South Of Polar Front Research Articles

Hydrography of the Southern Ocean fronts – The zooplankton link: A detailed study in the Indian Ocean sector

Despite its importance, zooplankton diversity, biomass and abundance are still poorly known in the remote Indian sector of the Southern Ocean (SO). Therefore, systematic observations of zooplankton and hydrography have been made in the Indian sector of the SO, with a focus on Copepoda. We collected the data along 57° E transect during austral summer and examined various oceanographic locations in the central Indian Ocean sector of the SO lying in the east of the Kerguelen Plateau. The contribution of copepod abundance was substantial (85 %) to the total zooplankton biomass across the entire study region. We observed surface stratification in the Subtropical Front (STF) and Subantarctic Front (SAF); on the other hand, subsurface stratification was encountered in the Polar Front (PF) and South of PF (S-PF). As a result, nutrients were lower in warmer waters than in colder waters. On the other hand, zooplankton abundance was higher in colder waters than in warmer waters. The highest zooplankton grazing impact was recorded in the PF zone, where zooplankton removed ∼ 81 % of the daily phytoplankton production, while north of the PF zooplankton grazing was generally low. Four copepod species were shown to be highly connected to oceanic fronts. The vertical distribution of zooplankton abundance and species composition was chiefly driven by physical and biological conditions in the Indian sector of the SO. As a result, our findings indicate that physical processes play an important role in organizing plankton diversity, which ultimately determines the fate of the foodweb structure and the biological carbon pump in the studied area.

Plankton diversity and dynamics in the upper surface of the Indian sector of the Southern Ocean ecosystem and biogeochemical processes

In the Southern Ocean (SO) marine ecosystem, the interplay between CO2 sink and climate change is closely linked to the shifting plankton communities, leading to potentially wide-ranging biogeochemical effects. Within the Indian sector of the Southern Ocean (ISSO), the phytoplankton community was predominantly composed of Fragilariopsis spp., followed by Chaetoceros sp., while the zooplankton consisted mainly of Pleuromamma spp. and Oithona similis. Plankton diversity is collectively influenced by variables such as temperature, photosynthetically available radiation (PAR), and macronutrient availability. During the austral summer of 2019, significant latitudinal variations in phytoplankton were observed at major fronts in the region, including the Subtropical Front (STF), Sub Antarctic Front (SAF), Polar Front (PF), and South of Polar Front (SPF). At the STF, flagellates dominated (87%), while their presence decreased to 12% at the SPF, while diatoms progressively increased from the STF (<1%) to the SPF (87%) through the SAF and PF. Chlorophyll a (Chl-a) concentrations at the surface were highest in the PF (0.62 mg m−3), followed by the SPF (0.56 mg m−3), SAF (0.51 mg m−3), and STF (0.28 mg m−3). The integrated Chl-a values across the water column were higher at the SPF (21.49 mg m−2) than at the STF (14.73 mg m−2). The dominant diatom species at the surface along the fronts, except for STF, was Fragilariopsis spp. (∼5 × 105 Cells L−1), followed by Chaetoceros sp. (∼1 × 105 Cells L−1), and Coscinodiscus sp. (∼5 × 104 Cells L−1). Conversely, at the STF, the most abundant flagellate species were Gyrodinium spp. (∼2.4 × 104 Cells L−1), followed by Protoperidinium sp. (∼2 × 104 Cells L−1), and Amphidinium sp. (∼1 × 104 Cells L−1). Zooplankton biovolume in the upper 200 m ranged from 0.71 to 10 ml 100 m−3, with relatively higher biovolume and numerical abundance found in the SAF compared to the PF. The higher Chl-a concentration in the PF and lower zooplankton biovolume could have significant negative consequences for higher trophic levels. PAR showed higher values up to 40 m than observed depths along the fronts. Additionally, variations in salinity and temperature were recorded, with a significant decline in temperature from the STF to the SPF. Nitrate (NO3−), phosphate (PO43−), and silicate (SiO44−) concentrations were higher at the SPF than at lower latitudes. Overall, these findings indicate that the plankton community composition in the study region is influenced by a diverse set of governing variables over time, highlighting the significant implications for the regional biogeochemical cycle.

Improved winter data coverage of the Southern Ocean CO2 sink from extrapolation of summertime observations

The Southern Ocean is an important sink of anthropogenic CO2, but it is among the least well-observed ocean basins, and consequentially substantial uncertainties in the CO2 flux reconstruction exist. A recent attempt to address historically sparse wintertime sampling produced ‘pseudo’ wintertime observations of surface pCO2 using subsurface summertime observations south of the Antarctic Polar Front. Here, we present an estimate of the Southern Ocean CO2 sink that combines a machine learning-based mapping method with an updated set of pseudo observations that increases regional wintertime data coverage by 68% compared with the historical dataset. Our results confirm the suggestion that improved winter coverage has a modest impact on the reconstruction, slightly strengthening the uptake trend in the 2000s. After also adjusting for surface boundary layer temperature effects, we find a 2004-2018 mean sink of −0.16 ± 0.07 PgC yr−1 south of the Polar Front and −1.27 ± 0.23 PgC yr−1 south of 35°S, consistent with independent estimates from atmospheric data.

Seasonal and interannual variability in abundance and population development of Calanus finmarchicus at the western entrance to the Barents Sea, 1995–2019

Data on copepodid abundance and stage composition of Calanus finmarchicus was collected with seasonal resolution (5–6 times a year) from four stations along an oceanographic transect across the western Barents Sea Opening over 25 years (1995–2019). The stations were located in the Norwegian Coastal Current (NCC), inflowing Atlantic water (two stations), and near the Arctic polar front south of Bear Island. Mean copepodite abundance in the water column for the four stations increased from a level around 1000–10,000 individuals m−2 in winter to 30,000–100,000 individuals m−2 in summer (maximum 670,000 m−2). The overwintering (G0) population was dominated by copepodite stage 5 (CV) (40–70%) but with a relatively high fraction also of stage CIV (25–35%) in January. During winter, there was a progressive development of CVs into adult males and females, with mean abundances from 1500 to 4000 adult females m−2 in April and May for the stations in the NCC and Atlantic water. Young copepodids of the new spring generation (G1) appeared with relatively high abundances in April at the NCC station and in May at the Atlantic water stations. The data showed a gradient of later development from south to north, reflected both in the maturation of the overwintering generation and in the development of the spring generation. Back-calculations based on temperature-dependent development time suggested peak spawning around late April to mid-May for the Atlantic water stations, and a month or so earlier (in March) for the NCC station, during an early phase of the spring phytoplankton growth. The spring generation developed as a distinct cohort but with large interannual variability in numbers. In Atlantic water, the composition of the copepodid developmental stages in August resembled the situation in June, although it was slightly more advanced. This is interpreted to reflect development of a second generation (G2) spawned upstream sometime in June or early July. The June data suggests that only a small fraction of the G1 cohort develops into adults that spawn and form the G2. There was a tendency of increased summer abundance of C. finmarchicus in the inflow region of the southwestern Barents Sea after about 2005. This may be related to the wind regime with more eastward wind-stress at the Barents Sea opening in recent ‘high Calanus’ years compared to ‘low Calanus’ years in the early 2000s.

Interannual Climate Variability in the West Antarctic Peninsula under Austral Summer Conditions

This study aimed to describe the interannual climate variability in the West Antarctic Peninsula (WAP) under austral summer conditions. Time series of January sea-surface temperature (SST) at 1 km spatial resolution from satellite-based multi-sensor data from Moderate Resolution Imaging Spectrometer (MODIS) Terra, MODIS Aqua, and Visible Infrared Imager Radiometer Suite (VIIRS) were compiled between 2001 and 2020 at localities near the Gerlache Strait and the Carlini, Palmer, and Rothera research stations. The results revealed a well-marked spatial-temporal variability in SST at the WAP, with a one-year warm episode followed by a five-year cold episode. Warm waters (SST &gt; 0 °C) reach the coast during warm episodes but remain far from the shore during cold episodes. This behavior of warm waters may be related to the regional variability of the Antarctic Circumpolar Current, particularly when the South Polar Front (carrying warm waters) reaches the WAP coast. The WAP can be divided into two zones representing two distinct ecoregions: the northern zone (including the Carlini and Gerlache stations) corresponds to the South Shetland Islands ecoregion, and the southern zone (including the Palmer and Rothera stations) corresponds to the Antarctic Peninsula ecoregion. The Gerlache Strait is likely situated on the border between the two ecoregions but under a greater influence of the northern zone. Our data showed that the Southern Annular Mode (SAM) is the primary driver of SST variability, while the El Niño Southern Oscillation (ENSO) plays a secondary role. However, further studies are needed to better understand regional climate variability in the WAP and its relation with SAM and ENSO; such studies should use an index that adequately describes the ENSO in these latitudes and addresses the limitations of the databases used for this purpose. Multi-sensor data are useful in describing the complex climate variability resulting from the combination of local and regional processes that elicit different responses across the WAP. It is also essential to continue improving SST approximations at high latitudes.

Warming temperatures and ectoparasitic sea lice impair internal organs in juvenile Atlantic salmon

Without insights into the threats affecting species across their distributions and throughout their annual cycles, effective conservation management cannot be applied. The Whenua Hou diving petrel Pelecanoides whenuahouensis (WHDP) is a Critically Endangered small seabird whose offshore habits and threats are poorly understood. We tracked WHDPs year-round in 2015/16, 2017/18, and 2018/19 using global location-sensing immersion loggers to identify offshore distribution, movements, behaviour, and overlap with commercial fishing effort. During the breeding period, WHDPs ranged from southern Aotearoa (New Zealand) to Maukahuka (Auckland Islands). After breeding, WHDPs migrated southwest towards the Polar Front south of Australia, exhibited clockwise movements, and returned to their breeding grounds via the Subantarctic Front. During the non-breeding period, WHDPs exhibited extreme aquatic behaviour and spent &gt;95% of their time on, or under, water. The core areas used consistently during breeding and non-breeding periods warrant listing as Important Bird and Biodiversity Areas. Spatiotemporal overlap of commercial fishing effort with breeding distributions was considerable, in contrast with non-breeding distributions. Spatiotemporal management of anthropogenic activity around the breeding colony during the breeding period could help protect WHDPs, but such measures should be subjected to a structured decision-making framework. Our results illustrate the importance of year-round studies to inform conservation of marine species.

Year-round offshore distribution, behaviour, and overlap with commercial fisheries of a Critically Endangered small petrel

Without insights into the threats affecting species across their distributions and throughout their annual cycles, effective conservation management cannot be applied. The Whenua Hou diving petrel Pelecanoides whenuahouensis (WHDP) is a Critically Endangered small seabird whose offshore habits and threats are poorly understood. We tracked WHDPs year-round in 2015/16, 2017/18, and 2018/19 using global location-sensing immersion loggers to identify offshore distribution, movements, behaviour, and overlap with commercial fishing effort. During the breeding period, WHDPs ranged from southern Aotearoa (New Zealand) to Maukahuka (Auckland Islands). After breeding, WHDPs migrated southwest towards the Polar Front south of Australia, exhibited clockwise movements, and returned to their breeding grounds via the Subantarctic Front. During the non-breeding period, WHDPs exhibited extreme aquatic behaviour and spent &gt;95% of their time on, or under, water. The core areas used consistently during breeding and non-breeding periods warrant listing as Important Bird and Biodiversity Areas. Spatiotemporal overlap of commercial fishing effort with breeding distributions was considerable (35%), in contrast with non-breeding distributions (0%). Spatial restrictions of anthropogenic activity around the breeding colony during the breeding period could help protect WHDPs, but such measures should be subjected to a structured decision-making framework. Our results illustrate the importance of year-round studies to inform conservation of marine species.

SST front anchored mesoscale feature of surface wind in the southern Indian Ocean

Using 28-year satellite-borne Special Sensor Microwave Imager observations, features of high-wind frequency (HWF) over the southern Indian Ocean are investigated. Climatology maps show that high winds occur frequently during austral winter, located in the open ocean south of Polar Front in subpolar region, warm flank of the Subantarctic Front between 55°E–78°E, and south of Cape Agulhas, where westerly wind prevails. The strong instability of marine atmospheric boundary layer accompanied by increased sensible and latent heat fluxes on the warmer flank acts to enhance the vertical momentum mixing, thus accelerate the surface winds. Effects of sea surface temperature (SST) front can even reach the entire troposphere by deep convection. HWF also shows distinct interannual variability, which is associated with the Southern Annual Mode (SAM). During positive phase of the SAM, HWF has positive anomalies over the open ocean south of Polar Front, while has negative anomalies north of the SST front. A phase shift of HWF happened around 2001, which is likely related to the reduction of storm tracks and poleward shift of westerly winds in the Southern Hemisphere.

Mercury and methylmercury in the Atlantic sector of the Southern Ocean

Oceans constitute one of the most important reservoirs for mercury. In order to provide a first insight into the concentrations of Hg species in the Atlantic sector of the Southern Ocean a sampling campaign was carried out south of the Polar Front.Water samples taken at discrete depths from the surface down to 300m at six stations were analysed for total Hg (HgT), methylmercury (MeHg) and other interpretative parameters such as salinity, temperature, dissolved and particulate organic carbon, dissolved oxygen, chlorophyll and inorganic nutrients.Results showed a high spatial variability in the concentrations of HgT and MeHg. HgT (0.93±0.69ngL−1) and MeHg (0.26±0.12ngL−1) levels were similar or higher than those reported in previous works in high latitude studies. The highest values were found at a location (−53°, 10°E) south of the South Polar Front, an area of strong gradients caused by the mixing of different water masses. Vertical profiles showed a great variability even for those stations sampled at the same location or an area dominated by the same oceanographic features. A decrease of HgT and a consequent increase in MeHg with depth was observed in some sites, suggesting the occurrence of Hg-methylation process, while at other stations, a concurrent decrease or increase of both mercury species was observed. In spite of these differences, an overall positive correlation between HgT and MeHg was observed. Differences between vertical profiles of Hg species were attributed to favourable environmental conditions for Hg methylation. The highest proportion of MeHg (% of HgT) was observed in sites with low dissolved oxygen or highest estimated remineralization rates.The results obtained in this study show that the Hg distribution and speciation in the Atlantic sector of the SO is comparable (or in some sites higher) to the ones published for the other open ocean regions. However, the concentrations of MeHg in this area are more dependent on the environmental conditions than on the total concentration of Hg present in the water.

Persistent climatic and oceanographic oscillations in the subpolar North Atlantic during the MIS 6 glaciation and MIS 5 interglacial

AbstractRapidly accumulating sediments from the Björn drift deposit south of Iceland are studied for comparison of glacial/interglacial climate changes related to millennial variability of the subpolar surface and deep ocean circulation in the North Atlantic. High‐resolution faunal, isotopic, and sedimentary analyses reveal a strong multimillennial climatic variability interpreted as oscillations in heat transport westward south of Iceland during marine isotope stage 6 (MIS 6), possibly related to the strength of the subpolar gyre (SPG). The oscillations persisted from MIS 6 through the following interglacial (MIS 5), although with diminished magnitude, and were respectively characterized by repeated advances of the polar front south of Iceland during MIS 6 and southward migrations of the Arctic front due to cold surface outflow through the East Greenland and East Iceland Currents during MIS 5. Incursions of cold, fresh surface waters, and drifting ice affected the dynamics of the SPG, episodically causing it to weaken and contract to the northwest. During these intervals of diminished SPG, the northward transport of subtropical heat and salt was strengthened and preferentially conveyed to the northeast past Iceland, enhancing deep‐water formation in the Nordic Seas. By contrast, when the SPG was strong, more subtropical water and its associated heat were entrained within the relatively warm Irminger Current flowing westward south of Iceland. These oceanographic oscillations were associated with repeated multimillennial cooling and warming episodes during the glacial stage MIS 6, equivalent to the Dansgaard‐Oeschger cycles of the last glaciation.

Tendencias espaciales y temporales en la distribución de macronutrientes en aguas superficiales del Pasaje Drake

The Drake Passage (DP) links the Southern Pacific and Atlantic Oceans, and acts as a physical narrowing for the largest current of the World Ocean, the Antarctic Circumpolar Current (ACC). This result in a constriction of the main circumpolar fronts related to the ACC: the Subantarctic Front, the Polar Front, the Southern Antarctic Circumpolar Current Front and the Continental Water Boundary Front. Previous studies showed that the DP is a high-nutrient low-chlorophyll (HNLC) region, but studies on the interannual and seasonal macronutrients’ fluctuations in relation with chlorophyll and frontal areas in the DP are scarce. This study analyzes the spatial and temporal trends of nitrate, phosphate, and silicate on surface samples from 113 oceanographic stations in the DP during summer and early fall of 2001 to 2004, in relation with the different fronts and chlorophyll-a concentration. The results revealed an increase of nutrients towards the south. Nitrate showed values from 3 μM at 55o S to 27 μM at 63o S, whereas values from 0.3 μM at 55o S to 2.6 μM at 63o S were observed for phosphate. Silicate increased considerably south of Polar Front, from 0 μM at 55o S to 74 μM at 62o S. The nutrients concentration was significantly higher in summer than in early fall and showed a trend to decrease over the studied years. A complex sum of biogeochemical processes would generate the main seasonal trends, while atmospheric-oceanic processes such as Southern Annular Mode and El Nino Southern Oscillation, would be responsible of observed nutrients interannual variability.

Are boundary conditions in surface productivity at the Southern Polar Front reflected in benthic activity?

In austral summer 2012, during the expedition ANT-XXVIII/3 on board RV Polarstern, two sites were sampled 1600km apart in the South Polar Front area (52°S) at the boundary of different productivity regimes for meio- and macrobenthos using a multiple-corer and an epibenthic sledge, respectively. Patterns in density and abundance data were compared between different size classes of the benthos and interpreted in relation to surface primary productivity data and sediment oxygen consumption. We tested the hypothesis that long-term satellite-derived surface phytoplankton biomass, in situ real time biomass, and productivity measurements at the surface and throughout the euphotic zone are reflected in abyssal benthos densities, abundances and activity. Specifically, we investigated the effect of boundary conditions for lower and higher surface productivity. Surface and integrated to 100m depth biomass and primary productivity measurements vary stations, with the lowest values at station 85 (0.083mgChl-am−3 at surface, 9mgChl-am−2 and 161mgCm−2 d−1− integrated over the first 100m depth), and the highest values at station 86 (2.231mgChl-am−3 at surface, 180mgChl-am−2 and 2587mgCm−2d−1 integrated over first 100m depth). Total meiofaunal densities varied between 102 and 335individuals/10cm². Densities were the highest at station 86-30 (335 individuals) and lowest at station 81-13 (102 individuals). Total macrofaunal densities (individuals/1000m²) varied between 26 individuals at station 81-17 and 194 individuals at station 86-24. However, three EBS hauls were taken at station 86 with a minimum of 80 and a maximum of 194 individuals. Sediment oxygen consumption did not vary significantly between stations from east to west. Bentho-pelagic coupling of meio- and macrobenthic communities could not be observed in the South Polar Front at the boundary conditions from low to high surface productivity between stations 81 and 86.

Composition and abundance of epibenthic-sledge catches in the South Polar Front of the Atlantic

An epibenthic sledge (EBS) was deployed at seven different deep-sea stations along the South Polar Front of the Atlantic in order to explore the composition and abundance of macrofaunal organisms and to identify the most abundant taxa in this transition zone to the Southern Ocean. In total 3,130 specimens were sampled by means of the EBS on board of RV Polarstern during the expedition ANT-XXVIII/3 in the austral summer of 2012. Benthic and suprabenthic Crustacea occurred to be most frequent in the samples. Among those, copepods were by far most numerous, with 1,585 specimens followed by the peracarid taxa Isopoda (236ind.), Amphipoda (103ind.), Tanaidacea (78ind.) and Cumacea (50ind.). Annelida were represented by a high number of specimens belonging to different polychaete taxa (404ind.). The molluscan fauna was clearly dominated by Bivalvia (255ind.), followed in numbers of specimens by Gastropoda (47ind.). The deep-sea benthos sampled along the Southern Polar Front occurred in surprisingly low abundances, contrasting the largely high surface productivity of the area. Numbers of specimens across different macrofaunal taxa and especially of peracarid crustaceans underscored by far those from South Ocean sites at higher latitudes in the Weddell Sea.

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.

Analysis of the variability of temperature gradient in the ocean frontal zones based on satellite data

AVHRR MCSST data for the periods 1982–2000 (mean weekly data) were used to calculate mean gradient fields in the ocean for different periods of time. Three-month averaged sea surface temperature gradients (SSTG) and their mean seasonal variations have been studied for 25 points in the large-scale oceanic fronts zones. Major oceanic fronts in the Atlantic and Pacific have been identified and compared in literature. In the North Atlantic and Pacific, the areas under study were the North Polar Front and Subpolar Fronts. In the South Atlantic and Pacific we studied the region of the Antarctic Circumpolar Current (ACC) and the fronts formed by this current, known as the South Polar Front, and the Subantarctic Front. SSTG were also calculated for El Niño (Southeast Pacific) and Benguela Current (Southeast Atlantic).In warm periods seasonal SSTG in the North Atlantic markedly increased and exhibit some interannual cycles. The correlation between the North Atlantic Oscillation index and seasonal SSTG for a single point in the Gulf Stream zone can be the key point for evaluation of heat transfer by the currents to the coast of East Europe.In the Southern Atlantic, the SSTG values are low during the cold period (summer in the southern hemisphere) in the ACC zone and increase in the warm season (winter in the southern hemisphere). It also exhibits interannual cycles.In the Northwest Pacific for some points in the Subpolar Front the SSTG values are high in the cold period (winter). Here at seven points in the spring of 1993 and 2000 the calculations disclosed significant increase of the gradient. In these years, the anomalous SSTG in Subpolar Front and South Polar Front were found to vary synchronously in both hemispheres, with maximum intensity in spring (North Pacific) and in summer (South Pacific).Mean annual SSTG in the El Niño zone and south oscillation index have been found to exhibit some correlation.Major jet currents periodically form high-gradient temperature fields and from the temperature satellite data we can derive information about variation in the large-scale fronts in the Global Ocean.

Genetic variability of the Metridia lucens complex (Copepoda) in the Southern Ocean

Analysis of the fragment of the mtDNA gene СО1 has revealed two genetically distinct groups of the Metridia lucens complex in the South part of the Atlantic. While the intragroup polymorphism was less than 1%, the intergroup difference was about 9.5%. These two groups may be considered as representing two cryptic species within the M. lucens complex: M. lucens North and M. lucens South. These forms are found mainly to the North and to the South of the South Polar Front. The results are confirmed by an analysis of the nuclear rDNA ITS1-5.8S-ITS2 fragment. No hybrids between the two forms were detected. M. lucens North inhabits the waters >4°C that allow us to discuss the temperature as putative limiting factor for the distribution.

남극 하계 드레이크 해협의 미세생물 먹이망에서 종속영양 미소형 및 소형플랑크톤의 역할

To elucidate the trophic role of heterotrophic nano- and microplankton (HNMP), we investigated their biomass, community structure, and herbivory in three different water masses, namely, south of Polar Front (SPF), Polar Front Zone (PFZ), the Sub-Antarcitc Front (SAF) in the Drake Passage in the Southern Ocean, during the austral summer in 2002. We observed a spatial difference in the relative importance of the dominant HNMP community in these water masses. Ciliates accounted for 34.7% of the total biomass on an average in the SPF where the concentration of chlorophyll-a was low with the dominance of pico- and nanophytoplankton. Moreover, the importance of ciliates declined from the SPF to the SAF. In contrast, heterotrophic dinoflagellates (HDFs) were the most dominant grazers in the PFZ where the concentration of chlorophyll-a was high with the dominance of net phytoplankton. HNMP biomass ranged from 321.9 to 751.4 ㎎Cm?² and was highest in the PFZ and lowest in the SPF. This result implies that the spatial dynamic of HNMP biomass and community was significantly influenced by the composition and concentration of phytoplankton as a food source. On an average, 75.6%, 94.5%, and 78.9% of the phytoplankton production were consumed by HNMP in the SPF, PFZ, and SAF, respectively. The proportion of phytoplankton grazed by HNMP was largely determined by the composition and biomass of HNMP, as well as the composition of phytoplankton. However, the herbivory of HNMP was one of the most important loss processes affecting the biomass and composition of phytoplankton, particularly in the PFZ. Our results suggest that the bulk of the photosynthetically fixed carbon was likely reprocessed by HNMP rather than contributing to the vertical flux in Drake Passage during the austral summer in 2002.

Tracking the Polar Front south of New Zealand using penguin dive data

Nearly 36,000 vertical temperature profiles collected by 15 king penguins are used to map oceanographic fronts south of New Zealand. There is good correspondence between Antarctic Circumpolar Current (ACC) front locations derived from temperatures sampled in the upper 150 m along the penguin tracks and front positions inferred using maps of sea surface height (SSH). Mesoscale features detected in the SSH maps from this eddy-rich region are also reproduced in the individual temperature sections based on dive data. The foraging strategy of Macquarie Island king penguins appears to be influenced strongly by oceanographic structure: almost all the penguin dives are confined to the region close to and between the northern and southern branches of the Polar Front. Surface chlorophyll distributions also reflect the influence of the ACC fronts, with the northern branch of the Polar Front marking a boundary between low surface chlorophyll to the north and elevated values to the south.

Width of the North Cape Current and location of the Polar Front in the western Barents Sea

Earlier results have shown that stronger southwesterly winds give higher temperatures, stronger velocities and higher transports in the North Cape Current (NCaC) in the western Barents Sea. This investigation shows that the wind field causing the stronger southwesterlies also causes a wider NCaC, i.e., the NCaC becomes warmer and wider at the same time. In warm periods, the NCaC seems to be a two‐core current system, while it has only one wider core in cold periods. The reason for the fluctuations in the width of the NCaC is a non‐uniform wind field across the current. The varying width of the NCaC affects the location of the Polar Front south of Bear Island. The location of this front is not as stationary as earlier believed and in warm periods with strong wind, the front is located further upslope than in cold periods.

The Southern Ocean's biological pump during the Last Glacial Maximum

Ice core records from Antarctica show large (∼80 ppm) and regular climate-related changes in atmospheric CO2, with minimum values during glacial periods and maximum values during peak interglacials. The suggested role of the Southern Ocean in driving these changes is based on either the potential for increased utilization of surface nutrients or the potential for decreased ventilation of deep waters during glacial times. Several recent studies have invoked increased stratification of the Southern Ocean to explain lower glacial atmospheric CO2 levels in terms of reduced exchange of CO2 between the deep sea and the atmosphere. A northward displacement and/or substantial weakening of the westerly winds during glacial periods are implicit in the scenarios that invoke enhanced stratification. However, both circulation models and proxy results argue against a weakening of the westerlies. In fact, the mean flow of the Antarctic Circumpolar Current and wind-driven upwelling during the Last Glacial Maximum (LGM) are thought to be at least as vigorous as those which exist today. Given these boundary conditions, we offer two (competing) scenarios for ecosystem structure and export production of the glacial Southern Ocean. The first scenario satisfies all proxy records for nutrient utilization and phytoplankton growth rate, and requires increased (relative to today) nitrate utilization south of the Antarctic Polar Front (APF) by phytoplankton other than diatoms, together with a shift in the zone of maximum diatom growth from south (interglacials) to north (glacials) of the APF. The second scenario has reduced growth of all phytoplankton species south of the APF during glacials, and a shift in the zone of maximum export production to the north of the Polar Front. The principal weakness of the first scenario is that there is little sedimentary evidence to support the increased export of particulate organic carbon required by the inferred increase in nitrate utilization south of the APF. The principal weakness of the second scenario is that it requires a change in the isotopic fractionation of nitrogen by phytoplankton growing south of the Polar Front for reasons presently unknown. The Southern Ocean's biological pump at the LGM has yet to be characterized unequivocally. On observe dans les carottes de glace provenant du continent Antarctique des cycles de grandes amplitudes (∼80 ppm) dans la concentration atmosphérique du CO2, qui sont reliés aux cycles climatiques, avec des valeurs minimum durant les époques glaciaires, et des valeurs maximum durant les époques inter-glaciaires. Un rôle de l’océan austral dans ces cycles climatiques a été suggéré, compte tenu du grand surplus de nutriments dans les eaux de surface, et de la possibilité que les eaux profondes auraient été moins ventilées pendant les époques glaciaires. Dans les dernières années, plusieurs études ont associées les concentrations moins importantes de CO2 dans l’atmosphère durant les époques glaciaires à une stratification plus importante de l’océan austral, ce qui aurait réduit l’échange de CO2 entre l’atmosphère et l’océan profond. En même temps, on remarque un déplacement vers le nord, et/ou une réduction importante de la force des vents d’ouest durant les époques glaciaires. Cependant, des simulations numériques, ainsi que des données proxys, ne supportent pas l’hypothèse d’une réduction des vents d’ouest. Au contraire, pendant le Dernier Maximum Glaciaire (DMG), le forcage des vents auraient entrainé un courant antarctique circumpolaire (ACC), ainsi que des courants ascendants au moins aussi vigoureux que ceux d’aujourd’hui. En tenant compte de ces conditions aux bords, nous proposons deux scénarios possibles de l’écologie et de l’éxportation de carbone dans l’océan austral pendant la dernière époque glaciaire. Le premier scénario est en accord avec toutes les données proxys ayant attrait a l’utilisation des éléments nutritifs, ainsi que le taux de croissance des phytoplanctons. Ce scénario implique que l’utilisation de nitrate par les phytoplanctons non-diatomés est plus importante durant les époques glaciaires qu’aujourd’hui, et aussi que la zone préférée de diatomées soit au sud du Front Polaire Antarctique (FPA), durant les époques interglaciaires, et au nord du FPA durant les époques glaciaires. Le deuxième scénario, par contre, implique une réduction de la productivité de toutes les espèces de phytoplanctons au sud du FPA durant le DMG, ainsi qu’un déplacement de la zone maximale d’éxportation de carbone au nord du PFA. Le principal point faible du premier scénario est lié à l’absence de données suggérant une augmentation dans l’exportation de carbone organique particulaire, dans les sédiments au sud du PFA. Le principal point faible du deuxième scénario est qu’il requière un fractionnement isotopique de l’azote par les phytoplanctons, au sud du PFA, durant la dernière époque glaciaire qui est différent de celui d’aujourd’hui. Bref, on est toujours à la recherche d’une caracterisation non-équivoque de la “pompe biologique” dans l’océan austral durant le DMG.