surface temperature (SST), sea surface height (SSH), and mixed layer depth (MLD). This nonparametric approach is based entirely on the observed statistical relationships between NCP and the predictors and, therefore, is strongly constrained by observations. A thorough cross-validation yields three retained NCP predictors, Chl, PAR, and MLD. Our constructed NCP is further validated by good agreement with previously published, independent in situ derived NCP of weekly or longer temporal resolution through real-time and climatological comparisons at various sampling sites. The resulting November–March NCP climatology reveals a pronounced zonal band of high NCP roughly following the Subtropical Front in the Atlantic, Indian, and western Pacific sectors, and turns southeastward shortly after the dateline. Other regions of elevated NCP include the upwelling zones off Chile and Namibia, the Patagonian Shelf, the Antarctic coast, and areas surrounding the Islands of Kerguelen, South Georgia, and Crozet. This basin-scale NCP climatology closely resembles that of the satellite POC field and observed air–sea CO2 flux. The long-term mean area-integrated NCP south of 50° S from our dataset, 17.9 mmol C m−2 d−1, falls within the range of 8.3 to 24 mmol C m−2 d−1 from other model estimates. A broad agreement is found in the basin-wide NCP climatology among various models but with significant spatial variations, particularly in the Patagonian Shelf. Our approach provides a comprehensive view of the Southern Ocean NCP climatology and a potential opportunity to further investigate interannual and intraseasonal variability.

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