Southern Antarctic Circumpolar Current Front Research Articles

CO2 sink and source zones delimited by marine fronts in the Drake Passage

Net sea-air CO2 fluxes (FCO2) in the Drake Passage (DP) were studied at a climatological scale (1999–2019) using observations from the Surface Ocean CO2 Atlas (SOCAT) database. Based on the monthly climatological position of the main circumpolar fronts of the DP (the Subantarctic Front (SAF), the Polar Front (PF) and the Southern Antarctic Circumpolar Current Front (SACCF)) and the thermal and nonthermal contributions to FCO2, we present a regional subdivision into different regimes that provide new insights into the processes controlling these fluxes. Our results indicate that the region in the north of SAF (R1) behaves as an annual CO2 sink (-1.3 ± 1.0 mmol m−2 d−1); this sink weakens between SAF-PF (R2) and PF-SACCF (R3) and the region south of SACCF (R4) acts as an annual CO2 source (2.2 ± 3.3 mmol m−2 d−1). The annual mean CO2 uptake in DP is 1.3 ± 15.5 Tg C yr-1. Analysis of thermal (TE) and nonthermal (nonTE) effects on seasonal sea surface CO2 partial pressure (pCO2sw) variability indicates that DP is mainly dominated by nonTE. Results emphasize that carbon fluxes are driven by mesoscale and submesoscale processes north of the PF and by the upwelling of Upper Circumpolar Deep Waters in the Antarctic boundary of the DP, while seasonal patterns are mostly modulated by local factors such as nutrient availability, biological activity and ice cover.

Size-fractionated phytoplankton biomass and primary production in the eastern Indian sector of the Southern Ocean in the austral summer 2018/2019

The Southern Ocean significantly contributes to primary production in the global ocean. In this study, size-fractionated primary production and chlorophyll a (Chl a), which are not well elucidated, were determined in the eastern Indian sector in the austral summer of 2018–2019. The Southern Antarctic Circumpolar Current Front, Southern Boundary of the Antarctic Circumpolar Current (SB), and Antarctic Slope Front (ASF) were set as study areas, and sampling stations were divided into the three areas: to the north of the SB, the area between the SB and ASF, and to the south of the ASF. Primary production was 85.1–385.7 mg C m−2 day−1 north of the SB, 131.4–192.5 mg C m−2 day−1 between the SB and ASF, and 95.6–472.7 mg C m−2 day−1 south of the ASF. The Chl a biomass integrated within the euphotic zone in each area was 19.8–46.1 mg m−2, 10.3–19.9 mg m−2, and 17.0–77.9 mg m−2, respectively. Although no significant difference was observed in the primary production and Chl a biomass among the three areas, the biomass tended to be lower between the SB and ASF than in the other two areas. The large phytoplankton (>10 μm) significantly contributed to the primary production and the Chl a biomass at every station north of the SB, accounting for approximately 65–90%. In the other two areas, there were some stations where large phytoplankton accounted for 60–85%, and others where large phytoplankton did not dominate. The flow to the secondary production was one order of magnitude higher to the north of the SB than to the south of the SB. The time since sea-ice melt possibly plays an important role in determining the dominant phytoplankton size and hence size structure of phytoplankton in the survey area. The features in phytoplankton size structure observed north and south of the SB may not be unique to the areas. High phytoplankton biomass and primary production were also observed in the west of 130°E, which was similar to the results of the previous study. A time lag between the west and east surveys, about one month, may have caused the high-west and low-east patterns.

Biomass of Antarctic krill (Euphausia superba) in the eastern Indian sector of the Southern Ocean (80–150°E) in the 2018–19 austral summer

Biomass of Antarctic krill (Euphausia superba) in the eastern Indian sector of the Southern Ocean (80–150°E, corresponding to the longitudinal range of CCAMLR Division 58.4.1) in the austral summer of 2018–19 was estimated using data from a calibrated echosounder and biological sampling. This study was carried out as a part of the multidisciplinary ecosystem survey (KY1804) by Japanese research vessel Kaiyo-maru. The krill biomass was estimated using an up-to-date standard method of CCAMLR, the “swarm-based method”. Biomass of krill in the area had not been estimated since 1996 (BROKE survey). KY1804 covered a total of 4680 km of the surveyed distance in the surveyed area of 0.909 million km2. Krill were mainly distributed between the Southern Antarctic Circumpolar Current Front and the Antarctic Slope Front. The krill biomass during KY1804 was estimated as 4.325 million tonnes (CV = 17.0%), comparable with the 1996 BROKE estimate (4.83 million tonnes, CV = 17.0%); however, direct comparison is difficult because of differences in (1) biomass estimation methods; (2) timing of the surveys (KY1804 commenced about 40 days earlier in the year than BROKE); and (3) areal coverage, primarily because of different positions of the sea ice edge, especially in the western part of the area. There was no noticeable difference of krill biomass between day and night. This study provides a 2018–19 biomass estimate for Antarctic krill in the eastern Indian sector of the Southern Ocean, which is a basis for studies of this species as well as the Southern Ocean ecosystem.

The Role of the Pacific‐Antarctic Ridge in Establishing the Northward Extent of Antarctic Sea‐Ice

AbstractMonitoring the Antarctic sea‐ice is essential for improving our knowledge of the Southern Ocean. We used satellite sea‐ice concentration data for the 2002–2020 period to retrieve the sea‐ice extent (SIE) and analyze its variability in the Pacific sector of the Southern Ocean. Results provide observational evidence of the recurring formation of a sea‐ice protrusion that extends to 60°S at 150°W during the winter season. Furthermore, we discuss how the Pacific‐Antarctic Ridge (PAR) influences this phenomenon. Our findings show that the northward deflection of the southern Antarctic Circumpolar Current front is driven by the PAR and is associated with the enhanced sea‐ice advance. The PAR also constrains eddy trajectories, limiting their interaction with the sea‐ice edge. These factors, within the 160°W–135°W sector, cause an average SIE increase of 61,000 km2 and 46,293 km2 per year more than the upstream and downstream areas, respectively.

Impact of the climate regime shift around 2000 on recruitment of Antarctic krill at the Antarctic Peninsula and South Georgia

We examined how the climate regime shift around 2000 affected Antarctic krill recruitment variability at the North Antarctic Peninsula (NAP) and South Georgia (SG) during the period of 1990s through to 2000s. We used sea surface temperature (SST) as a proxy of the monthly sea ice extent and related it to the recruitment index (proportion of 1-year-old krill) of each succeeding year for NAP and SG by examining the correlation of the time series before and after 2000. For NAP, recruitment index was negatively correlated with winter SST at NAP in the 1990s but with autumn SST in the 2000s, suggesting that good recruitment was supported by extensive winter sea-ice in the 1990s but by extensive autumn sea-ice in the 2000s. This was attributed to the regime shift featured by winter warming and autumn cooling after 2000. For SG, recruitment was negatively correlated with winter SST at NAP in the 1990s but positively correlated with autumn SST in the southern Scotia Sea (SSS) in the 2000s. According to particle tracking experiment from NAP to SG in relation to the sea-ice extent and position of SACCF (Southern Antarctic Circumpolar Current Front), less sea-ice extent in 2000s meant less larvae distributing as north as SACCF, resulting in much fewer larval being transported to SG. Hence, we hypothesized that the regime shift weakened the connection between NAP and SG, but instead SSS became the main driver of recruitment at SG. We presume the krill population demonstrated adaptability to the regime shift by offsetting the unfavorable winter condition with the favorable autumn condition at NAP and for SG, replacing the source of recruits with those closer to it.

Baleen whales in the Scotia Sea during January and February 2003

Different species of baleen whales display distinct spatial distribution patterns in the Scotia Sea during the austral summer. Passive acoustic and visual surveys for baleen whales were conducted aboard the RRS James Clark Ross in the Scotia Sea and around South Georgia in January and February 2003. Identified calls from four species were recorded during the acoustic survey including southern right (Eubalaena australis), blue (Balaenoptera musculus), fin (B. physalus) and humpback whales (Megaptera novaeangliae). These acoustic data included up calls made by southern right whales, downswept D and tonal calls by blue whales, two possible types of fin whale downswept calls and humpback whale moans and grunts. Visual detections included southern right, fin, humpback and Antarctic minke whales (B. bonaerensis sp.). Most acoustic and visual detections occurred either around South Georgia (southern right and humpback whales) or south of the southern boundary of the Antarctic Circumpolar Current (ACC) and along the outer edge of the ice pack (southern right, blue, humpback and Antarctic minke whales). Fin whales were the exception, being the only species acoustically and visually detected primarily in the central Scotia Sea, along the southern ACC front. In addition to identifiable calls from these species, two types of probable baleen whale calls were detected: 50Hz upswept and pulsing calls. It is proposed that minke whales may produce the pulsing calls, based on their similarities with minke whale calls recorded in the North Atlantic Ocean. There was an overlap between locations of fin whale sightings and recordings and locations of 50Hz upswept calls in the central Scotia Sea, but these calls were most similar to calls attributed to blue whales in other parts of Antarctica. More study is required to determine if baleen whales produce these two call types, and if so, which species. The efficiency of acoustics and visual surveys varied by species, with blue whales being easier to detect using acoustics, Antarctic minke whales being best detected during visual surveys and other species falling in between these two extremes.

Variability of the Shelf Circulation Around South Georgia, Southern Ocean

AbstractA high‐resolution ocean model is used to characterize the variability of the shelf circulation and cross‐shelf transport around the South Georgia island (SG). The time‐mean shelf circulation consists of a counterclockwise flow with a net onshelf mass flow in the south and a net offshelf mass flow in the north. In the south, the cross‐shelf exchanges show a two‐layer structure with an offshelf flow below 350 m and onshelf flow above. In the north, the cross‐shelf exchanges show a three‐layer structure with the onshelf flow found only between 350 and 50 m. Correlation analysis shows that winds and the Southern Antarctic Circumpolar Current Front (SACCF) current modulate the variability of the shelf circulation and cross‐shelf transport. Local wind stress is significantly correlated with the coastal currents, mid‐shelf jet, and cross‐shelf transports in the upper layer, while the SACCF modulates the shelf and cross‐shelf transports in the southwestern shelf. Likewise, an Empirical Orthogonal Function analysis indicates that the first mode of shelf circulation variability is highly correlated with the SACCF, while the second mode is explained by the local wind stress and significantly correlated with the Antarctic Oscillation. The El Niño Southern Oscillation does not significantly contribute to the shelf circulation but is significantly correlated with the surface temperature variability. The atmospheric teleconnection drives changes in local heat flux, such that warm El Niño conditions over the equatorial Pacific are associated with a cooling of the SG waters. This superposes local signals onto temperature anomalies advected from upstream in the ACC found in previous studies.

Diapycnal and isopycnal mixing along the continental rise in the Australian–Antarctic Basin

Shipboard measurements of temperature, salinity and horizontal current velocity were used to estimate diapycnal and isopycnal diffusivities in the Australian sector of the Southern Ocean between 120∘E and 150∘E near 64∘S, along the climatological positions of the Southern Antarctic Circumpolar Current Front and the Southern Boundary. The diapycnal diffusivities estimated by the finescale parameterization on the neutral density surfaces of 1028.05, 1028.2, and 1028.3 kg m−3 are (9.5±6.8)×10−5, (10.7±2.6)×10−5, and (13.8±3.7)×10−5 m2s−1, respectively (mean ± one standard deviation). At these densities, salinity and temperature decreased with depth but salt fingering was unlikely. Using the water mass transformation framework and a box inverse model, the isopycnal diffusivities were estimated from the diapycnal diffusivities. Climatological estimates of the isopycnal diffusivities in the density ranges of Upper Circumpolar Deep Water, Lower Circumpolar Deep Water, and Antarctic Bottom Water were −30±116, 16±319, and −30±83 m2s−1, respectively. These diffusivities were likely underestimates by 10 to 20 m2s−1 because eddy transport could not be considered in the climatological data. The weak isopycnal diffusivities were not inconsistent with diffusivity suppressed by the frontal jets. The large uncertainties were caused by the uncertainty in the box inverse model and the fact that the size of the box was comparable to the advection speed times the diffusion time scale.

Shoaling of abyssal ventilation in the Eastern Indian Sector of the Southern Ocean

Antarctic Bottom Water formation, which ventilates the abyssal layers of the Southern Ocean, is an integral component of the global ocean meridional overturning circulation. Considering evident freshening and density decreases in the source waters, widespread warming in the Southern Ocean suggests a weakening in the Antarctic Bottom Water supply. We demonstrate that the weakening is robust based on water mass warming in the deep and abyssal layers of the Australian-Antarctic Basin, which remained after removing the southward shift effect of the Southern Antarctic Circumpolar Current Front. Moreover, a decrease in apparent oxygen utilisation and reduced warming in the intermediate density layer below Circumpolar Deep Water extended further from the Australian-Antarctic Basin to the South Australian Basin. We suggest that a concurrent weakening in the densest portion and strengthening in the less dense portion shape the multi-basin change in the meridional overturning circulation that originates from the Southern Ocean.

Contrasting life-history traits of two toothfish (Dissostichus spp.) species at their range edge around the South Sandwich Islands

The South Sandwich Islands (SSI), a chain of volcanic islands in the Atlantic sector of the Southern Ocean, are home to two large notothenoid species: the Patagonian toothfish Dissostichus eleginoides and the Antarctic toothfish Dissostichus mawsoni. Both species support valuable fisheries throughout the Southern Ocean under management of the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). The SSI region, which is located south of the Southern Antarctic Circumpolar Current Front, has a diverse and distinct biodiversity and it represents a range edge for the distribution of both toothfish species. In this paper we have updated and expanded previous biological analyses with recent data, explored the stock hypotheses and links of these species to other regions, and investigated the role of the SSI archipelago in the life cycles of both toothfish species, where they overlap in their distribution. We conclude that Patagonian toothfish around the SSI are linked to the adjacent South Georgia population, but have some unique characteristics, including faster growth and better somatic condition, possibly reflecting ‘Bergmann's rule’ which states that body size increases with decreasing temperature and increasing latitude. By comparison, the Antarctic toothfish at the SSI appear to be the northern extent of a larger stock connecting further south towards the Antarctic continent. Finally, we consider the relative importance of the SSI in the life cycle of both species, including in the context of climatic changes to this region.

Changes of Circumpolar Deep Water between 2006 and 2020 in the south-west Indian Ocean, East Antarctica

The climatological distribution of circumpolar deep water (CDW) and its changes in austral summer between 2006 and 2020 are documented using historical hydrographic data and recent measurements in the south-west Indian Ocean, East Antarctica. The CDW is primarily distributed in the northeast of the Cooperation Sea, north of the southern Antarctic Circumpolar Current Front (sACCF), as shown by the CDW thickness (on average > 800 m) and mean depth (maxima >900 m). The CDW layer becomes thinner to the south of sACCF and is less than 100 m thick near the Southern Boundary (SB). The CDW distribution shows significant changes between years. Its extension reached further south in 2020 than in 2006, and there was a positive thickness anomaly with a magnitude of 50–200 m. There was also evidence of warmer (0.1–0.2 °C) and more saline (0.005) CDW in 2020 than 2006 along 70°E and between 62.8° and 63.7°S. Those warm and saline signals extended further south. Warmer and more saline water occupied the subsurface layers between 35° and 69°E at 63.6°S and 64°S, and occurred to the east of 55°E. The region between 55° and 65°E is a major entrance for the CDW poleward extension. Furthermore, we found that the changes of CDW between 2006 and 2020 were dominated by the sACCF shift due to enhanced northwest summer winds. The wind anomalies raised the sea surface height by the strengthening northward Ekman transport. Thus the ACC shifted southward (∼120 km in average), carrying more CDW poleward. A tongue-shaped southward shift of sACCF generated anomalous anticyclonic eddies, advecting CDW southward as far as 66° in 2020.

High latitude Southern Ocean phytoplankton have distinctive bio-optical properties.

Studying the biogeochemistry of the Southern Ocean using remote sensing relies on accurate interpretation of ocean colour through bio-optical and biogeochemical relationships between quantities and properties of interest. During the Antarctic Circumnavigation Expedition of the 2016/2017 Austral Summer, we collected a spatially comprehensive dataset of phytoplankton pigment concentrations, particulate absorption and particle size distribution and compared simple bio-optical and particle property relationships as a function of chlorophyll a. Similar to previous studies we find that the chlorophyll-specific phytoplankton absorption coefficient is significantly lower than in other oceans at comparable chlorophyll concentrations. This appears to be driven in part by lower concentrations of accessory pigments per unit chlorophyll a as well as increased pigment packaging due to relatively larger sized phytoplankton at low chlorophyll a than is typically observed in other oceans. We find that the contribution of microphytoplankton (>20 µm size) to chlorophyll a estimates of phytoplankton biomass is significantly higher than expected for the given chlorophyll a concentration, especially in higher latitudes south of the Southern Antarctic Circumpolar Current Front. Phytoplankton pigments are more packaged in larger cells, which resulted in a flattening of phytoplankton spectra as measured in these samples when compared to other ocean regions with similar chlorophyll a concentration. Additionally, we find that at high latitude locations in the Southern Ocean, pheopigment concentrations can exceed mono-vinyl chlorophyll a concentrations. Finally, we observed very different relationships between particle volume and chlorophyll a concentrations in high and low latitude Southern Ocean waters, driven by differences in phytoplankton community composition and acclimation to environmental conditions and varying contribution of non-algal particles to the particulate matter. Our data confirm that, as previously suggested, the relationships between bio-optical properties and chlorophyll a in the Southern Ocean are different to other oceans. In addition, distinct bio-optical properties were evident between high and low latitude regions of the Southern Ocean basin. Here we provide a region-specific set of power law functions describing the phytoplankton absorption spectrum as a function of chlorophyll a.

Hydrodynamics of the choke point between Cape Town and Antarctica during the austral summer of 2019

This study addresses the hydrodynamics inferred from the expendable conductivity-temperature-depth (XCTD) observations carried out in the southwestern Indian Ocean sector of the Southern Ocean along two transects: the Lazarev Sea to Cape Town (track-1) and Cape Town to Prydz bay (track-2) during the austral summer of 2019. The vertical temperature and salinity structures revealed an eddy extending up to 44°S (45°S) on track-1 (track-2). South of the eddy, we encountered frontal zones extending to 53°S (59°S) on track-1(track-2). The frontal locations identified from XCTD and satellite-based sea surface temperature and absolute dynamic topography coincided, with the latter placed within the latitudinal limits identified from the XCTD data. Meandering of the Polar front (PF), the southern Antarctic Circumpolar Current (ACC) Front, and the Southern Boundary of the ACC was observed from 90 to 550 km southward. The Winter Water which was confined to the south of 50°S was detected at deeper depth (~350 m) on track-1, compared to a depth of 100 m on track-2, and its thickness varied from zero to 1.2 m on track-1 and from 0.5 to 2.5 m on track-2. The vertical thermohaline structure revealed the northward subduction of a mixture of Antarctic Surface Water and Subantarctic Surface Water up to 45.5°S and down to 500 (320) m on track-1 (track-2). The volume transport (relative to 1000 m) accounted for 87% of that estimated in the literature (90 ± 2.4 Sv) and 34.7 Sv across track-1 and -2, respectively. It was found that 70% of the volume transport was confined to the ACC frontal region and 26% of the total transport occurred in the 100–500 m slab. The cumulative heat and salt content was 2% and 1.2% higher between 39° and 66°S, compared to that estimated from 2008 data along track-1. We used a satellite-based absolute dynamic topography field, to trace out Agulhas current (AC) and its retroflection current, and eddies detached from the meanders. A higher dynamic topography gradient across the polar front facilitates enhanced transport. Sea surface temperature fields revealed that the meanders in the AC propagate southwest with an offshore extent of 30–300 km.

Modeling the Impact of Ocean Circulation on Chlorophyll Blooms Around South Georgia, Southern Ocean

AbstractThe northeast periphery of the Scotia Sea hosts one of the largest chlorophyll‐a blooms of the Southern Ocean. This bloom peaks to the northwest of the island of South Georgia, extending eastward for hundreds of kilometers. Although the Southern Ocean has many islands of similar size, South Georgia is ecologically one of the most significant: It not only sustains one of the Southern Ocean's largest and most diverse ecosystems but also constitutes its single most important region for biological carbon sequestration. While the exceptional nature of South Georgia's blooms has been recognized widely, both the physical processes that contribute to their fertilization and the reasons why these blooms are larger than those of other similar regions (e.g., Kerguelen or Crozet Islands) are poorly understood. We use the results of a high‐resolution ocean model to investigate the physical processes that mediate the entrainment of deep, iron‐rich waters into the surface layers of the South Georgia region. We show that the Southern Antarctic Circumpolar Current Front, the southernmost jet of the Antarctic Circumpolar Current (ACC), pumps iron‐enriched waters from the deep ocean onto the bottom layers of South Georgia's shelf. These waters are upwelled along the northern coast of the island and are then exported into the Georgia Basin, where topographically steered circulation shields them from the dispersive effects of local currents and eddies, thus allowing the bloom development.

WITHDRAWN: Abundance and characteristics of microplastics in the Bellingshausen Sea, Antarctica, are distinguished by the Southern Antarctic Circumpolar Current Front

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Antarctic sea-ice and palaeoproductivity variation over the last 156,000 years in the Indian sector of Southern Ocean

Antarctic sea ice plays a vital role in global climate via its impact on ocean circulation, biological productivity and CO 2 partitioning between the ocean and the atmosphere. However, very little is known about its past history, especially in the southwestern Indian sector of the Southern Ocean (SO). We here provide new quantitative records of winter sea-ice concentration (WSIC) and duration (WSID), sea-surface temperatures (SST) and productivity in sediment core SK 200/33 (55°S – 45°E) from the Permanently Open Ocean Zone over the last 156,000 years. The new records, combined with regional records, indicate that the hydrological structures migrated northward by a few degrees of latitude during all glacial periods with the Southern Antarctic Circumpolar Current Front reaching the core site, the Antarctic Polar Front located at ~46°S and the winter sea ice (WSI) probably extending to ~49°S. In contrast, hydrological fronts and WSI edge migrated poleward by a couple of degrees of latitude during the early Holocene and last interglacial. Comparison to SST and WSI records from different sectors of the SO suggests higher amplitude variations in WSI in the Atlantic sector as compared to the Indian and western Pacific sectors over the last glacial-interglacial cycle, which we attribute to the presence of the Weddell Gyre transporting far to the north the sea ice produced in the Weddell Sea. The new records also suggest a drop in productivity in the POOZ during glacial periods, probably related to greater WSI extent and reduced growing season. • New quantitative records for the Antarctic Indian Sector over the last 156,000 years. • Winter sea-ice limit north of 50°S, APF at ~46°S and SACCF at ~55°S during glacial stages. • Ocean thermodynamics governed sea-ice dynamic. • Winds and ocean currents drove zonal sea-ice amplitude changes. • Contrasted paleoproductivity patterns across the Antarctic Polar Front.

Zooplankton community structure and dominant copepod population structure on the southern Kerguelen Plateau during summer 2016

The influence of environmental factors on the horizontal community structure of zooplankton over the southern Kerguelen Plateau was investigated during summer in 2016. Zooplankton abundance ranged from 1490 to 363,484 ind. 1000 m-3, with highest numbers observed in the eastern and central areas. Based on cluster analysis the zooplankton were divided into six groups (A−F), and these were only distinguished based on water masses and frontal systems. Groups A to C had abundant zooplankton and were consistent with areas of high chlorophyll a concentration. Group D represented low abundance near the southern Antarctic Circumpolar Current front, while group E was clustered south of the Southern Boundary and group F comprised two stations to the east of the Fawn Trough. Generalised linear model (GLM) highlighted both fronts and chlorophyll a concentration as drivers of overall zooplankton distribution. However, the population structures of key species were more likely a result of species-specific life cycles rather than water masses and frontal systems.

Salpa thompsoni in the Indian Sector of the Southern Ocean: Environmental drivers and life history parameters

The Southern Ocean ecosystem is thought to be experiencing a long-term increase in Salpa thompsoni. Uncertainty surrounds the environmental drivers behind variable S. thompsoni abundances, particularly within the East Antarctic region. In this study, S. thompsoni populations were sampled in the Indian Sector of the Southern Ocean, as part of the January–February 2016 Kerguelen Axis voyage. These recent data were compared against historical density records in the broader Kerguelen Plateau region from voyages during 1985–2006. Results show that 2016 maximum S. thompsoni densities across the Kerguelen Plateau were higher, and more southerly located, than those previously sampled in the area. The highest 2016 S. thompsoni abundances exceeded 2500 individuals 1000 m-3 and were located between the Southern Boundary of the Antarctic Circumpolar Current and the Southern Antarctic Circumpolar Current Front. The life-stage composition of S. thompsoni comprised approximately 90% aggregates (blastozooids) and 10% solitaries (oozoids). Generalised Additive Models associated low chlorophyll-a concentration and low solar elevation (outside of peak daylight) with higher S. thompsoni abundances. In addition, elevated abundances occurred in locations from where the sea ice retreated at least eight weeks previously. These abundance-environment relationships are consistent with results from several previous surveys in the region and the West Antarctic. Due to the complex life cycle of S. thompsoni, and the unpredictability of their distribution patterns, multi-seasonal and multi-year surveys are needed to confirm whether the 2016 distribution patterns are indicative of a long-term increase or southerly shift in abundances. The new information from this study provides much needed baseline abundance and distribution data (which can be used to assess future change in a currently understudied oceanographic region) on a species capable of shaping the future ecosystem structure and function in the Indian Sector of the Southern Ocean.

Individual variation in migratory movements of chinstrap penguins leads to widespread occupancy of ice-free winter habitats over the continental shelf and deep ocean basins of the Southern Ocean.

A goal of tracking migratory animals is to characterize the habitats they use and to interpret population processes with respect to conditions experienced en route to, and within, overwintering areas. For migratory seabirds with broad breeding ranges, inferring population-level effects of environmental conditions that are experienced during migratory periods would benefit by directly comparing how birds from different breeding aggregations disperse, characterizing the physical conditions of areas they use, and determining whether they occupy shared foraging areas. We therefore tracked 41 adult and juvenile chinstrap penguins (Pygoscelis antarctica) from three breeding locations in the northern Antarctic Peninsula region during the austral winter of 2017. The satellite tracking data revealed overlap of individuals over continental shelf areas during autumn months (Mar-May), shared outbound corridors that track the southern Antarctic circumpolar current front, followed by occupancy of progressively colder, deeper, and ice-free waters that spanned the entire western hemisphere south of the Polar Front. Despite broadly similar physical environments used by individuals from different colonies, the proportion of birds from each colony that remained within 500km of their colony was positively correlated with their local population trends. This suggests that local migration strategies near the Antarctic Peninsula may benefit breeding populations. However, the magnitude of inter-colony and intra-colony overlap was generally low given the broad scale of habitats occupied. High individual variation in winter movements suggests that habitat selection among chinstrap penguins is more opportunistic, without clear colony-specific preference for fine-scale foraging hotspots. Mixing of individuals from multiple colonies across broad regions of the Southern Ocean would expose chinstrap penguins from the Antarctic Peninsula to a shared environmental experience that helps explain the regional decline in their abundance.

Finding mesopelagic prey in a changing Southern Ocean

Mesopelagic fish and squid occupy ocean depths extending below the photic zone and their vertical migrations represent a massive pathway moving energy and carbon through the water column. Their spatio-temporal distribution is however, difficult to map across remote regions particularly the vast Southern Ocean. This represents a key gap in understanding biogeochemical processes, marine ecosystem structure, and how changing ocean conditions will affect marine predators, which depend upon mesopelagic prey. We infer mesopelagic prey vertical distribution and relative abundance in the Indian sector of the Southern Ocean (20° to 130°E) with a novel approach using predator-derived indices. Fourteen years of southern elephant seal tracking and dive data, from the open ocean between the Antarctic Polar Front and the southern Antarctic Circumpolar Current front, clearly show that the vertical distribution of mesopelagic prey is influenced by the physical hydrographic processes that structure their habitat. Mesopelagic prey have a more restricted vertical migration and higher relative abundance closer to the surface where Circumpolar Deep Water rises to shallower depths. Combining these observations with a future projection of Southern Ocean conditions we show that changes in the coupling of surface and deep waters will potentially redistribute mesopelagic prey. These changes are small overall, but show important spatial variability: prey will increase in relative abundance to the east of the Kerguelen Plateau but decrease to the west. The consequences for deep-diving specialists such as elephant seals and whales over this time scale will likely be minor, but the changes in mesoscale vertical energy flow have implications for predators that forage within the mesopelagic zone as well as the broader pelagic ecosystem.