Subantarctic Zone Research Articles

Decadal evolution of carbon sink within a strong bloom area in the subantarctic zone

[1] The fate of the Southern Ocean atmospheric CO2 sink is under question. Here we assess seasonal to decadal changes of surface fCO2 within an extended sink area along the track between Kerguelen and Amsterdam islands in the subantarctic zone. Data from 17 oceanographic cruises were used, from 1991 to 2011 and two distinct regions were examined, separated by the Subantarctic Front (SAF). The region south of the SAF displays a strong summer phytoplankton bloom of up to −28 mmol C m−2 d−1within a calm area, constrained by physics and topography. On an annual basis, this region is a 6-fold more important sink than that deduced from Takahashi climatology, highlighting the importance of key-areas separate examination before proceeding to spatial integration. Our data point towards a decadal decline of the CO2 sink in the Southern part of the SAF, most probably due to both warming and less Fe input to surface waters from reduced water mixing.

Sphaeronectes pughi sp. nov., a new species of sphaeronectid calycophoran siphonophore from the subantarctic zone

A new species of sphaeronectid calycophoran siphonophore, Sphaeronectes pughi sp. nov. is described from material collected in the subantarctic zone, south of Australia. An identification key for sphaeronectid nectophores is provided, with illustrations of the eleven recognized species.

Anticyclonic and cyclonic eddies of subtropical origin in the subantarctic zone south of Africa

Two eddies, one anticyclonic and the other cyclonic, intersected in the Subantarctic Zone south of South Africa during a hydrographic transect, are described using a large set of measurements including full depth hydrography, Acoustic Doppler Current Profiler velocities, biogeochemical tracers, air‐sea fluxes and altimetric sea surface height. Both eddies have a subtropical origin. The anticyclone is an Agulhas ring with convected core water of ∼12°C, and swirl velocities of 1 m s−1. It was 9.5 months old when sampled and had crossed the Agulhas Ridge. Though sampled in summer, it was releasing ∼200 W m−2 (sensible plus latent heat flux) to the atmosphere. It was observed adjacent to the Subantarctic Front, illustrating the usual encounters of such structures with this front. The cyclone, marked by pronounced low oxygen and CFC anomalies revealing an origin at the continental slope, was 4.5 months old. It had swirl speeds of 0.3 m s−1, and was coupled with the anticyclone when observed. From their kinematics and water mass properties both structures were found to transport subtropical water down to ∼900 m, the water trapped below this depth being either from the northern Subantarctic Zone, or local water. The two structures illustrate the capacity of eddies in the region to transfer subtropical and alongslope water properties into the Subantarctic Zone.

Nitrogen uptake by phytoplankton in the Atlantic sector of the Southern Ocean during late austral summer

Abstract. As part of the Bonus-GoodHope (BGH) campaign, 15N-labelled nitrate, ammonium and urea uptake measurements were made along the BGH transect from Cape Town to ~60° S in late austral summer, 2008. Our results are categorised according to distinct hydrographic regions defined by oceanic fronts and open ocean zones. High regenerated nitrate uptake rate in the oligotrophic Subtropical Zone (STZ) resulted in low f-ratios (f = 0.2) with nitrogen uptake being dominated by ρurea, which contributed up to 70 % of total nitrogen uptake. Size fractionated chlorophyll data showed that the greatest contribution (>50 %) of picophytoplankton (<2 μm) were found in the STZ, consistent with a community based on regenerated production. The Subantarctic Zone (SAZ) showed the greatest total integrated nitrogen uptake (10.3 mmol m−2 d−1), mainly due to enhanced nutrient supply within an anticyclonic eddy observed in this region. A decrease in the contribution of smaller size classes to the phytoplankton community was observed with increasing latitude, concurrent with a decrease in the contribution of regenerated production. Higher f-ratios observed in the SAZ (f = 0.49), Polar Frontal Zone (f= 0.41) and Antarctic Zone (f = 0.45) relative to the STZ (f = 0.24), indicate a higher contribution of NO3−-uptake relative to total nitrogen and potentially higher export production. High ambient regenerated nutrient concentrations are indicative of active regeneration processes throughout the transect and ascribed to late summer season sampling. Higher depth integrated uptake rates also correspond with higher surface iron concentrations. No clear correlation was observed between carbon export estimates derived from new production and 234Th flux. In addition, export derived from 15N estimates were 2–20 times greater than those based on 234Th flux. Variability in the magnitude of export is likely due to intrinsically different methods, compounded by differences in integration time scales for the two proxies of carbon export.

Extraordinarily high biomass benthic community on Southern Ocean seamounts

We describe a previously unknown assemblage of seamount-associated megabenthos that has by far the highest peak biomass reported in the deep-sea outside of vent communities. The assemblage was found at depths of 2–2.5 km on rocky geomorphic features off the southeast coast of Australia, in an area near the Sub-Antarctic Zone characterised by high rates of surface productivity and carbon export to the deep-ocean. These conditions, and the taxa in the assemblage, are widely distributed around the Southern mid-latitudes, suggesting the high-biomass assemblage is also likely to be widespread. The role of this assemblage in regional ecosystem and carbon dynamics and its sensitivities to anthropogenic impacts are unknown. The discovery highlights the lack of information on deep-sea biota worldwide and the potential for unanticipated impacts of deep-sea exploitation.

Efficient silicon recycling in summer in both the Polar Frontal and Subantarctic Zones of the Southern Ocean

We measured biogenic silica (bSiO(2)) dissolution and production rates at 3 Southern Ocean sites with contrasting biogeochemical regimes (SAZ-Sense cruise, January to February 2007). Two sites were located in the Subantarctic Zone (SAZ) southeast and southwest of Tasmania, and 1 site was in the Polar Frontal Zone (PFZ). The measurements were repeated 3 times within a time frame of 4 to 7 d. In the PFZ and the western SAZ, the release of silicon from dissolving bSiO(2), following a productive period, appeared sufficient to entirely sustain Si demand in surface waters. Biogenic silica dissolution was more efficient in the SAZ, likely owing to stronger microzooplankton grazing and bacterial activity. The degree of iron co-limitation correlated to bSiO(2) growth rates (V-Si, d(-1)). Highest rates were observed in the SAZ, southeast of Tasmania, a region influenced by iron-enriched East Australian Current water advecting through the Subtropical Front. The diatom communities appeared differently adapted and conditioned, in terms of uptake ability, for growth under low Si(OH) 4 availability. Combining our results with existing bSiO(2) dissolution and production data, we estimate a spring to summer net bSiO(2) production for the circumpolar SAZ and PFZ regions of 7.4 and 3.6 Tmol yr(-1), respectively, representing 4.3 and 2.1% of the global net bSiO(2) production for each region. Furthermore, a clear seasonal pattern emerges with (1) higher bSiO(2) production in spring (0.9 to 12.7 and 6.8 to 60.7 mmol Si m(-2) d(-1) for the SAZ and PFZ, respectively) compared to summer (0.1 to 6.6 and 0.3 to 9.1 mmol Si m(-2) d(-1) for the SAZ and PFZ, respectively) and (2) a bSiO(2) dissolution to production ratio of 1 in summer (similar to 1.7).

Southern Ocean dust–climate coupling over the past four million years

Dust has the potential to modify global climate by influencing the radiative balance of the atmosphere and by supplying iron and other essential limiting micronutrients to the ocean. Indeed, dust supply to the Southern Ocean increases during ice ages, and 'iron fertilization' of the subantarctic zone may have contributed up to 40 parts per million by volume (p.p.m.v.) of the decrease (80-100 p.p.m.v.) in atmospheric carbon dioxide observed during late Pleistocene glacial cycles. So far, however, the magnitude of Southern Ocean dust deposition in earlier times and its role in the development and evolution of Pleistocene glacial cycles have remained unclear. Here we report a high-resolution record of dust and iron supply to the Southern Ocean over the past four million years, derived from the analysis of marine sediments from ODP Site 1090, located in the Atlantic sector of the subantarctic zone. The close correspondence of our dust and iron deposition records with Antarctic ice core reconstructions of dust flux covering the past 800,000 years (refs 8, 9) indicates that both of these archives record large-scale deposition changes that should apply to most of the Southern Ocean, validating previous interpretations of the ice core data. The extension of the record beyond the interval covered by the Antarctic ice cores reveals that, in contrast to the relatively gradual intensification of glacial cycles over the past three million years, Southern Ocean dust and iron flux rose sharply at the Mid-Pleistocene climatic transition around 1.25 million years ago. This finding complements previous observations over late Pleistocene glacial cycles, providing new evidence of a tight connection between high dust input to the Southern Ocean and the emergence of the deep glaciations that characterize the past one million years of Earth history.

Role of the seasonal cycle in coupling climate and carbon cycling in the subantarctic zone

Workshop on the Seasonal Cycle of the Carbon‐Climate System in the Southern Ocean; Cape Town, South Africa, 23–25 August 2010; There is increasing evidence in the Southern Ocean that mesoscales and seasonal scales play an important role in the coupling of ocean carbon cycling and climate. The seasonal cycle is one of the strongest modes of variability in different components of the carbon cycle in the Southern Ocean. It is also the mode that couples climate forcing to ecosystem responses such as productivity and ultimately biogeochemical signals including carbon export. However, not only are these scales of coupling poorly understood, but also there appear to be important regional differences in the way they couple climate to carbon. With this as an overarching theme, a workshop in South Africa brought together scientists working in the Southern Ocean, the waters south of Australia, New Zealand, and South Africa. The importance of the Subantarctic Zone (SAZ) as a carbon sink made it an ideal system on which to focus the workshop.

Simulation of chlorophyll and iron supplies in the Sub Antarctic Zone South of Australia

The Sub Antarctic Zone (SAZ) is part of the high nutrient low chlorophyll region of the Southern Ocean where low iron availability limits phytoplankton productivity. In the Tasman Sea, the SAZ displays a unique and large zonal gradient in the surface chlorophyll concentrations. In situ measurements show higher dissolved iron in the east than in the west which could explain the zonal chlorophyll gradient. To assess and quantify the major sources of iron in this region, we use an eddy-resolving model to simulate the dissolved Fe distribution arising from both the shelf and the dust supply of iron. The simulation showed that dissolved Fe from the shelf is only transported eastward to about 155°E and the shelf source of dissolved iron is unable to supply iron to the entire Tasman Sea SAZ region even when we assume that there no iron losses. The simulation with the atmospheric source of iron showed that through a combination of atmospheric transport and ocean transport, enough dissolved Fe can be supplied to the entire Tasman Sea SAZ region to relieve iron limitation. From the model simulations, we conclude that atmospheric iron supply is essential to generating the observed gradient of the chlorophyll in the SAZ region of the Tasman Sea.

Heat budget of the surface mixed layer south of Africa

ARGO hydrographic profiles, two hydrographic transects and satellite measurements of air–sea exchange parameters were used to characterize the properties and seasonal heat budget variations of the Surface Mixed Layer (SML) south of Africa. The analysis distinguishes the Subtropical domain (STZ) and the Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ) and Antarctic Zone (AZ) of the Antarctic Circumpolar Current. While no Subantarctic Mode Water forms in that region, occurrences of deep SML (up to ∼450 m) are observed in the SAZ in anticyclones detached from the Agulhas Current retroflection or Agulhas Return Current. These are present latitudinally throughout the SAZ, but preferentially at longitudes 10–20° E where, according to previous results, the Subtropical Front is interrupted. Likely owing to this exchange window and to transfers at the Subantarctic Front also enhanced by the anticyclones, the SAZ shows a wide range of properties largely encroaching upon those of the neighbouring domains. Heat budget computations in each zone reveal significant meridional changes of regime. While air–sea heat fluxes dictate the heat budget seasonal variability everywhere, heat is mostly brought through lateral geostrophic advection by the Agulhas Current in the STZ, through lateral diffusion in the SAZ and through air–sea fluxes in the PFZ and AZ. The cooling contributions are by Ekman advection everywhere, lateral diffusion in the STZ (also favoured by the ∼10° breach in the Subtropical Front) and geostrophic advection in the SAZ. The latter likely reflects an eastward draining of water warmed through mixing of the subtropical eddies.

Distribution, abundance and seasonal flux of pteropods in the Sub-Antarctic Zone

Pteropods were identified from epipelagic net and trawl samples in the Sub-Antarctic Zone during the 2007 mid-summer (January 17–February 20) Sub-Antarctic Zone Sensitivity to Environmental Change (SAZ-Sense) voyage, as well as in a moored sediment trap in the same region. Overall pteropod densities during SAZ-Sense were lower than those reported for higher-latitude Southern Ocean waters. The four major contributors to the Sub-Antarctic Zone pteropod community during the SAZ-Sense voyage, Clio pyramidata forma antarctica, Clio recurva, Limacina helicina antarctica and Limacina retroversa australis, accounted for 93% of all pteropods observed. The distribution of the two dominant pteropods collected in the Sub-Antarctic Zone, L. retroversa australis and C. pyramidata forma antarctica, is strongly related to latitude and depth. L. retroversa australis is typical of cold southern (50–54°S) polar waters and C. pyramidata forma antarctica is typical of shallow (top 20 m) Sub-Antarctic Zone waters. A moored sediment trap deployed to 2100 m at 47°S, 141°E in 2003/04 showed the pteropod flux in the Sub-Antarctic Zone had late-Spring and mid-summer peaks. The diversity, abundance and distribution of pteropods collected during SAZ-Sense provide a timely benchmark against which to monitor future changes in SAZ ocean pteropod communities, particularly in light of predictions of declining aragonite saturation in the Southern Ocean by the end of the century.

Oceanography of the subantarctic and Polar Frontal Zones south of Australia during summer: Setting for the SAZ-Sense study

This paper provides a description of the physical and chemical properties (temperature, salinity, macro-nutrient, and oxygen concentrations) and bulk biomass indicators (chlorophyll and beam attenuation) prevailing in the subantarctic zone and polar front zones south of Tasmania (Australia) during the ‘Sensitivity of the subantarctic zone to environmental change’ (SAZ-Sense) expedition carried out in the austral summer of 2007. Phytoplankton biomass showed a characteristic north–south gradient of decreasing chlorophyll from the subantarctic zone to Polar Frontal Zone, as well as a zonal gradient in the northern subantarctic zone, with an increase in chlorophyll from southwest to southeast of Tasmania. The representativeness of the observations was assessed by comparison to previous studies including satellite observation of chlorophyll biomass over a 10-year period. We consider the possible role of spatial differences in: (i) ocean water masses and frontal systems, (ii) upper mixed layer stratification at three process stations, and (iii) nutrient availability, in controlling the observed variations in phytoplankton biomass in the region. Zonal gradients of the basic oceanographic physical and chemical conditions in the subantarctic zone were relatively small and therefore unlikely to control the three-fold west-to-east differences observed in the accumulation of phytoplankton biomass. The zonal variation in subantarctic zone chlorophyll biomass appears to be driven at least partly by greater micro-nutrient (iron) supply to the waters east of Tasmania, as reported also by others ( Bowie et al., 2009; Mongin et al., 2011a). Despite this condition, the region of higher phytoplankton biomass to the southeast of Tasmania was only marginally more productive than the region of lower biomass west of Tasmania and south of the polar front, and exported less particulate carbon than the lower biomass waters ( Jacquet et al., 2011).

Marine microbial ecology in the sub-Antarctic Zone: Rates of bacterial and phytoplankton growth and grazing by heterotrophic protists

The sub-Antarctic zone (SAZ) of the Southern Ocean is considered one of the largest sinks for atmospheric CO 2 and as such is an important region for climate change research. To determine the importance of micro- and nano-heterotrophs in controlling microbial abundance within this region, we determined microbial standing stocks and rates of herbivory and bacterivory in relation to changes in the water masses south of Tasmania. The SAZ-Sense (‘Sensitivity of the sub-Antarctic zone to environmental change’) cruise traversed the SAZ during mid-late austral summer and focussed on process stations to the southeast (45°S, 153°E) and southwest (46°S, 140°E) of Tasmania and at the Polar Front (54°S, 147°E). Growth and grazing mortality of phytoplankton and bacteria were estimated by the grazing dilution technique using seawater from 10 m depth at 15 sites along the survey, along with concentrations of heterotrophic nanoflagellates (HNF), microzooplankton, bacteria, cyanobacteria and size fractionated (pico-, nano- and micro-sized) chlorophyll a (Chl a). Rates of herbivory ranged from 0.12 to 1.39 d −1 and were highest in the north-eastern SAZ (NE-SAZ) where concentrations of prey (as indicated by Chl a) and microzooplankton were also highest. Rates of herbivory were correlated with total rates of phytoplankton growth, bacterial growth and concentrations of microzooplankton. On average 82%, 67% and 42% primary production d −1 was consumed by microzooplankton and HNF at process stations in the north-western SAZ (NW-SAZ), NE-SAZ and polar frontal zone (PFZ), respectively. In the NW-SAZ, grazing pressure was highest on the pico-sized Chl a fraction, whereas in the NE-SAZ, grazing pressure was more evenly distributed across all three size fractions of Chl a. Bacterivory removed 77%, 93% and 39% of bacterial production d −1 in the NW-SAZ, NE-SAZ and PFZ, respectively, and rates of bacterivory ranged from 0.12 to 1.03 d −1. Rates of bacterivory were highest in the NE-SAZ where concentrations of bacteria were significantly higher than elsewhere in the region and bacterivory was correlated with bacterial growth rates and rates of cyanobacterivory. Cluster analysis of the concentrations of marine microbes and their rates of growth and grazing mortality identified 5 groups of sampling sites that differed in community structure. Analysis distinguished between high nutrient, low Chl a (HNLC) communities in the NW-SAZ that were iron-limited; iron-limited low Chl a PFZ communities; and iron-replete NE-SAZ communities where high rates of remineralisation correlated with higher concentrations of Chl a. Our findings show that much of the carbon sequestered by photosynthesis in the SAZ during summer is reprocessed via the microbial loop rather than contributing to vertical flux, particularly to the southeast of Tasmania. This suggests strong seasonality in carbon export in the region and that future climate-driven changes in oceanography may reduce carbon export from the region in summer.

Protistan communities in the Australian sector of the Sub-Antarctic Zone during SAZ-Sense

Protistan species composition and abundance in the Sub-Antarctic Zone (SAZ) and Polar Front Zone (PFZ) south of Tasmania were determined by microscopy and pigment analysis from samples collected during the Sub-Antarctic Zone—Sensitivity of the sub-Antarctic Zone to Environmental Change (SAZ-Sense) voyage, in January and February of 2007. A primary goal of this voyage was to determine the potential effects of climate change-induced natural iron fertilisation of the SAZ on the protistan community by exploring differences between communities in waters west of Tasmania, which are low in iron, and eastern waters, which are fertilised by continental iron input and mixing across the subtropical front. The SAZ is a sink for anthropogenic CO 2 in spring, but the magnitude of this may vary depending on seasonal changes in protistan abundance, composition and trophodynamics. Protistan species composition and abundance in the western Sub-Antarctic Zone at process station 1 (P1) showed a community in which low carbon biomass was dominated by a Thalassiosira sp., which was very weakly silicified under strong silica limitation. Protistan cell carbon was dominated by diatoms and nano-picoflagellates at process station 2 (P2) in the Polar Front Zone (PFZ), while dinoflagellates dominated in the iron-enriched waters of eastern SAZ at station 3 (P3). Iron enrichment enhanced production and favoured proliferation of small flagellates during summer in the silica-depleted eastern SAZ rather than large diatoms, though the effect this may have on the vertical export of particulate organic carbon (POC) is still unclear.

Distributions of dissolved and particulate iron in the sub-Antarctic and Polar Frontal Southern Ocean (Australian sector)

This paper presents iron (Fe) profiles in the upper 1000 m from nine short-term (transect) stations and three long-term (process) stations occupied in the Australian sector of the Southern Ocean during the SAZ-Sense expedition in austral summer (January–February) 2007. Strong vertical and horizontal gradients in Fe concentrations were observed between the 18 sampled profiles (i.e. 0.09–0.63 nmol/l dissolved Fe (dFe)). Average dFe concentrations in surface gggwaters in the northern Sub-Antarctic Zone (SAZ-N) West (station P1) were 0.27±0.04 nmol/l. This is lower in the SAZ-N East region (station P3 and around) where average dFe values in the mixed layer were 0.48±0.10 nmol/l. The Polar Front (PF) station (P2) exhibited the lowest average surface Fe values (i.e., 0.22±0.02 nmol/l). Iron concentrations in deep waters down to 1000 m were more uniform (0.25–0.37 nmol/l dFe), which is in accordance with values reported elsewhere in remote waters of the Southern Ocean, but lower than those observed in the North Atlantic and North Pacific basins. A strong decoupling was observed between dFe and nutrient cycles at all stations. Particulate Fe levels were generally very low for all SAZ stations (<0.08–1.38 nmol/l), with higher values observed at stations collected near Tasmania and in the SAZ-N East region. The intrusion of subtropical waters, enriched with Fe from sediments or dust further north, is thought to mediate Fe input to the SAZ-N and STZ areas, while input from below would be the main source of Fe in the PF region. We applied the tracer Fe ⁎ (Fe ⁎= [dFe]− R Fe:P×[PO 4 3−], where R Fe:P is the algal uptake ratio) to estimate the degree to which the water masses were Fe limited. In this study, Fe ⁎ tended to be negative and decreased with increasing depths and latitude. Positive Fe ⁎ values, indicating Fe sufficiency, were observed in the (near-)surface waters collected in the SAZ-N East and near continental sources, where primary production was higher and ultimately limited by the lack of macro-nutrients, not Fe. Micro-organisms residing in the SAZ-N West and PF on the other hand experienced negative Fe ⁎, indicating a strong co-limitation by low silicic acid concentration and Fe supply (and light in the case of PF).

Controls on mesopelagic particle fluxes in the Sub-Antarctic and Polar Frontal Zones in the Southern Ocean south of Australia in summer—Perspectives from free-drifting sediment traps

The SAZ-Sense project examined ecosystem controls on Southern Ocean carbon export during austral summer (January–February 2007) at three locations: P1 in the low biomass Subantarctic Zone (SAZ) west of Tasmania, P3 in a region of elevated biomass in the SAZ east of Tasmania fuelled by enhanced iron supply, and P2 in High-Nutrient/Low Chlorophyll (HNLC) Polar Frontal Zone (PFZ) waters south of P1 and P3. Sinking particles were collected using (i) a cylindrical time-series (PPS3/3) trap for bulk geochemical fluxes, (ii) indented rotating sphere (IRS) traps operated as in-situ settling columns to determine the flux distribution across sinking-rate fractions, and (iii) cylindrical traps filled with polyacrylamide gels to obtain intact particles for image analysis. Particulate organic carbon (POC) flux at 150 m (PPS3/3 trap) was highest at P1, lower at P2, and lowest at P3 (3.3±1.8, 2.1±0.9, and 0.9±0.4 mmol m −2 d −1, respectively). Biogenic silica (BSi) flux was very low in the SAZ (0.2±0.2 and 0.02±0.005 mmol m −2 d −1 at P1 and P3, respectively) and much higher in the PFZ (2.3±0.5 mmol m −2 d −1 at P2). Hence, the high biomass site P3 did not exhibit a correspondingly high flux of either POC or BSi. Separation of sinking-rate fractions with the IRS traps (at 170 and 320 m depth) was only successful at the PFZ site P2, where a relatively uniform distribution of flux was observed with ∼1/3 of the POC sinking faster than 100 m d −1 and 1/3 sinking slower than 10 m d −1. Analysis of thousands of particles collected with the gel traps (at 140, 190, 240, and 290 m depth) enabled us to identify 5 different categories: fluff-aggregates (low-density porous or amorphous aggregates), faecal-aggregates (denser aggregates composed of different types of particles), cylindrical and ovoid faecal pellets, and isolated phyto-cells (chains and single cells). Faecal-aggregates dominated the flux at all sites, and were larger in size at P1 in comparison to P3. The PFZ site P2 differed strongly from both SAZ sites in having a much higher abundance of diatoms and relatively small-sized faecal-aggregates. Overall, the particle images suggest that grazing was an important influence on vertical export at all three sites, with differences in the extents of large aggregate formation and direct diatom export further influencing the differences among the sites.

Twilight zone organic carbon remineralization in the Polar Front Zone and Subantarctic Zone south of Tasmania

We report on the distribution of excess, non-lithogenic particulate barium (Ba xs), a proxy for twilight zone remineralization of organic matter, in the Australian sector of the Southern Ocean during the mid-austral summer 2007. This study was part of a broader investigation focusing on macro- and micronutrient availability controlling ecosystem functioning in this area. Mesopelagic particulate excess Ba (Ba xs) in the twilight zone (focus was on the 100–600 m depth layer) proved to be significantly controlled by the vertical distribution of bacterial activity, with higher Ba xs contents in situations where significant bacterial activity extended deeper in the water column. However, despite this covariation, the magnitude of the carbon fluxes as estimated from Ba xs and bacterial activity did not match well, with carbon demand based on bacterial activity largely exceeding organic carbon remineralization estimated from Ba xs. Possible reasons for this discrepancy are discussed. It appeared that the magnitude of the mesopelagic carbon remineralization flux obtained from Ba xs was realistic when weighted against primary, new and export productions. Our findings corroborate earlier results indicating that mesopelagic carbon remineralization in the 100–600 m depth layer increases from the Subantarctic Zone (SAZ) toward the Polar Front, and from spring to late summer. We observed furthermore that the iron-replete Subantarctic Zone east of the Tasman Plateau had a higher mesopelagic remineralization efficiency (on average 91±20% of the carbon exported from the upper 100 m was remineralized before reaching 600 m) compared to the Fe-limited SAZ area west of the plateau and the Polar Front Zone, where mesopelagic remineralization efficiencies ranged between 21±5% and 64±24%, respectively. SAZ-East and SAZ-West sites thus differed in their efficiency of carbon sequestration into the deep (>600 m) water column, with SAZ-West exceeding the sequestration capacity of SAZ-East.

Parameter optimisation of a marine ecosystem model at two contrasting stations in the Sub-Antarctic Zone

SeaWiFS surface chlorophyll estimates in the Sub-Antarctic Zone show low concentrations south west of Tasmania and high concentrations south east of Tasmania. Data assimilation experiments were performed using simulated annealing to obtain parameter estimates of a simple nitrogen based mixed-layer marine ecosystem model at two locations in this region (station P1 at 140°E, 46.5°S and station P3 at 152°E, 45.5°S). The assimilation methods and parameter sensitivities are assessed in a twin experiment. This assessment determined that inversion method was successful at estimating the correct parameters but that only a sub-set of the model parameters can be uniquely determined using chlorophyll a observations. An analysis of parameter uncertainties shows at both stations accurate parameterisations of phytoplankton growth and zooplankton mortality, and the biological recycling processes are required to realistically model chlorophyll. Applying the inversion method to the climatological SeaWiFS chlorophyll a observations from the two sites we estimate model parameters at these two sites. The most significant differences in parameters between the two stations are the parameters relating to phytoplankton growth and zooplankton mortality. The difference in growth parameters results in spring time productivity estimates of 659 mg C m −2d −1 at P1 and 203 mg C m −2d −1 at P3. In situ estimates from the SAZ-Sense cruise do not support such dramatic differences in primary production between the two stations. We conclude that the same ecosystem model structure is not applicable at both stations and we need additional processes at P3 to reproduce the observed seasonality of phytoplankton and the observed primary productivity. We hypothesise that the missing processes in the ecosystem model at P3 are iron limitation of phytoplankton and the seasonal variations in atmospheric deposition of iron.

Carbon export production in the subantarctic zone and polar front zone south of Tasmania

We studied the water column distribution of total 234Th in subantarctic (SAZ) and polar front (PFZ) zone systems south of Tasmania during mid-austral summer 2007. The objective was to assess whether the observed zonal differences in biomass in this sector translated into variability of the carbon export and sequestration potential, and to identify possible causes inducing this variability. This study is part of a broader investigation focusing on macro- and micronutrient availability controlling ecosystem functioning in this area. Surface deficits in 234Th activities were observed at every station. 234Th export fluxes calculated from the 234Th activity deficits assuming steady state conditions showed higher 234Th export fluxes in the western than eastern SAZ, both of which were higher than those in the PFZ. 234Th fluxes sampled by free-drifting IRS and PPS-3 sediment traps at 150 and 170 m during short-term deployments (∼ 6 days) at the three process stations were significantly lower than those obtained by the 234Th-deficit method. Possible reasons for this discrepancy are discussed. Carbon export fluxes were calculated based on the total 234Th fluxes ( 234Th deficit method) and the C:Th ratio for the >54 μm particle size fraction from the appropriate export depth. The >54 μm C:Th ratio was significantly lower in the eastern SAZ than the western SAZ or PFZ, resulting in carbon export fluxes that were lowest in the eastern SAZ. Overall, export fluxes range from 3.6±1.5 to 13.2±3.1 mmol C m −2 d −1. Carbon export fluxes are compared with gross primary production, new production and mesopelagic remineralization fluxes obtained by others during the same cruise. Contrary to expectations, we found higher export production in the PFZ and the western SAZ where biomass and dissolved Fe were lower than in the eastern SAZ. Significant differences in community structure of both primary producers and consumers likely contributed to this difference between the three regions. This pattern of higher shallow export continued into the mesopelagic, with lower remineralization efficiency in PFZ and SAZ-West compared to SAZ-East. Within the SAZ, east and west sites thus differed in their efficiency of carbon sequestration into the deep (>600 m) water column, with SAZ-West exceeding the sequestration capacity of SAZ-East. If the SAZ-East region can be used as an analog for a future climate warming scenario, our results suggest that export production at high latitudes could decrease, despite increased primary production.

Potential climate change impacts on microbial distribution and carbon cycling in the Australian Southern Ocean

Changes in oceanic circulation and physiochemical parameters due to climate change may alter the distribution, structure and function of marine microbial communities, thereby altering the action of the biological carbon pump. One area of current and predicted future change is the sub-Antarctic zone (SAZ) to the southeast of Tasmania, Australia, where a southward shift in westerly winds appears to be forcing warmer and macronutrient-poor subtropical waters into the sub-Antarctic zone (SAZ). We investigated the impact of these subtropical waters on the microbial community of the SAZ on the SAZ-Sense cruise during the austral summer of 2007. The abundance of pico- and nanoeukaryotic algae, cyanobacteria, heterotrophic nanoflagellates, bacteria and viruses was determined by flow cytometry at stations in the Polar Frontal Zone (PFZ), the SAZ and in Subtropical Zone (STZ). Using cluster and similarity profile analyses on integrated microbial abundances over the top 200 m, we found that microbial communities located in the potential future SAZ to the southeast of Tasmania formed two distinct groups from those of the remainder of the SAZ and the PFZ. In the waters of the potential future SAZ, shallow mixed layers and increased iron concentrations elevated cyanobacterial, bacterial and viral abundances and increased percentage high DNA bacteria, resulting in communities similar to those of subtropical waters. Conversely, waters of the PFZ exhibited relatively low concentrations of autotrophic and heterotrophic microbes and viruses, indicative of the iron limitation in this region. A Distance Based Linear Model determined that salinity and nitrogen availability (nitrate, nitrite and ammonia concentrations) were the most influential environmental parameters over the survey, explaining 72% of the variation in microbial community structure. The microbial community of the potential future SAZ showed a shift away from particulate carbon export from the photic zone towards increased production by smaller cells, increased significance of the microbial loop and viral lysis. These changes would promote carbon recycling within the photic zone, thereby potentially decreasing the capacity of the future SAZ to absorb CO 2.