Polar Frontal Zone Research Articles

Modelling the life cycle of Salpa thompsoni

Salpa thompsoni is an important grazer in the Southern Ocean. It is found from the Subtropical Convergence southward to the coastal Antarctic Seas but being most abundant in the Antarctic Polar Frontal Zone. Low temperatures appear to negatively affect their development, limiting their ability to occur in the krill dominated high Antarctic ecosystems. Yet reports indicate that with ocean warming S. thompsoni have experienced a southward shift in their distribution. As they are efficient filter feeders, this shift can result in large-scale changes in the Southern Ocean ecosystem by increasing competitive or predatory interactions with Antarctic krill. To explore salp bloom dynamics in the Southern Ocean a size-structured S. thompsoni population model was developed with growth, consumption, reproduction and mortality rates dependent on temperature and chlorophyll a conditions. The largest uncertainties in S. thompsoni population ecology are individual and population growth rates, with a recent study identifying the possibility that the life cycle could be much shorter than previously considered. Here we run a suite of hypothesis scenarios under various environmental conditions to determine the most appropriate growth rate. Temperature and chlorophyll a were sufficient drivers to recreate seasonal and interannual dynamics of salp populations at two locations. The most suitable growth model suggests that mean S. thompsoni growth rates are likely to be ∼1 mm body length d−1, 2-fold higher than previous calculations. S. thompsoni biomass was dependent on bud release time, with larger biomass years corresponding to bud release occurring during favorable environmental conditions; increasing the survival and growth of blastozooids and resulting in higher embryo release. This model confirms that it is necessary for growth and reproductive rates to be flexible in order for the salp population to adapt to varying environmental conditions and provides a framework that can examine how future salp populations might respond to climate change.

A survival pack for escaping predation in the open ocean: amphipod – pteropod associations in the Southern Ocean

Hyperiidean amphipods are a major prey for fish and seabirds. In the Southern Ocean, they are particularly abundant, with distributions ranging from the Polar Frontal Zone to Antarctic shelf waters. The species Hyperiella dilatata has previously been reported to show a peculiar anti-predatory behaviour: It captures chemically protected, gymnosome pteropods in the water column and carries them on its dorsum, like a backpack. We report this association at four oceanic sampling sites between latitudes 45° and 71° S. Molecular barcodes of both hosts and pteropods are provided and compared with those of other hyperiidean and pteropod specimens. Morphological identifications as well as molecular analyses show a so far undocumented association of Hyperiella antarctica with the pteropod Spongiobranchaea australis in the Polar Frontal Zone (Lazarev Sea). H. dilatata carried Clione limacina antarctica specimens in the Weddell Sea, as recorded previously for the Ross Sea. Lengths of the abducted pteropods varied between 1 and 5 mm, with the biggest pteropod measuring more than half the host’s size. One of the abducting amphipods was a female carrying eggs. The formation of such tandem is known to be very efficient as protection from visually hunting icefish in the crystal-clear coastal waters around the Antarctic continent; however, in the open ocean, this behaviour was so far undocumented. Here, we develop hypotheses on its origin and function.

Nitrous oxide variability at sub-kilometre resolution in the Atlantic sector of the Southern Ocean.

The Southern Ocean is an important region for global nitrous oxide (N2O) cycling. The contribution of different source and sink mechanisms is, however, not very well constrained due to a scarcity of seawater data from the area. Here we present high-resolution surface N2O measurements from the Atlantic sector of the Southern Ocean, taking advantage of a relatively new underway setup allowing for collection of data during transit across mesoscale features such as frontal systems and eddies. Covering a range of different environments and biogeochemical settings, N2O saturations and sea-to-air fluxes were highly variable: Saturations ranged from 96.5% at the sea ice edge in the Weddell Sea to 126.1% across the Polar Frontal Zone during transit to South Georgia. Negative sea-to-air fluxes (N2O uptake) of up to −1.3 µmol m−2 d−1 were observed in the Subantarctic Zone and highest positive fluxes (N2O emission) of 14.5 µmol m−2 d−1 in Stromness Bay, coastal South Georgia. Although N2O saturations were high in areas of high productivity, no correlation between saturations and chlorophyll a (as a proxy for productivity) was observed. Nevertheless, there is a clear effect of islands and shallow bathymetry on N2O production as inferred from supersaturations.

Habitat modelling of tracking data from multiple marine predators identifies important areas in the Southern Indian Ocean

AbstractAimThe distribution of marine predators is driven by the distribution and abundance of their prey; areas preferred by multiple marine predator species should therefore indicate areas of ecological significance. The Southern Ocean supports large populations of seabirds and marine mammals and is undergoing rapid environmental change. The management and conservation of these predators and their environment relies on understanding their distribution and its link with the biophysical environment, as the latter determines the distribution and abundance of prey. We addressed this issue using tracking data from 14 species of marine predators to identify important habitat.LocationIndian Ocean sector of the Southern Ocean.MethodsWe used tracking data from 538 tag deployments made over a decade at the Subantarctic Prince Edward Islands. For each real track, we simulated a set of pseudo‐tracks that allowed a presence‐availability habitat modelling approach that estimates an animal's habitat preference. Using model ensembles of boosted regression trees and random forests, we modelled these tracks as a response to a set of 17 environmental variables. We combined the resulting species‐specific models to evaluate areas of mean importance.ResultsReal tracking locations covered 39.75 million km2, up to 7,813 km from the Prince Edward Islands. Areas of high mean importance were located broadly from the Subtropical Zone to the Polar Frontal Zone in summer and from the Subantarctic to Antarctic Zones in winter. Areas of high mean importance were best predicted by factors including wind speed, sea surface temperature, depth and current speed.Main conclusionsThe models and predictions developed here identify important habitat of marine predators around the Prince Edward Islands and can support the large‐scale conservation and management of Subantarctic ecosystems and the marine predators they sustain. The results also form the basis of future efforts to predict the consequences of environmental change.

Abundances and test weights of living planktic foraminifers across the Southwest Indian Ocean: Implications for carbon fluxes

Calcifying marine organisms include planktic foraminifers that contribute to the marine carbon turnover by generating inorganic carbon production (CaCO3, shell) and flux. In this study, we have analyzed assemblages at the morphospecies level and abundances of living planktic foraminifers (LPF) from ten stations located in the Southwest Indian Ocean, and sampled in austral summer 2012. LPF density ranges from zero in the Subtropical Zone (STZ) to 944 individuals m−3 in the Polar Frontal Zone (PFZ), and is composed by up to 80% by the four species Neogloboquadrina pachyderma, Neogloboquadrina incompta, Globigerina bulloides, and Globigerinita uvula. For the entire region, we measured the individual morphometry and test mass (CaCO3) of 454 tests of living planktic foraminifers in order to calculate inorganic carbon standing stocks, as well as carbon flux. In the STZ, the average daily planktic foraminifer CaCO3 flux at 100m water depth is low (< 0.22mgm−2 d−1), whereas south of the Sub-Antarctic Zone (SAZ), in the PFZ and Antarctic Zone (AAZ), it reaches up to 49.41mgm−2 d−1, and 2.20mgm−2 d−1, respectively. The large regional variability in CaCO3 production and flux of LPF assemblages affects the marine carbonate system to varying degrees, depending on hydrological conditions. We conclude that recent changes in the position of hydrological fronts could induce a decrease in the LPF-related carbonate counter pump, which increases the oceanic uptake of CO2, and counteracts climate warming.

Modelled estimates of spatial variability of iron stress in the Atlantic sector of the Southern Ocean

Abstract. The Atlantic sector of the Southern Ocean is characterized by markedly different frontal zones with specific seasonal and sub-seasonal dynamics. Demonstrated here is the effect of iron on the potential maximum productivity rates of the phytoplankton community. A series of iron addition productivity versus irradiance (PE) experiments utilizing a unique experimental design that allowed for 24 h incubations were performed within the austral summer of 2015/16 to determine the photosynthetic parameters αB, PBmax and Ek. Mean values for each photosynthetic parameter under iron-replete conditions were 1.46 ± 0.55 (µg (µg Chl a)−1 h−1 (µM photons m−2 s−1)−1) for αB, 72.55 ± 27.97 (µg (µg Chl a)−1 h−1) for PBmax and 50.84 ± 11.89 (µM photons m−2 s−1) for Ek, whereas mean values under the control conditions were 1.25 ± 0.92 (µg (µg Chl a)−1 h−1 (µM photons m−2 s−1)−1) for αB, 62.44 ± 36.96 (µg (µg Chl a)−1 h−1) for PBmax and 55.81 ± 19.60 (µM photons m−2 s−1) for Ek. There were no clear spatial patterns in either the absolute values or the absolute differences between the treatments at the experimental locations. When these parameters are integrated into a standard depth-integrated primary production model across a latitudinal transect, the effect of iron addition shows higher levels of primary production south of 50° S, with very little difference observed in the subantarctic and polar frontal zone. These results emphasize the need for better parameterization of photosynthetic parameters in biogeochemical models around sensitivities in their response to iron supply. Future biogeochemical models will need to consider the combined and individual effects of iron and light to better resolve the natural background in primary production and predict its response under a changing climate.

Biogeographic distribution of extant Coccolithophores in the Indian sector of the Southern Ocean

Abstracts Water samples from nine vertical profiles down to 110 m water depth and 19 samples from the sea-surface were studied for coccolithophore abundance and distribution across oceanic frontal regions of the Indian sector of the Southern Ocean. Sampling was performed along a north-south transect (between 39°S and 65.49°S, ~ 57.3°E) during the 4th Indian Southern Ocean Expedition (between 31st January and 18th February 2010). Coccospheres and coccoliths were counted separately using a Scanning Electron Microscope (SEM). A total of 39 taxa (including morphotypes, types and subspecies) were recorded as intact coccospheres with abundances reaching up to 750 × 10 3 coccospheres/l. In addition, 85 taxa (including varieties, morphotypes) were counted as coccoliths reaching up to 900 × 10 5 coccoliths/l. Emiliania huxleyi was recognized as the most abundant species, accounting for > 86% of the total coccolithophore assemblage at each station. Elevated coccolithophore diversity was observed in the subtropical zone whereas high coccolithophore abundance was observed in the Subantarctic zone. A monospecific Emiliania huxleyi assemblage was recorded within and south of the Polar frontal zone. Three assemblages were recognized based on coccolithophore abundance and diversity. The assemblage of the Agulhas Retroflection frontal zone and Subtropical zone is highly diverse (39 taxa) and can be linked to relatively warm, high saline and oligotrophic waters. The Subantarctic zone assemblage is characterized by a reduced number (thirteen) of coccolithophore taxa, whereas the Polar Frontal zone comprises a monospecific E. huxleyi assemblage (preferentially morphotypes C and B/C). Multivariate statistics indicated that regions with elevated temperature and low nutrient concentration show high coccolithophore diversity whereas regions with high nutrient concentrations and low temperature show a strongly reduced coccolithophore diversity with abundant monospecific E. huxleyi (morphotypes B/C and C) assemblages.

A pivotal role for ocean eddies in the distribution of microbial communities across the Antarctic Circumpolar Current.

Mesoscale variability and associated eddy fluxes play crucial roles in ocean circulation dynamics and the ecology of the upper ocean. In doing so, these features are biologically important, providing a mechanism for the mixing and exchange of nutrients and biota within the ocean. Transient mesoscale eddies in the Southern Ocean are known to relocate zooplankton communities across the Antarctic Circumpolar Current (ACC) and are important foraging grounds for marine top predators. In this study we investigated the role of cyclonic and anti-cyclonic eddies formed at the South-West Indian Ridge on the spatial variability and diversity of microbial communities. We focused on two contrasting adjacent eddies within the Antarctic Polar Frontal Zone to determine how these features may influence the microbial communities within this region. The water masses and microbiota of the two eddies, representative of a cyclonic cold core from the Antarctic zone and an anti-cyclonic warm-core from the Subantarctic zone, were compared. The data reveal that the two eddies entrain distinct microbial communities from their points of origin that are maintained for up to ten months. Our findings highlight the ecological impact that changes, brought by the translocation of eddies across the ACC, have on microbial diversity.

Low trophic level diet of juvenile southern elephant seals Mirounga leonina from Marion Island: a stable isotope investigation using vibrissal regrowths

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 577:237-250 (2017) - DOI: https://doi.org/10.3354/meps12240 Low trophic level diet of juvenile southern elephant seals Mirounga leonina from Marion Island: a stable isotope investigation using vibrissal regrowths Nico Lübcker1,*, Ryan R. Reisinger1,4, W. Chris Oosthuizen1, P. J. Nico de Bruyn1, André van Tonder1, Pierre A. Pistorius2,3, Marthán N. Bester1 1Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Hatfield 0028, Pretoria, South Africa 2Marine Apex Predator Research Unit, Institute for Coastal and Marine Research, Nelson Mandela Metropolitan University, South Campus, Port Elizabeth 6031, South Africa 3DST/NRF Centre of Excellence at the Percy FitzPatrick Institute, Department of Zoology, Nelson Mandela Metropolitan University, South Campus, Port Elizabeth 6031, South Africa 4Present address: Centre d’Etudes Biologiques de Chizé, UMR 7372 du CNRS-Université de La Rochelle, 79360 Villiers-en-Bois, France *Corresponding author: nlubcker@zoology.up.ac.za ABSTRACT: Insight into the trophic ecology of marine predators is vital for understanding their ecosystem role and predicting their responses to environmental change. Juvenile southern elephant seals (SES) Mirounga leonina are considered generalist predators within the Southern Ocean. Although mesopelagic fish and squid dominate their stomach lavage samples, the stable isotope profile captured along the length of sampled vibrissae of young SES at Macquarie Island, southwest Pacific Ocean (54.5°S, 158.9°E) recently emphasized the contribution of crustaceans to their diet (likely Euphausia superba). Herein, we used the stable isotope values of sampled vibrissal regrowths with known growth histories to assess the diet of juvenile SES at Marion Island, southern Indian Ocean (46.8°S, 37.8°E) on a temporally integrated basis. We specifically aimed to quantify the possible contribution of crustaceans to the diet of juvenile SES. Sequentially (chronologically) sampled vibrissal regrowths of 14 juvenile SES produced fine-scale dietary information spanning up to 9 mo. The depleted stable isotope signatures of nitrogen (δ15N) (8.5 ± 0.6‰) and carbon (δ13C) (-20.3 ± 0.1‰) measured during the period of independent foraging suggested the use of a lower trophic level diet within the Polar Frontal Zone. A mixing model predicted that up to 76% of juvenile SES diet comprised crustaceans, consisting of 2 crustacean groups, each contributing 26% (credible interval, CI: 13-39%) and 50% (CI: 35-64%) to their diets, presumably representing subantarctic krill species. This first utilisation of the isotopic signature captured along the length of vibrissal regrowths confirms the inclusion and importance of crustaceans in the diet of juvenile SES. KEY WORDS: Crustaceans · Diet · Marine mammals · Pinnipeds · Stable isotopes · Vibrissae · Whiskers Full text in pdf format Supplementary material PreviousNextCite this article as: Lübcker N, Reisinger RR, Oosthuizen WC, de Bruyn PJN, van Tonder A, Pistorius PA, Bester MN (2017) Low trophic level diet of juvenile southern elephant seals Mirounga leonina from Marion Island: a stable isotope investigation using vibrissal regrowths. Mar Ecol Prog Ser 577:237-250. https://doi.org/10.3354/meps12240 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 577. Online publication date: August 18, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

Coccolithophore variability across Marine Isotope Stage 11 in the Pacific sector of the Southern Ocean and its potential impact on the carbon cycle

AbstractProxy‐based reconstructions of past changes in the marine biological carbon pumps are limited, especially in the Southern Ocean. This work provides new insights into the productivity variations in the Pacific sector of the Southern Ocean. We present new data derived from three sediment cores that show glacial/interglacial coccolithophore variability across Marine Isotope Stage 11 (MIS 11). The cores were retrieved during R/V Polarstern cruise PS75 from the Subantarctic Zone and Polar Front Zone at the western flank of the East Pacific Rise and in the vicinity of the Antarctic‐Pacific Ridge. Coccolithophore assemblages were overwhelmingly dominated by the species Gephyrocapsa caribbeanica and small Gephyrocapsa. Total numbers of coccoliths, coccolith accumulation rates, coccolith fraction (CF; &lt;20 μm fraction) Sr/Ca data, and temperature‐corrected CF Sr/Ca records consistently showed an increase in coccolithophore productivity during Termination V (MIS 12‐11 boundary), highest productivity throughout MIS 11 (~424–374 kyr), and a decrease during late MIS 11 in all the cores. We end with a discussion of back‐calculated coccolith calcification rate in the surface ocean and its potential contribution to changes in the concentration of atmospheric CO2.

Impact of icebergs on net primary productivity in the Southern Ocean

Abstract. Productivity in the Southern Ocean (SO) is iron-limited, and supply of iron dissolved from aeolian dust is believed to be the main source from outside the marine environment. However, recent studies show that icebergs could provide a comparable amount of bioavailable iron to the SO as aeolian dust. In addition, small-scale areal studies suggest increased concentrations of chlorophyll, krill, and seabirds surrounding icebergs. Based on previous research, this study aims to examine whether iceberg occurrence has a significant impact on marine productivity at the scale of the SO, using remote sensing data of iceberg occurrences and ocean net primary productivity (NPP) covering the period 2002–2014. The impacts of both large and small icebergs are examined in four major ecological zones of the SO: the continental shelf zone (CSZ), the seasonal ice zone (SIZ), the permanent open ocean zone (POOZ), and the polar front zone (PFZ). We found that the presence of icebergs is associated with elevated levels of NPP, but the differences vary in different zones. Grid cells with small icebergs on average have higher NPP than other cells in most iron-deficient zones: 21 % higher for the SIZ, 16 % for the POOZ, and 12 % for the PFZ. The difference is relatively small in the CSZ where iron is supplied from meltwater and sediment input from the continent. In addition, NPP of grid cells adjacent to large icebergs on average is 10 % higher than that of control cells in the vicinity. The difference is larger at higher latitudes, where most large icebergs are concentrated. From 1992 to 2014, there is a significant increasing trend for both small and large icebergs. The increase was most rapid in the early 2000s and has leveled off since then. As the climate continues to warm, the Antarctic Ice Sheet is expected to experience increased mass loss as a whole, which could lead to more icebergs in the region. Based on our study, this could result in a higher level of NPP in the SO as a whole, providing a possible negative feedback for global warming in near future.

Polar Frontal Zone of the Barents Sea Western Trough Based on the Direct Measurements in 2007

Polar Frontal Zone of the Barents Sea Western Trough Based on the Direct Measurements in 2007

Polyarnaya frontal'naya zona Zapadnogo zheloba Barentseva morya po dannym kontaktnykh nablyudeniy 2007 goda

Polyarnaya frontal'naya zona Zapadnogo zheloba Barentseva morya po dannym kontaktnykh nablyudeniy 2007 goda

Eddy properties in the Pacific sector of the Southern Ocean from satellite altimetry data

Mesoscale eddies play a key role in the ocean dynamics of the Southern Ocean, and eddy response to the climate changes has also been widely noted. Both eddy kinetic energy (EKE) and eddy detection algorithm are used to study the eddy properties in the Pacific sector of the Southern Ocean. Consistent with previous works, the maps of the EKE illustrate that higher energy confines to the Antarctic Polar Frontal Zone (APFZ) and decreases progressively from west to east. It also shows that the most significant increase in the EKE occurs in the western and central parts of the Pacific sector, where the baroclinicity of the Antarctic Circumpolar Current (ACC) is much stronger. Statistical eddy properties reveal that both of the spatial pattern and interannual variation of the EKE are primarily due to the eddy amplitude and the eddy rotational speed, rather than the eddy number or the eddy radius. In general, these results furtherly confirm that anomalous westerly wind forcing associated with the positive Southern Annular Mode (SAM) index enhances the Southern Ocean eddy activity by strengthening the eddy properties.

Thalassiosira lentiginosa size variation and associated biogenic silica burial in the Southern Ocean over the last 42 kyrs

The diatom Fragilariopsis kerguelensis (O′Meara) Hustedt is thought to be the main carrier of biogenic silica to the Southern Ocean sea-floor, thereby attracting most investigations of diatom biometry. Thalassiosira lentiginosa (Janisch) Fryxell, a large centric diatom which is the second most abundant species in the Southern Ocean, conversely received very little attention to date. We here present the first study on modern sediments to explore the variability of the mean valve area of T. lentiginosa in relation to productivity and modern environmental conditions. Larger T. lentiginosa are observed around the Polar Front Zone with size decreasing both northward and southward as previously observed for F. kerguelensis. Such a pattern neither corresponds to the species productivity changes nor with the iron concentration in surface waters. Conversely, this pattern may be partly due to the low efficiency of Si uptake, internal transport and deposition of silicic acid under conditions which are outside of the species specific temperature range, i.e. at the lower and upper limit of the diatom ecological preferences. We also propose that T. lentiginosa mean valve area variations in four deep-sea cores across the frontal zones in the Atlantic and Indian sectors of the Southern Ocean resulted from changes in sea-surface temperatures and sea-ice presence modulating the species ability to make use nutrient stocks and controlling the length of its growing season, respectively. Our results also indicate that T. lentiginosa exported more biogenic silica to the sea-floor than F. kerguelensis over the last 42 kyrs.

Intra-annual seasonal variability of surface zooplankton distribution patterns along a 110°E transect of the Southern Ocean in the austral summer of 2011/12

Seasonal cycles can provide insight into the interactions between zooplankton and the environment. However, few intra-annual seasonal studies have been undertaken in the Southern Ocean. We investigated the composition, distribution, and abundance of micro- and meso-zooplankton along the 110°E meridian with three transects in December 2011, January and March 2012 using a Continuous Plankton Recorder. High zooplankton abundance was recorded in the Polar Frontal Zone (PFZ) and the Antarctic Zone (AZ) in both day and night at all transects with 179.0–300.9 ind. m−3. The small copepods Oithona similis, Ctenocalanus citer, and copepodites indet (copepod indeterminable) were dominant in the PFZ and AZ communities. Total zooplankton abundance was comparatively consistent among transects. Nighttime abundance levels remained high in the AZ in March with high abundance of copepodites indet. This seasonal fluctuation appeared to be influenced by recruitment of new populations. Most core species/taxa, except for O. similis, C. citer, and foraminiferans in the AZ area in early January, exhibited a diel decrease in abundance. A multi-ship intra-annual seasonal survey will help detect their various regional and/or seasonal distribution patterns, and the impacts of environmental change on Southern Ocean pelagic ecosystems.

The fate of diatom valves in the Subantarctic and Polar Frontal Zones of the Southern Ocean: Sediment trap versus surface sediment assemblages

An array of deep ocean sediment traps was initiated in 1997 within the Australian sector of the Southern Ocean, and serviced annually for over a decade (the Subantarctic site is ongoing as part of the Southern Ocean Time Series). Here, we expand on previous findings obtained from the shallow traps (~1000m depth) in the Polar Frontal Zone (PFZ) and Subantarctic Zone (SAZ) by examining the chemical composition and diatom assemblages collected by sediment traps deployed at bathypelagic depths in the PFZ (1500m depth; years 1997–1998, 1999–2000 and 2003–2004) and SAZ (2000 and 3800m; year 1999–2000). Additionally, the diatom assemblages of the surface sediments below the traps were analyzed to document how the seasonal signal is recorded in the sedimentary record. Analysis of the changes of the BSi:POC ratios with depth at both sites confirms previous work suggesting that the decoupling between Si and C cycles in the Southern Ocean is not different from other biogeochemical provinces of the world's ocean. Comparison of the seasonal flux pattern registered by the traps indicates that the diatom assemblages in the sediments of the Antarctic Zone and PFZ mainly represent the summer period. In contrast, the assemblages found in the sediments of the SAZ are a reflection of the spring and summer months, a period characterized by a larger variability in chemical and physical parameters. The strong correlation between the POC fluxes and the relative abundance of a group of diatom species (mainly Pseudo-nitzschia and small Fragilariopsis) in the PFZ traps suggest that there is an intimate relationship between the development of these species and the POC pulses to the deepest layers of the water column in this region. Interestingly, analysis of the diatom assemblages in the surface sediments reveals that the strong dissolution in the sediment-water interface removed the signal of these “carbon sinkers” from the sedimentary record leaving the sediment assemblage enriched in heavily silicified species.

Bottom pressure torque and the vorticity balance from observations in Drake Passage

AbstractThe vorticity balance of the Antarctic Circumpolar Current in Drake Passage is examined using 4 years of observations from current‐ and pressure‐recording inverted echo sounders. The time‐varying vorticity, planetary and relative vorticity advection, and bottom pressure torque are calculated in a two‐dimensional array in the eddy‐rich Polar Frontal Zone (PFZ). Bottom pressure torque is also estimated at sites across Drake Passage. Mean and eddy nonlinear relative vorticity advection terms dominate over linear advection in the local (50‐km scale) vorticity budget in the PFZ, and are balanced to first order by the divergence of horizontal velocity. Most of this divergence comes from the ageostrophic gradient flow, which also provides a second‐order adjustment to the geostrophic relative vorticity advection. Bottom pressure torque is approximately one‐third the size of the local depth‐integrated divergence. Although the cDrake velocity fields exhibit significant turning with depth throughout Drake Passage even in the mean, surface vorticity advection provides a reasonable representation of the depth‐integrated vorticity balance. Observed near‐bottom currents are strongly topographically steered, and bottom pressure torques grow large where strong near‐bottom flows cross steep topography at small angles. Upslope flow over the northern continental slope dominates the bottom pressure torque in cDrake, and the mean across this Drake Passage transect, 3 to m s−2, exceeds the mean wind stress curl by a factor of 15–20.

Indices based on silicoflagellate assemblages offer potential for paleo-reconstructions of the main oceanographic zones of the Southern Ocean

This study reports detailed silicoflagellate assemblage composition and annual seasonal flux from sediment traps at four locations along a transect across the Southern Ocean frontal systems. The four traps sampled the central Subantarctic Zone (SAZ, 47°S site), the Subantarctic Front (SAF, 51°S site), the Polar Frontal Zone (54°S site) and the Antarctic Zone (61°S site) across the 140°E longitude. Annual silicoflagellate fluxes to the deep ocean exhibited a similar latitudinal trend to those of diatom fluxes reported in previous work, with maxima in the Antarctic Zone and minima in the Subantarctic Zone. The data suggest that, along with diatoms, silicoflagellates are important contributors to biogenic silica export at all sites, particularly in the Subantarctic Zone. Two main silicoflagellate genera were observed, with Stephanocha sp. (previously known as Distephanus) dominating polar waters and Dictyocha sp. important in sub-polar waters. This is consistent with previous use of the Dictyocha / Stephanocha ratio to infer paleotemperatures and monitor shifts in the position of the Polar Frontal Zone in the sedimentary record. It appears possible to further refine the application of this approach by using the ratio between two Dictyocha species, because Dictyocha aculeata dominated at the Subantarctic Front, while Dictyocha stapedia dominated in the central Subantarctic Front. Given the well-defined environmental affinities of both species, a new SAF silicoflagellate index (SAF-SI) based on this ratio is proposed as a useful diagnostic for SAF and SAZ water mass signatures in the Plio-Pleistocene and Holocene sedimentary record.

Coccolithophore assemblage distribution along a temperate to polar gradient in theWest Pacific sector of the Southern Ocean (January 2005)

We document surface coccolithophore species composition along a N-S transect from New Zealand to the Ross Sea, across the fronts of the Antarctic Circumpolar Current. Emiliania huxleyi is the most abundant coccolithophore species throughout, with high concentration in the northernmost samples, in the Polar Frontal Zone and at the Polar Front. Three E. huxleyi morphotypes, A, B/C and O were recognized and show different biogeographic distributions along the investigated transect. Coccolithophore distribution appears to be strongly controlled by the location of oceanographic fronts, with minor species (Calcidiscus leptoporus, Syracosphaera spp., Acanthoica quattrospina, Umbellosphaera tenuis, Corisphaera strigilis and other holococcolithophores) largely restricted to the northern Sub-Antarctic Zone, only showing minor occurrence in the southern Sub-Antarctic Zone and being very rare in the Polar Frontal Zone. Our data confirm previous findings and add new information on the biogeography of minor coccolithophore species in relation with oceanographic features. Overall, integration with previous data on the southern extent distribution of E. huxleyi showed no significant temporal shifts in the polar dynamic perspective of the species since early observations.