Polar Frontal Zone Research Articles

Pigments, elemental composition (C, N, P, and Si), and stoichiometry of particulate matter in the naturally iron fertilized region of Kerguelen in the Southern Ocean

Abstract. The particulate matter distribution and phytoplankton community structure of the iron-fertilized Kerguelen region were investigated in early austral spring (October–November 2011) during the KEOPS2 cruise. The iron-fertilized region was characterized by a complex mesoscale circulation resulting in a patchy distribution of particulate matter. Integrated concentrations over 200 m ranged from 72.2 to 317.7 mg m−2 for chlorophyll a 314 to 744 mmol m−2 for biogenic silica (BSi), 1106 to 2268 mmol m−2 for particulate organic carbon, 215 to 436 mmol m−2 for particulate organic nitrogen, and 29.3 to 39.0 mmol m−2 for particulate organic phosphorus. Three distinct high biomass areas were identified: the coastal waters of Kerguelen Islands, the easternmost part of the study area in the polar front zone, and the southeastern Kerguelen Plateau. As expected from previous artificial and natural iron-fertilization experiments, the iron-fertilized areas were characterized by the development of large diatoms revealed by BSi size–fractionation and high performance liquid chromatography (HPLC) pigment signatures, whereas the iron-limited reference area was associated with a low biomass dominated by a mixed (nanoflagellates and diatoms) phytoplankton assemblage. A major difference from most previous artificial iron fertilization studies was the observation of much higher Si : C, Si : N, and Si : P ratios (0.31 ± 0.16, 1.6 ± 0.7 and 20.5 ± 7.9, respectively) in the iron-fertilized areas compared to the iron-limited reference station (0.13, 1.1, and 5.8, respectively). A second difference is the patchy response of the elemental composition of phytoplankton communities to large scale natural iron fertilization. Comparison to the previous KEOPS1 cruise also allowed to address the seasonal dynamics of phytoplankton bloom over the southeastern plateau. From particulate organic carbon (POC), particulate organic nitrogen (PON), and BSi evolutions, we showed that the elemental composition of the particulate matter also varies at the seasonal scale. This temporal evolution followed changes of the phytoplankton community structure as well as major changes in the nutrient stocks progressively leading to silicic acid exhaustion at the end of the productive season. Our observations suggest that the specific response of phytoplankton communities under natural iron fertilization is much more diverse than what has been regularly observed in artificial iron fertilization experiments and that the elemental composition of the bulk particulate matter reflects phytoplankton taxonomic structure rather than being a direct consequence of iron availability.

A glimpse into the deep of the Antarctic Polar Front – Diversity and abundance of abyssal molluscs

Our knowledge of the biodiversity and distribution patterns of benthic deep-sea faunas is still limited, with large parts of the world׳s abyss unexplored, lacking α-taxonomic data across oceans basins and especially of biogeographic transition zones between oceans. The Antarctic Polar Frontal Zone has been discussed as major biogeographic barrier hindering faunal exchange between Subantarctic and Antarctic provinces and conserving high rates of endemism in the Southern Ocean benthos. In the present study we report first, exploratory α-taxonomy on the malacofauna sampled by means of an epibenthic sledge from four bathyal respectively abyssal stations (2732–4327m depth) in the vicinity of the Antarctic Polar Front during the SYSTCO II expedition (SYSTem COupling in the Southern Ocean, RV Polarstern cruise ANT XXVIII/3). We identified 58 distinct molluscan taxa based on external morphology (‘morphospecies’); of the 33 taxa successfully assigned to described species 94% were previously reported from the Southern Ocean, but 24% exhibit distribution ranges crossing the Polar Front. One North Atlantic scaphopod is reported for the first time in Antarctic waters. Our study supports that the Antarctic Polar Front does not serve as effective barrier preventing gene flow in deep-sea molluscs. The present dataset shows the general characteristics of deep-sea sampling: patchiness in distribution and a high degree of singletons. Overall molluscan abundances were generally low ranging between 3.60 and 24.65ind./1000m², but in comparison with equatorial and subtropic abyssal basins, gastropod species richness and abundance were reaching high values similar to high Antarctic stations. Comparison between high productivity and low productivity zones along the Polar Front suggests increased abundances and species richness in high productivity zones. Intensified sampling is needed, however, to outweigh stochastic errors and to evaluate the influence of carbon flux as driving factor to faunal composition and abundances of abyssal molluscs.

Settling fluxes of diatoms to the interior of the Antarctic circumpolar current along 170 °W

An array of four sediment trap moorings recorded the particulate flux across the Antarctic Circumpolar Current (ACC) at 170°W, between November 1996 and January 1998, as part of the US JGOFS-Antarctic Environment and Southern Ocean Process Study (AESOPS) program. The trap locations represent sampling within the Polar Frontal Zone, the Antarctic Polar Front, the Antarctic Zone and the Southern Antarctic Zone. Here we report observations from 1000m below the sea-surface compared to seafloor and surface water distributions. Sub-sample splits from each trap were obtained and total diatom flux and species composition were determined. The diatom fluxes were quantified using both a dilution and a ‘spike’ method to allow for the rapid repeatability of measurements. Diatom flux was found to be highly seasonal across the ACC particularly at higher latitudes. Marine snow aggregates of intact diatom cells and chains were the major components of the biogenic flux. Siliceous particle size was noted to decrease with increasing latitude, which could be aligned with a shift of the diatom assemblage to small-size species/sea-ice affiliated species. A ‘double-structured’ diatom flux was recorded at the location of the Antarctic Polar Front trap, with a shift in the diatom assemblage from larger to smaller diatoms in the second flux episode. The sediment trap assemblage shows deviations from the surface water assemblage, while surface sediment samples indicate that significant dissolution occurs after 1000m and at the sediment–water interface. Estimation of diatom biovolumes across the ACC shows that large diatoms have the potential to greatly impact biogenic fluxes to the ocean interior despite their low fluxes. Small species of the genus Fragilariopsis could potentially export as much Corg as Fragilariopsis kerguelensis near the retreating ice edge. However, their low abundance in the surface sediments also suggests that these diatoms are a shallow export species.

Diverse trends in shell weight of three Southern Ocean pteropod taxa collected with Polar Frontal Zone sediment traps from 1997 to 2007

The impact of ocean acidification on key ocean calcifiers is predicted to be imminent, particularly in high-latitude ecosystems. Long-term field observations are essential to ground truth predictions of change in regional ecosystems. Here, we report on aragonitic pteropods collected to sediment traps at 800 m depth at 54°S, 140°E in the Polar Frontal Zone (PFZ) of the Southern Ocean from 1997 to 2007. Statistically significant trends were not identified in either mass or number flux from 1997 to 2007; however, differences emerged in decadal trends seen in shell weight for each of the three common taxa collected: Limacina helicina antarctica forma antarctica shells became significantly lighter (P < 0.05), L. retroversa australis shells became significantly heavier (P < 0.05) and L. helicina antarctica forma rangi shells did not change significantly. These results suggest that factors other than ocean acidification affect pteropod population variations on decadal timescales, with the potential to either amplify or counter the impact of decreasing aragonite saturation state, at least in the short term. Comparison to sea surface temperature and chlorophyll biomass did not identify these as significant drivers of the observed changes, and attribution across these multiple variables requires better understanding of pteropod physiology and ecology. Our PFZ pelagic pteropod observations provide a reference for evaluation of southern polar pteropod responses to changing ocean conditions in coming decades. Importantly, these data also raise the issue of taxonomic care when monitoring the region for impacts of ocean acidification on calcifiers.

Sedimentation in the Angola and Cape basins during the quaternary

Reference cores of sediments from the Angola and Cape basins were subjected to lithological, geochemical, and micropaleontological investigations. Based on nannofossil and diatom assemblages, the examined sediments represented by oxidized miopelagic clays, calcareous (nannofossil-foraminiferal, foraminiferal-nannofossil), calcareous-clayey, and clayey-calcareous mud are estimated to be the Pleistocene-Holocene in age. The Cape Basin section includes surface and buried ferromanganese nodules. There are indications of sedimentary material redeposition by the Antarctic Bottom Water and turbidity currents. The South Polar Front zone changed repeatedly its geographic position and migrated northward from the presentday one. As a whole, the Pleistocene-Holocene sedimentation in the Angola and Cape basins corresponds to the incomplete pelagic (miopelagic) type of oceanic lithogenesis.

Evidence of a southward eddy corridor in the South-West Indian ocean

Mesoscale eddies and meanders have been shown to be one of the dominant sources of flow variability in the world׳s ocean. One example of an isolated eddy hotspot is the South-West Indian Ridge (SWIR). Several investigations have shown that the SWIR and the corresponding planetary potential vorticity field (f/H) exert a strong influence on the location and dynamics of the Antarctic Circumpolar Current (ACC), resulting in substantial fragmentation of the jets downstream of the ridge. The easterly extension of this eddy corridor appears to be restricted to the deep channel separating the Conrad Rise from the Del Cano and Crozet Plateau. However, while the fate of eddies formed at the SWIR has been widely investigated and the frontal character of this eastward extension is well known, the zone of diminishing variability that extends southwards to approximately 60°S remains poorly sampled. Using a combination of Argo, AVISO and NCEP/NCAR datasets, the character of this eddy corridor as a conduit for warm core eddies to move across the ACC into the Antarctic zone is investigated. In this study, we track a single warm-core eddy as it moves southwards from an original position of 31°E, 50°20′S to where it dissipates 10 months later in the Enderby Basin at 56°20′S. An Argo float entrained within the eddy confirms that its water masses are consistent with water found within the Antarctic Polar Frontal Zone north of the APF. Latent and sensible heat fluxes are on average 8W/m2 and 10W/m2 greater over the eddy than directly east of this feature. It is estimated that the eddy lost an average of 5W/m2 of latent heat and 5W/m2 of sensible heat over a 1-year period, an amount capable of melting approximately 0.92m of sea ice. In addition, using an eddy tracking algorithm a total of 28 eddies are identified propagating southwards, 25 of which are anti-cyclonic in rotation. Based on the new Argo float data, combined with AVISO and NCEP/NCAR datasets, these results suggest that the southward passage of warm-core eddies act as vehicles transporting heat, salt and biota southwards across the ACC and into the eastern boundary of the Weddell gyre.

Seasonal variability of phytoplankton biomass and composition in the major water masses of the Indian Ocean sector of the Southern Ocean

Long-term changes in phytoplankton biomass and community composition are important in the ecosystem and biogeochemical cycle in the Southern Ocean. We aim to ultimately evaluate changes in phytoplankton assemblages in this region on a decadal scale. However, yearly continuous data are lacking, and long-term datasets often include seasonal variability. We evaluated the seasonal changes in phytoplankton abundance/composition across latitudes in the Indian Ocean sector of the Southern Ocean via multi-ship observations along the 110°E meridian from 2011 to 2013. The chlorophyll a concentration was 0.3–0.5 mg m−3 in the Subantarctic Zone (40–50°S) and 0.4–0.6 mg m−3 in the Polar Frontal Zone (50–60°S); pico-sized phytoplankton (<10 μm), mainly haptophytes, were dominant in both zones. In the Antarctic Divergence area (60–65°S), the chlorophyll a concentration was 0.6–0.8 mg m−3, and nano-sized phytoplankton (>10 μm), mainly diatoms, dominated. Chlorophyll a concentrations and phytoplankton community compositions were the same within a latitudinal zone at different times, except during a small but distinct spring bloom that occurred north of 45°S and south of 60°S. This small seasonal variation means that this part of the Southern Ocean is an ideal site to monitor the long-term effects of climate change.

Biogeographic distribution of living coccolithophores in the Pacific sector of the Southern Ocean

Abstract This paper adds to a series of studies addressing the distribution of living coccolithophores in the Southern Ocean (SO). We investigated plankton samples collected during RV Polarstern cruise ANT-XXVI/2 (from 27th November 2009 to 27th January 2010) along a broad E–W transect in the Pacific sector of the SO during austral summer. One hundred and fifty samples from twenty-nine stations were collected from the upper 150 m of the water column. Both coccoliths and coccospheres per sample were counted separately using a scanning electron microscope (SEM). The highest abundances of 640 · 103 coccospheres/l were reached close to the Subtropical Front (STF) and increases in the numbers of coccospheres and coccoliths were found both at the Subantarctic Front (SAF) and the Polar Front (PF). However, the numbers decrease southward until almost a monospecific assemblage and sporadic record of Emiliania huxleyi (types B/C and C) south of the PF. Thirty-three coccolithophore species, including sixteen species found as isolated coccoliths, were identified of which E. huxleyi is clearly the most dominant coccolithophore taxon in the studied samples. Two main coccolithophore assemblages were established coincident with areas bounded by the oceanographic fronts: the Polar Front Zone (PFZ) and Subantarctic Zone (SAZ). In the upper photic zone of the SAZ, Acanthoica quattrospina, Calcidiscus leptoporus, Coccolithus pelagicus (sensu lato) HOL, E. huxleyi type A, Ophiaster spp. and Syracosphaera spp. among others were found. The PFZ was characterized by a reduced number of species, i.e., Calciopappus caudatus, E. huxleyi types B, B/C and C, as well as Pappomonas spp. and Papposphaera spp. The sea surface temperature measured in situ was the most prominent factor influencing coccolithophore diversity, distribution and assemblage compositions in the Pacific sector of the SO during austral summer. Coccolithophore biogeography in the study area showed marked differences with the northern high latitudes; the reduced presence of the cold water species Coccolithus pelagicus, abundant in the (sub) Arctic region, and the dominance of E. huxleyi type B/C and C in the SO contrasts with the dominance of E. huxleyi types A and B in the North Atlantic. Findings such as these cover existing gaps in an unexplored area of the SO as well as supporting previous research performed in neighboring areas. The current coccolithophore numbers and assemblage distribution in relation to the frontal dynamics of the SO provide valuable information for potential future paleoceanographic reconstructions.

Computation of Geostrophic Streamfunction, Its Derivatives, and Error Estimates from an Array of CPIES in Drake Passage

AbstractCurrent and pressure-recording inverted echo sounders (CPIES) were deployed in an eddy-resolving local dynamics array (LDA) in the eddy-rich polar frontal zone (PFZ) in Drake Passage as part of the cDrake experiment. Methods are described for calculating barotropic and baroclinic geostrophic streamfunction and its first, second, and third derivatives by objective mapping of current, pressure, or geopotential height anomaly data from a two-dimensional array of CPIES like the cDrake LDA.Modifications to previous methods result in improved dimensional error estimates on velocity and higher streamfunction derivatives. Simulations are used to test the reproduction of higher derivatives of streamfunction and to verify mapping error estimates. Three-day low-pass-filtered velocity in and around the cDrake LDA can be mapped with errors of 0.04 m s−1 at 4000 dbar, increasing to 0.13 m s−1 at the sea surface; these errors are small compared to typical speeds observed at these levels, 0.2 and 0.65 m s−1, respectively. Errors on vorticity are 9 × 10−6 s−1 near the surface, decreasing with depth to 3 × 10−6 s−1 at 4000 dbar, whereas vorticities in the PFZ eddy field are 4 × 10−5 s−1 (surface) to 1.3 × 10−5 s−1 (4000 dbar). Vorticity gradient errors range from 4 × 10−10 to 2 × 10−10 m −1 s−1, just under half the size of typical PFZ vorticity gradients. Comparisons between cDrake mapped temperature and velocity fields and independent observations (moored current and temperature, lowered acoustic Doppler current profiler velocity, and satellite-derived surface currents) help validate the cDrake method and results.

Protist community composition in the Pacific sector of the Southern Ocean during austral summer 2010

Knowledge about the protist diversity of the Pacific sector of the Southern Ocean is scarce. We tested the hypothesis that distinct protist community assemblages characterize large-scale water masses. Therefore, we determined the composition and biogeography of late summer protist assemblages along a transect from the coast of New Zealand to the eastern Ross Sea. We used state of the art molecular approaches, such as automated ribosomal intergenic spacer analysis and 454-pyrosequencing, combined with high-performance liquid chromatography pigment analysis to study the protist assemblage. We found distinct biogeographic patterns defined by the environmental conditions in the particular region. Different water masses harbored different microbial communities. In contrast to the Arctic Ocean, picoeukaryotes had minor importance throughout the investigated transect and showed very low contribution south of the Polar Front. Dinoflagellates, Syndiniales, and small stramenopiles were dominating the sequence assemblage in the Subantarctic Zone, whereas the relative abundance of diatoms increased southwards, in the Polar Frontal Zone and Antarctic Zone. South of the Polar Front, most sequences belonged to haptophytes. This study delivers a comprehensive and taxon detailed overview of the protist composition in the investigated area during the austral summer 2010.

Latitudinal shifts in the Polar Front in Indian sector of the Southern Ocean: evidences from silicoflagellate assemblage

We used silicoflagellate assemblage records to describe the polar frontal variability over the last 48kyr in the Indian sector of the Southern Ocean. The studied core was collected onboard ORV Sagar Nidhi from within the Polar frontal zone (PFZ) during the 4th Indian Scientific Expedition to Southern Ocean. The Polar front is dominated by silica-rich sediments (diatoms and silicoflagellates). Silicoflagellates were dominated by Distephanus speculum and Dictyocha fibula species. The biostratigraphic record of these silicoflagellates was used qualitatively to examine past changes in polar frontal variability in the Southern Ocean. Warming is indicated by an increase (decrease) in Dictyocha sp. (Distephanus sp.) from the LGM to the Holocene. Dictyocha sp. abundance indicates warmer temperatures during 43–45 kyr and is nearly synchronous with the warming event recorded in an Antarctic ice core. Dictyocha/Distephanus ratio also suggests a northern and southern shift in the polar front during LGM and 43–45 kyr respectively. The southward displacement of the frontal system is linked to an increase in sea surface temperature as evidenced from the δ18O Byrd Antarctic ice core data and solar insolation data. The low dust flux, higher δ18O and absence of an upwelling indicator diatom, Thallasionema nitzchoides during the Antarctic warming event also suggest stronger thermal stratification during the Antarctic warming event as compared to LGM. The present study would improve our understanding of the frontal variability under future warming scenarios.

Use of the Southern Ocean by breeding Short-tailed shearwaters (Puffinus tenuirostris)

During the breeding season, seabirds act as central place foragers, constrained to foraging locations nearer to the breeding colony. Variability in energetic demands during phases of the breeding period has the capacity to shape foraging behaviour and influence at-sea foraging behaviour of seabirds due to the changing requirements of the young. Short-tailed shearwaters (Puffinus tenuirostris) (STSH) are a migratory species that migrates between foraging grounds in the northern Pacific Ocean and breeding colonies in south-east Australia. They are known to traverse large sectors of the Southern Ocean to find prey; however the at-sea foraging locations of this species are not spatially and temporally well defined during the breeding season, when energetic requirements are at their peak. We aimed to define key foraging areas of STSH over the pre-laying, incubation and chick-rearing phases of the breeding cycle and identify associations with broad scale oceanographic features. Using light-level geolocation archival tag technology, 27 adults from a southern Tasmanian colony were tracked from their arrival in October to March (2010–2011), providing information on a total of 77 pre-laying exodus, incubation and chick-rearing flights throughout the Southern Ocean. During the pre-laying exodus and incubation phases, the birds visited waters of the sub-Antarctic Front and Polar Frontal Zone. After hatching, the birds made direct flights to the Marginal Ice Zone between 155°E and 90°E. The Antarctic pack ice is a key nursery ground for Antarctic krill (Euphasia superba) over the winter months, and as the ice retreats they become more accessible to STSH when energetic demands, due to chick provisioning, are at their greatest. During late chick-rearing, the birds used waters in the region of the Antarctic Divergence. A foraging index created from activity data collected by the tags indicated that adults spent the greatest amount of time foraging at the furthest extent of each trip. This study has shown that STSH alter their foraging movements over the course of the breeding season, using areas that are likely to provide predictable food resources.

Mesozooplankton in the Southern Ocean: Spatial and temporal patterns from Discovery Investigations

Mesozooplankton samples taken during the Discovery Investigations in the Southern Ocean in the 1930s were analysed from a series of 5 transects along 80°W. The samples provide a unique level of depth-discrete resolution across large spatial scales, over most of the productive austral season. Stratified net hauls were taken between 0 and 1000m within the period December 1933 to November 1934. Within the epipelagic (0–100m), median zooplankton abundance (278ind. m−3) was ∼22 times greater than at 1000m. A 3–4-fold variability of abundance in the epipelagic contrasted with depths >250m where variability was <1-fold. Depth was the strongest factor separating samples (ANOSIM, R=0.66 p=0.1%), with a clear biological distinction between epipelagic and upper and lower mesopelagic horizons. Results from multi-dimensional scaling indicated that, when plankton abundance was integrated over all depth horizons, 3 different groups could be identified. These ‘communities’ were consistent with the spatial extents of Antarctic, Polar Frontal Zone, and sub-Antarctic water-mass regimes. Such groupings became less distinct when considering only deeper horizons (500–1000m) and excluding seasonal migrants. Seasonal signals across all data became less distinct with depth. Rarefaction analysis indicated that diversity increased with depth. Although depth alone was the most important influence on sample diversity, (r2=0.60), water mass regime and month improved the fit (r2=0.71). Overall plankton diversity was highest in the sub-Antarctic zone. Following atmospheric and ocean warming that has taken place close to the study area in the last 80years we hypothesise that species richness may increase in the Antarctic water masses as sub-Antarctic species increasingly encroach south.

Response of Phytoplankton Photophysiology to Varying Environmental Conditions in the Sub-Antarctic and Polar Frontal Zone

Climate-driven changes are expected to alter the hydrography of the Sub-Antarctic Zone (SAZ) and Polar Frontal Zone (PFZ) south of Australia, in which distinct regional environments are believed to be responsible for the differences in phytoplankton biomass in these regions. Here, we report how the dynamic influences of light, iron and temperature, which are responsible for the photophysiological differences between phytoplankton in the SAZ and PFZ, contribute to the biomass differences in these regions. High effective photochemical efficiency of photosystem II (/ 0.4), maximum photosynthesis rate (), light-saturation intensity (), maximum rate of photosynthetic electron transport (1/), and low photoprotective pigment concentrations observed in the SAZ correspond to high chlorophyll and iron concentrations. In contrast, phytoplankton in the PFZ exhibits low / ( 0.2) and high concentrations of photoprotective pigments under low light environment. Strong negative relationships between iron, temperature, and photoprotective pigments demonstrate that cells were producing more photoprotective pigments under low temperature and iron conditions, and are responsible for the low biomass and low productivity measured in the PFZ. As warming and enhanced iron input is expected in this region, this could probably increase phytoplankton photosynthesis in this region. However, complex interactions between the biogeochemical processes (e.g. stratification caused by warming could prevent mixing of nutrients), which control phytoplankton biomass and productivity, remain uncertain.

Biogeochemical cycling of zinc and its isotopes in the Southern Ocean

We report Zn concentration and isotope data for seawater samples from the Atlantic sector of the Southern Ocean, collected during the IPY/GEOTRACES ANT-XXIV/III cruise along the Greenwich Zero Meridian. Data are reported for the full depth range of the water column at three stations, as well as a transect of surface samples, using a new analytical approach that is presented in detail here.Zn concentrations increase with depth, though due to proximity to upwelling sites, surface concentrations are not as low as in some parts of the ocean such as further northward into the Sub-Antarctic Zone. For two depth profiles south of the Polar Front Zone, the physical stratification of the upper water column is reflected in sudden near-surface changes in Zn concentration with depth. In contrast, beneath 100–300m Zn concentrations barely change with depth. Zn isotopic data beneath 1000m, for the Southern Ocean data presented here as well as published data from the North Atlantic and North Pacific, are strikingly homogeneous, with an average δ66Zn=+0.53±0.14‰ (2SD, 2SE=0.03, n=21). The surface Southern Ocean is more variable, with δ66Zn ranging from 0.07‰ to 0.80‰. Between the two is a thin horizon at 40–80m which, in the Southern Ocean as well as the North Atlantic and North Pacific, is characterised by distinctly light isotopic signatures, with δ66Zn about 0.3‰ lower than surface waters.Strong correlations between Si and Zn concentrations seen here and elsewhere, coupled to the lack of any systematic relationship between Si and Zn isotopes in the Southern Ocean, suggest that the removal of Zn associated with diatom opal involves little or no isotopic fractionation. Regeneration of this Zn also explains the homogeneous Zn isotopic composition of the global deep ocean so far sampled. However, the low Zn content of opal requires that deep ocean Zn does not directly come from the opal phase itself, but rather from associated organic material external to the diatom frustule during growth. Experimental data are consistent with little or no fractionation during incorporation of Zn into this material. On the other hand, the light zinc at 40–80m is most consistent with the regeneration of an intra-cellular pool that both culturing experiments and field data suggest will be isotopically light. The data thus imply two processes by which Zn is taken up in the surface ocean, that these pools have very different regeneration lengthscales, and that physical mixing of the oceans cannot eradicate their isotopic signatures. Finally, the deep δ66Zn ocean value is significantly higher than the current best estimate of the input to the oceans. The most obvious candidate for the required light sink is the survival of some of the cellular Zn to be buried in sediment.

Mitochondrial DNA polymorphism (CO1) of three dominant copepod species in the South Atlantic

The Southern Ocean is characterized by the complex system of oceanic fronts that maintain the latitudinal zonality of biotopes. These fronts are boundaries of water masses with different hydrophysical characteristics. We explore the genetic differentiation of the dominant zooplankton species in regards to the complex hydrophysical zonality of the Southern Ocean. The barcoding region of mitochondrial CO1 gene was sequenced for three copepod species, Calanus simillimus, Rhincalanus gigas, and Metridia lucens. These species are the most abundant in the Southern Ocean and form the basis of the zooplankton community. Genetic differentiation was found neither for Calanus simillimus nor for Rhincalanus gigas. The mitochondrial haplotypes of Metridia lucens cluster in two genetically distant groups (Subantarctic and Antarctic) found together only in the Polar Front Zone.

Phytoplankton growth in the Australian sector of the Southern Ocean, examined by optimising ecosystem model parameters

SeaWiFS surface chlorophyll estimates and Levitus nitrate estimates in the Southern Ocean south of Australia (140°E) show that this region is characterised by a high-nitrate low-chlorophyll (HNLC) regime typical of Southern Ocean waters. The HNLC conditions become more prominent moving south from the Sub-Antarctic Zone, with surface chlorophyll generally decreasing and nitrate increasing with latitude. Parameter optimisation experiments were performed using simulated annealing to fit a zero-dimensional nitrogen-based four-component ecosystem model to SeaWiFS surface chlorophyll data in the Sub-Antarctic Zone (SAZ), Polar Frontal Zone (PFZ) and Antarctic Zone (AZ). We hypothesise that bioavailability of iron limits phytoplankton growth in this region. A physiological indicator of iron availability was investigated by optimising three of the model parameters defining maximum photosynthetic growth and maximum photosynthetic efficiency of phytoplankton.The effect of zooplankton grazing and light, mixed layer depth and temperature forcing data on the optimisation results was investigated in further optimisation experiments. An error analysis of the optimised parameter estimates was performed by analysing the Hessian matrix of the cost function.The parameter optimisations indicate that phytoplankton growth rates in the Polar Frontal Zone and Antarctic Zone are limited by some process not explicitly included in this model, with iron availability being the most likely candidate. Based on these optimisations we support the theory that micronutrient availability is the primary cause of the HNLC conditions in the Australian sector of the Southern Ocean.

Subantarctic mode water in the southeast Pacific: Effect of exchange across the Subantarctic Front

[1] This study considered cross-frontal exchange as a possible mechanism for the observed along-front freshening and cooling between the 27.0 and 27.3 kg m − 3 isopycnals north of the Subantarctic Front (SAF) in the southeast Pacific Ocean. This isopycnal range, which includes the densest Subantarctic Mode Water (SAMW) formed in this region, is mostly below the mixed layer, and so experiences little direct air-sea forcing. Data from two cruises in the southeast Pacific were examined for evidence of cross-frontal exchange; numerous eddies and intrusions containing Polar Frontal Zone (PFZ) water were observed north of the SAF, as well as a fresh surface layer during the summer cruise that was likely due to Ekman transport. These features penetrated north of the SAF, even though the potential vorticity structure of the SAF should have acted as a barrier to exchange. An optimum multiparameter (OMP) analysis incorporating a range of observed properties was used to estimate the cumulative cross-frontal exchange. The OMP analysis revealed an along-front increase in PFZ water fractional content in the region north of the SAF between the 27.1 and 27.3 kg m − 3 isopycnals; the increase was approximately 0.13 for every 15° of longitude. Between the 27.0 and 27.1 kg m − 3 isopycnals, the increase was approximately 0.15 for every 15° of longitude. A simple bulk calculation revealed that this magnitude of cross-frontal exchange could have caused the downstream evolution of SAMW temperature and salinity properties observed by Argo profiling floats.

Late summer particulate organic carbon export and twilight zone remineralisation in the Atlantic sector of the Southern Ocean

Abstract. As part of the GEOTRACES Bonus-GoodHope (BGH) expedition (January–March 2008) in the Atlantic sector of the Southern Ocean, particulate organic carbon (POC) export was examined from the surface to the mesopelagic twilight zone using water column distributions of total 234Th and biogenic particulate Ba (Baxs). Surface POC export production was estimated from steady state and non steady state modelling of 234Th fluxes, which were converted into POC fluxes, using the POC/234Th ratio of large, potentially sinking particles (&gt; 53 μm) collected via in situ pumps. Deficits in 234Th activities were observed at all stations from the surface to the bottom of the mixed layer, yielding 234Th export fluxes from the upper 100 m of 496 &amp;amp;pm; 214 dpm m−2 d−1 to 1195 &amp;amp;pm; 158 dpm m−2 d−1 for the steady state model and of 149 &amp;amp;pm;517 dpm m−2 d−1 to 1217 &amp;amp;pm; 231 dpm m−2 d−1 for the non steady state model. Using the POC/234Thp ratio of sinking particles (ratios varied from 1.7 &amp;amp;pm; 0.2 μmol dpm−1 to 4.8 &amp;amp;pm; 1.9 μmol dpm−1) POC export production at 100 m was calculated to range between 0.9 &amp;amp;pm; 0.4 and 5.1 &amp;amp;pm; 2.1 mmol C m−2 d−1,assuming steady state and between 0.3 &amp;amp;pm; 0.9 m−2 d−1 and 4.9 &amp;amp;pm; 3.3 mmol C m−2 d−1, assuming non steady state. From the comparison of both approaches, it appears that during late summer export decreased by 56 to 16% for the area between the sub-Antarctic zone and the southern Antarctic Circumpolar Current Front (SACCF), whereas it remained rather constant over time in the HNLC area south of the SACCF. POC export represented only 6 to 54% of new production, indicating that export efficiency was, in general, low, except in the vicinity of the SACCF, where export represented 56% of new production. Attenuation of the POC sinking flux in the upper mesopelagic waters (100–600 m depth interval) was evidenced both, from excess 234Th activities and from particulate biogenic Ba (Baxs) accumulation. Excess 234Th activities, reflected by 234Th/238U ratios as large as 1.21 &amp;amp;pm; 0.05, are attributed to remineralisation/disaggregation of 234Th-bearing particles. The accumulation of excess 234Th in the 100–600 m depth interval ranged from 458 &amp;amp;pm; 633 dpm m−2 d−1 to 3068 &amp;amp;pm; 897 dpm m−2 d−1, assuming steady state. Using the POC/234Thp ratio of sinking particles (&gt; 53 μm), this 234Th accumulation flux was converted into a POC remineralisation flux which ranged between 0.9 &amp;amp;pm; 1.2 mmol C m−2 d−1 and 9.2 &amp;amp;pm; 2.9 mmol C m−2 d−1. Mesopelagic particulate biogenic Ba has been reported to reflect bacterial degradation of organic matter and to be related to oxygen consumption and bacterial carbon respiration. We observed that the highest Baxs contents (reaching up to &gt; 1000 pM), in general, occurred between 200 and 400 m. Depth-weighted average mesopelagic Baxs (meso-Baxs) values were converted into respired C fluxes, which ranged between 0.23 and 6.4 mmol C m−2 d−1, in good agreement with 234Th-based remineralisation fluxes. A major outcome from this study is the observed significant positive correlation between POC remineralisation as estimated from meso-Baxs contents and from 234Th excess (R2 = 0.73; excluding 2 outliers). Remineralisation of POC in the twilight zone was particularly efficient relative to POC export resulting in negligible bathypelagic (&gt; 600 m) POC export fluxes in the sub-Antarctic zone, the Polar Front zone and the northern Weddell Gyre, while the subtropical zone as well as the vicinity of the SACCF had significant deep POC fluxes.

Description of the biogeochemical features of the subtropical southeastern Atlantic and the Southern Ocean south of South Africa during the austral summer of the International Polar Year

Abstract. Meridional and vertical distributions of several biogeochemical parameters were studied along a section in the southeastern Atlantic and the Southern Ocean south of South Africa during the austral summer 2008 of the International Polar Year to characterize the biogeochemical provinces and to assess the seasonal net diatom production. Based on analyses of macro-nutrients, ammonium (NH4), chlorophyll a, (Chl a), phaeopigments, biogenic silica (BSi), particulate inorganic carbon (PIC), and particulate organic carbon and nitrogen (POC and PON, respectively), four biogeochemical domains were distinguished along the section: the subtropical Atlantic, the confluence zone of the subtropical and subantarctic domains, the Polar Frontal Zone (PFZ) in the Antarctic Circumpolar Current (ACC), and the north-eastern branch of the Weddell Gyre. The subtropical region displayed extremely low nutrient concentrations featuring oligotrophic conditions, and sub-surface maxima of Chl a and phaeopigments never exceeded 0.5 µg L−1 and 0.25 µg L−1, respectively. The anticyclonic and cyclonic eddies crossed in the Cape Basin were characterized by a deepening and a rise, respectively, of the nutrients isoclines. The confluence zone of the subtropical domain and the northern side of the ACC within the subantarctic domain displayed remnant nitrate and phosphate levels, whereas silicate concentrations kept to extremely low levels. In this area, Chl a level of 0.4–0.5 µg L−1 distributed homogenously within the mixed layer, and POC and PON accumulated to values up to 10 µM and 1.5 µM, respectively, indicative of biomass accumulation along the confluence zone during the late productive period. In the ACC domain, the Polar Frontal Zone was marked by a post-bloom of diatoms that extended beyond the Polar Front (PF) during this late summer condition, as primarily evidenced by the massive depletion of silicic acid in the surface waters. The accumulation of NH4 to values up to 1.25 µM at 100 m depth centred on the PF and the accumulation of BSi up to 0.5 µM in the surface waters of the central part of the PFZ also featured a late stage of the seasonal diatom bloom. The silica daily net production rate based on the seasonal depletion of silicic acid was estimated to be 11.9 ± 6.5 mmol m−2 d−1 in the domain of the vast diatom post-bloom, agreeing well with the previously recorded values in this province. The Weddell Gyre occasionally displayed relative surface depletion of silicic acid, suggesting a late stage of a relatively minor diatom bloom possibly driven by iceberg drifting releases of iron. In this domain the estimated range of silica daily net production rate (e.g. 21.1 ± 8.8 mmol m−2 d−1) is consistent with previous studies, but was not significantly higher than that in the Polar Front region.