Northern Scotia Sea Research Articles

Seasonal cycles of biogeochemical fluxes in the Scotia Sea, Southern Ocean: a stable isotope approach

Abstract. The biological carbon pump is responsible for much of the decadal variability in the ocean carbon dioxide (CO2) sink, driving the transfer of carbon from the atmosphere to the deep ocean. A mechanistic understanding of the ecological drivers of particulate organic carbon (POC) flux is key both to the assessment of the magnitude of the ocean CO2 sink and for accurate predictions as to how this will change with changing climate. This is particularly important in the Southern Ocean, a key region for the uptake of CO2 and the supply of nutrients to the global thermocline. In this study we examine sediment-trap-derived particle fluxes and stable isotope signatures of carbon (C), nitrogen (N), and biogenic silica (BSi) at a study site in the biologically productive waters of the northern Scotia Sea in the Southern Ocean. Both deep (2000 m) and shallow (400 m) sediment traps exhibited two main peaks in POC, particulate N, and BSi flux: one in austral spring and one in summer, reflecting periods of high surface productivity. Particulate fluxes and isotopic compositions were similar in both deep and shallow sediment traps, highlighting that most remineralisation occurred in the upper 400 m of the water column. Differences in the seasonal cycles of isotopic compositions of C, N, and Si provide insights into the degree of coupling of these key nutrients. We measured increasing isotopic enrichment of POC and BSi in spring, consistent with fractionation during biological uptake. Since we observed isotopically light particulate material in the traps in summer, we suggest physically mediated replenishment of lighter isotopes of key nutrients from depth, enabling the full expression of the isotopic fractionation associated with biological uptake. The change in the nutrient and remineralisation regimes, indicated by the different isotopic compositions of the spring and summer productive periods, suggests a change in the source region of material reaching the traps and associated shifts in phytoplankton community structure. This, combined with the occurrence of advective inputs at certain times of the year, highlights the need to make synchronous measurements of physical processes to improve our ability to track changes in the source regions of sinking particulate material. We also highlight the need to conduct particle-specific (e.g. faecal pellets, phytoplankton detritus, zooplankton moults) isotopic analysis to improve the use of this tool in assessing particle composition of the sinking material and to develop our understanding of the drivers of biogeochemical fluxes.

A seasonal transition in biological carbon pump efficiency in the northern Scotia Sea, Southern Ocean

The biological carbon pump (BCP) contributes to the oceanic CO2 sink by transferring particulate organic carbon (POC) into the deep ocean. The magnitude and efficiency of the BCP is likely to vary on timescales of days to seasons, however characterising this variability from shipboard observations is challenging. High resolution, sustained observations of primary production and particle fluxes by autonomous vehicles offer the potential to fill this knowledge gap. Here we present a 4 month, daily, 1 m vertical resolution glider dataset, collected in the high productivity bloom, downstream of South Georgia, Southern Ocean. The dataset reveals substantial temporal variability in primary production, POC flux and attenuation. During the pre-bloom peak phase we find high export efficiency, implying minimal heterotrophic POC consumption, i.e. productivity is decoupled from upper ocean remineralisation processes. As the bloom progresses from its peak through its declining phase, export flux decreases, but transfer efficiency within the upper 100 m of the mesopelagic increases. Conversely, transfer efficiency in the lower mesopelagic decreases in the post-bloom phase, implying that the flux attenuation processes operating in the upper and lower mesopelagic are effectively decoupled. This finding underscores an important limitation of using a single parameter, such as Martin's ‘b’, to characterise POC flux attenuation in a given location or season. Frequent pulses of export flux are observed throughout the deployment, indicating decoupling between primary production and the processes driving export of material from the upper ocean. The mechanisms underlying the observed seasonal changes in BCP magnitude and efficiency are unclear, as temperature and oxygen concentration changed minimally, although the nature of the sinking particles changed substantially as the bloom progressed. Our results highlight the difficulty of capturing temporal variability and episodic flux events with traditional shipboard observations, which affects our conceptual understanding of the BCP. The increasing use of autonomous vehicles to observe particle fluxes will be essential to characterising the temporal variability in magnitude and functioning of the BCP.

Deep carbon export peaks are driven by different biological pathways during the extended Scotia Sea (Southern Ocean) bloom

Estimating the amount of organic carbon leaving the upper water column and becoming sequestered in the deep ocean is a major challenge in our understanding the oceanic C cycle. This study investigate deep sediment trap material collected at a 5 day resolution over a 4 month period covering the bloom in the northern Scotia Sea. This region is characterised by extensive and long-lived phytoplankton blooms that collectively take up the greatest amount of atmospheric carbon dioxide yet measured in the Southern Ocean. In particular, the study resolved multiple peaks of POC flux during the northern Scotia Sea bloom resulting from distinctly different export processes. This work also examine the contribution of diatoms and calcifying species to the POC flux as well as determining FP (faecal pellet) characteristics, including biogeochemical composition and sinking velocity. Results showed that POC flux during the bloom was concentrated in three major POC peak events. The first two POC peaks were large, of similar intensity (24.5 mg C m−2 d−1 in early November and 22.5 mg C m−2 d−1 in early December), and were dominated by faecal pellets (FPs, 60–66%). FP biogeochemical composition during these two POC peaks was characterised by a dominance of carbonate (>48%), as well as a higher sinking speed (up to 347 m d−1) compared with other FP in the rest of the samples. Results suggest that the large presence of calcium carbonate in these FPs contributed to their relatively high sinking velocity which, in turn, promoted their higher level of POC export to bathypelagic depths. Intact single cell diatoms were relatively important in the first POC peak (mainly characterised by Fragilariopsis kerguelensis and Thalassionema nitzschioides), while detritus including semi-grazed phytodetritus were more abundant during the second POC peak, suggesting a change in the upper ocean C cycle between the two POC peaks. A third, smaller, POC peak (12.7 mg C m−2 d−1 in January) was dominated by diatom resting spores, mainly Chaetoceros Hyalochaete sp. (>70% of total diatoms contribution), which were, almost exclusively, fully intact cells. Our high resolution sampling allowed insights into short time-scale processes that influence the ultimate magnitude of the substantial annual POC flux in this region. Overall our findings highlights that the temporal sequence of biological events in the surface layers during the bloom has a strong influence on both the magnitude and the composition of the fluxes.

Observing the disintegration of the A68A iceberg from space

Icebergs impact the physical and biological properties of the ocean where they drift, depending on the degree of melting. We use satellite imagery and altimetry to quantify the area, thickness, and volume change of the massive A68A iceberg from its calving off the Larsen-C Ice Shelf in July 2017 until January 2021, when it disintegrated. A68A thinned from 235 ± 9 to 168 ± 10 m, on average, and lost 802 ± 34 Gt of ice in 3.5 years, 254 ± 17 Gt of which was through basal melting (a lower bound for the immediate fresh water input into the ocean). Basal melting peaked at 7.2 ± 2.3 m/month in the Northern Scotia Sea and an estimated 152 ± 61 Gt of freshwater was released off South Georgia, potentially altering the local ocean properties, plankton occurrence and conditions for predators.

Ecological Networks in the Scotia Sea: Structural Changes Across Latitude and Depth

The Scotia Sea is a productive pelagic ecosystem in the Southern Ocean, which is rapidly changing as a consequence of global warming. Species range shifts are particularly evident, as sub-Antarctic species expand their range from North to South, potentially rearranging the structure of this ecosystem. Thus, studies are needed to determine the current extent of variation in food web structure between these two biogeographic regions of the Scotia Sea and to investigate whether the observed patterns are consistent among depth zones. We compiled a database of 10,888 feeding interactions among 228 pelagic taxa, underpinned by surveys and dietary studies conducted in the Scotia Sea. Network analysis indicated that the Northern Scotia Sea (NSS), relative to the Southern Scotia Sea (SSS), is more complex: with higher species richness (more nodes) and trophic interactions (more links) is more connected overall (greater connectance and linkage density). Moreover, the NSS is characterised by more groups of strongly interacting organisms (greater node clustering) than the SSS, suggesting a higher trophic specialisation of Antarctic compared to sub-Antarctic species. Depth also played a key role in structuring these networks, with higher mean trophic position and more dietary generalism in the mesopelagic and bathypelagic zones relative to the epipelagic zones. This suggests that direct access to primary producers is a key factor influencing the trophic structure of these communities. Our results suggest that under current levels of warming the SSS ecosystem will likely become more connected and less modular, resembling the current structure of the NSS.

The Transfer of Antarctic Circumpolar Waters to the Western South Atlantic Ocean

AbstractSouthern Ocean waters enter the South Atlantic Ocean through the Scotia Sea along pathways constrained by the bathymetry of the northern Scotia Sea passages. We use the Argo profiling‐float data set to calculate the water transports in and out of the region, focusing on the water balances down to the deepest isoneutral sampled in all passages (γn = 28.0 kg m−3, located between about 500 and 2,000 m in the Drake Passage and even shallower in the Northern Passages). Down to this reference level, the water inflow through the Drake Passage is 140.8 ± 7.4 Sv and the water outflow through the deeper portions of the Northern Passages is 115.9 ± 8.3 Sv, implying a leakage of about 25 ± 11.1 Sv over topography shallower than 1,000 m. Below the reference isoneutral and down to 2,000 m, an additional 23.4 Sv enter through the Drake Passage; when added to reported inputs of about 20 Sv through the South Scotia Ridge, this accounts well for the observed 43.4 Sv outflow–from 28.0 kg m−3 to 2,000 m–through the Northern Passages. Relative to the 2,000 m reference level, the mean barotropic contribution always represents over half the total transports. We also observe substantial seasonal and moderate interannual variations in the water transports and composition (peak differences occur seasonally in the Drake Passage, with a range of 111–174 Sv), associated with changes in water exchange across the frontal systems. Two independent measures set the water mean‐residence time in the Scotia Sea at about 6–8 months.

Thirty years of marine debris in the Southern Ocean: Annual surveys of two island shores in the Scotia Sea

We report on three decades of repeat surveys of beached marine debris at two locations in the Scotia Sea, in the Southwest Atlantic sector of the Southern Ocean. Between October 1989 and March 2019 10,112 items of beached debris were recovered from Main Bay, Bird Island, South Georgia in the northern Scotia Sea. The total mass of items (data from 1996 onwards) was 101 kg. Plastic was the most commonly recovered item (97.5% by number; 89% by mass) with the remainder made up of fabric, glass, metal, paper and rubber. Mean mass per item was 0.01 kg and the rate of accumulation was 100 items km−1 month−1. Analyses showed an increase in the number of debris items recovered (5.7 per year) but a decline in mean mass per item, suggesting a trend towards more, smaller items of debris at Bird Island. At Signy Island, South Orkney Islands, located in the southern Scotia Sea and within the Antarctic Treaty area, debris items were collected from three beaches, during the austral summer only, between 1991 and 2019. In total 1304 items with a mass of 268 kg were recovered. Plastic items contributed 84% by number and 80% by mass, with the remainder made up of metal (6% by number; 14% by mass), rubber (4% by number; 3% by mass), fabric, glass and paper (<1% by number; 3% by mass). Mean mass per item was 0.2 kg and rate of accumulation was 3 items km−1 month−1. Accumulation rates were an order of magnitude higher on the western (windward) side of the island (13–17 items km−1 month−1) than the eastern side (1.5 items km−1 month−1). Analyses showed a slight decline in number and slight increase in mean mass of debris items over time at Signy Island. This study highlights the prevalence of anthropogenic marine debris (particularly plastic) in the Southern Ocean. It shows the importance of long-term monitoring efforts in attempting to catalogue marine debris and identify trends, and serves warning of the urgent need for a wider understanding of the extent of marine debris across the whole of the Southern Ocean.

Geochronology and geochemistry of the northern Scotia Sea: A revised interpretation of the North and West Scotia ridge junction

Understanding the tectonic evolution of the Scotia Sea is critical to interpreting how ocean gateways developed during the Cenozoic and their influence on ocean circulation patterns and water exchange between the Atlantic and Southern oceans. We examine the geochronology and detrital age history of lithologies from the prominent, submerged Barker Plateau of the North Scotia Ridge. Metasedimentary rocks of the North Scotia Ridge share a strong geological affinity with the Fuegian Andes and South Georgia, indicating a common geological history and no direct affinity to the Antarctic Peninsula. The detrital zircon geochronology indicates that deposition was likely to have taken place during the mid – Late Cretaceous. A tonalite intrusion from the Barker Plateau has been dated at 49.6 ± 0.3 Ma and indicates that magmatism of the Patagonian–Fuegian batholith continued into the Eocene. This was coincident with the very early stages of Drake Passage opening, the expansion of the proto Scotia Sea and reorganization of the Fuegian Andes. The West Scotia Ridge is an extinct spreading center that shaped the Scotia Sea and consists of seven spreading segments separated by prominent transform faults. Spreading was active from 30–6 Ma and ceased with activity on the W7 segment at the junction with the North Scotia Ridge. Reinterpretation of the gravity and magnetic anomalies indicate that the architecture of the W7 spreading segment is distinct to the other segments of the West Scotia Ridge. Basaltic lava samples from the eastern flank of the W7 segment have been dated as Early – mid Cretaceous in age (137–93 Ma) and have a prominent arc geochemical signature indicating that seafloor spreading did not occur on the W7 segment. Instead the W7 segment is likely to represent a downfaulted block of the North Scotia Ridge of the Fuegian Andes continental margin arc, or is potentially related to the putative Cretaceous Central Scotia Sea.

Respiration rates and active carbon flux of mesopelagic fishes (Family Myctophidae) in the Scotia Sea, Southern Ocean

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 610:149-162 (2019) - DOI: https://doi.org/10.3354/meps12861 Respiration rates and active carbon flux of mesopelagic fishes (Family Myctophidae) in the Scotia Sea, Southern Ocean Anna Belcher*, Ryan A. Saunders, Geraint A. Tarling British Antarctic Survey, Natural Environment Research Council, Cambridge, CB3 0ET, UK *Corresponding author: annbel@bas.ac.uk ABSTRACT: Mesopelagic fish have recently been highlighted as an important, but poorly studied component of marine ecosystems, particularly regarding their role in the marine pelagic food webs and biogeochemical cycles. Myctophids (Family Myctophidae) are one of the most biomass-dominant groups of mesopelagic fishes, and their large vertical migrations provide means of rapid transfer of carbon to the deep ocean where it can be sequestered for centuries or more. In this study, we develop a simple regression for the respiration rate of myctophid fish using literature-based wet mass and habitat temperature data. We apply this regression to net haul data collected across the Scotia-Weddell sector of the Southern Ocean to estimate respiration rates of the biomass-dominant myctophid species. Electrona carlsbergi, Electrona antarctica, and Gymnoscopelus braueri made a high contribution (up to 85%) to total myctophid respiration. Despite the lower temperatures of the southern Scotia Sea (-1.46 to 0.95°C), total respiration here was as high (reaching 1.1 mg C m-2 d-1) as in the warmer waters of the mid and northern Scotia Sea. The maximum respiratory carbon flux of the vertically migrating community was 0.05 to 0.28 mg C m-2 d-1, equivalent to up to 47% of the gravitational particulate organic carbon flux in some parts of the Scotia-Weddell region. Our study provides the first baseline estimates of respiration rates and carbon flux of myctophids in the Southern Ocean. However, direct measurements of myctophid respiration, and of mesopelagic fish generally, are needed to constrain these estimates further and incorporate these fluxes into carbon budgets. KEY WORDS: Myctophid · Respiration · Lantern fish · Carbon · Active flux · Southern Ocean Full text in pdf format Supplementary materialCorrigendum PreviousNextCite this article as: Belcher A, Saunders RA, Tarling GA (2019) Respiration rates and active carbon flux of mesopelagic fishes (Family Myctophidae) in the Scotia Sea, Southern Ocean. Mar Ecol Prog Ser 610:149-162. https://doi.org/10.3354/meps12861 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 610. Online publication date: February 01, 2019 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2019 Inter-Research.

Threatened species drive the strength of the carbonate pump in the northern Scotia Sea

The efficiency of deep-ocean CO2 sequestration is regulated by the relative balance between inorganic and organic carbon export respectively acting through the biological carbon pump (BCP) and the carbonate counter pump (CCP). The composition and abundance of calcifying species in the prevailing oceanic plankton community plays a major role in driving the CCP. Here we assess the role of these calcifying organisms in regulating the strength of the CCP in a Southern Ocean region (northern Scotia Sea) known to be a major hotspot for the drawdown of atmospheric CO2. We show that, when shelled pteropods dominate the calcifying community, the total annual reduction of CO2 transferred to the deep ocean doubles (17%) compared to when other plankton calcifiers dominate (3–9%). Furthermore, predation enhances their contribution through the removal of organic soft tissue. Pteropods are threatened in polar regions by ocean warming and acidification. We determine that their potential decline would have major implications to the comparative strengths of the BCP and CCP.

Stepping stones to isolation: Impacts of a changing climate on the connectivity of fragmented fish populations.

In the marine environment, understanding the biophysical mechanisms that drive variability in larval dispersal and population connectivity is essential for estimating the potential impacts of climate change on the resilience and genetic structure of populations. Species whose populations are small, isolated and discontinuous in distribution will differ fundamentally in their response and resilience to environmental stress, compared with species that are broadly distributed, abundant and frequently exchange conspecifics. Here, we use an individual‐based modelling approach, combined with a population genetics projection model, to consider the impacts of a warming climate on the population connectivity of two contrasting Antarctic fish species, Notothenia rossii and Champsocephalus gunnari. Focussing on the Scotia Sea region, sea surface temperatures are predicted to increase significantly by the end of the 21st century, resulting in reduced planktonic duration and increased egg and larval mortality. With shorter planktonic durations, the results of our study predict reduced dispersal of both species across the Scotia Sea, from Antarctic Peninsula sites to islands in the north and east, and increased dispersal among neighbouring sites, such as around the Antarctic Peninsula. Increased mortality modified the magnitude of population connectivity but had little effect on the overall patterns. Whilst the predicted changes in connectivity had little impact on the projected regional population genetic structure of N. rossii, which remained broadly genetically homogeneous within distances of ~1,500 km, the genetic isolation of C. gunnari populations in the northern Scotia Sea was predicted to increase with rising sea temperatures. Our study highlights the potential for increased isolation of island populations in a warming world, with implications for the resilience of populations and their ability to adapt to ongoing environmental change, a matter of high relevance to fisheries and ecosystem‐level management.

Impacts of rising sea temperature on krill increase risks for predators in the Scotia Sea.

Climate change is a threat to marine ecosystems and the services they provide, and reducing fishing pressure is one option for mitigating the overall consequences for marine biota. We used a minimally realistic ecosystem model to examine how projected effects of ocean warming on the growth of Antarctic krill, Euphausia superba, might affect populations of krill and dependent predators (whales, penguins, seals, and fish) in the Scotia Sea. We also investigated the potential to mitigate depletion risk for predators by curtailing krill fishing at different points in the 21st century. The projected effects of ocean warming on krill biomass were strongest in the northern Scotia Sea, with a ≥40% decline in the mass of individual krill. Projections also suggest a 25% chance that krill biomass will fall below an established depletion threshold (75% of its unimpacted level), with consequent risks for some predator populations, especially penguins. Average penguin abundance declined by up to 30% of its unimpacted level, with up to a 50% chance of falling below the depletion threshold. Simulated krill fishing at currently permitted harvest rates further increased risks for depletion, and stopping fishing offset the increased risks associated with ocean warming in our model to some extent. These results varied by location and species group. Risk reductions at smaller spatial scales also differed from those at the regional level, which suggests that some predator populations may be more vulnerable than others to future changes in krill biomass. However, impacts on predators did not always map directly to those for krill. Our findings indicate the importance of identifying vulnerable marine populations and targeting protection measures at appropriate spatial scales, and the potential for spatially-structured management to avoid aggravating risks associated with rising ocean temperatures. This may help balance tradeoffs among marine ecosystem services in an uncertain future.

Life cycle, distribution and trophodynamics of the lanternfish Krefftichthys anderssoni (Lönnberg, 1905) in the Scotia Sea

Myctophid fish play an important role in the Southern Ocean pelagic food web. The lanternfish Krefftichthys anderssoni is one of the most common myctophids in the region, but its ecology is poorly known. This study examines spatial and temporal patterns in the species distribution of density, life cycle, population structure and diet using samples collected by mid-water trawl nets deployed in different seasons across the Scotia Sea. Virtually absent from the sea-ice zone, the species was most abundant in the northern Scotia Sea around the Georgia Basin at depths below 400 m that are associated with the Circumpolar Deep Water. The species migrated during night from waters deeper than 700 m to waters above the 400 m following their main prey species: the copepods Rhincalanus gigas and Calanoides acutus and euphausiids of the Thysanoessa genus. Larvae length distribution and post-larvae length-frequency analyses suggested a life cycle of ~3 years with spawning and recruitment strongly connected with APF and the South Georgia shelf. Our results show that species spatial distribution, population structure and diet changed both seasonally and ontogenetically. This study is the most comprehensive examination of the ecology of K. anderssoni in the Southern Ocean to date and contributes to resolving how pelagic food webs and ecosystems operate in the region.

Constraining the dating of late Quaternary marine sediment records from the Scotia Sea (Southern Ocean)

In Southern Ocean sediments south of the Antarctic Polar Front, the scarcity of calcareous microfossils hampers the development of sediment chronologies based on radiocarbon dating and oxygen isotope stratigraphy established from carbonate. In this study, radiometric dating, magnetic susceptibility (MS), biogenic opal content, diatom abundance fluctuation, and paleomagnetic information were investigated on a north–south transect of central Scotia Sea sediment cores to verify their reliability as stratigraphic tools in the study area. Radiocarbon dating on organic carbon humic acid fraction can be used to establish the stratigraphy of upper core sections, but regional comparison and correlation are needed to verify a possible bias by fossil carbon contamination. For the long-term stratigraphy, MS, which can be correlated to the Antarctic ice core dust/climate signal, represents the most valuable parameter. Fine-grained single domain magnetite, probably of biogenic origin, makes a significant contribution to the interglacial MS signal, while major contributions from detrital material affect the glacial MS record. The core from the southern Scotia Sea contains significant proportions of biogenic magnetite also in glacial sediments, suggesting depositional environments different from those of the northern Scotia Sea. Our data suggest low contributions of high-coercive minerals to the overall magnetic intensity of glacial and interglacial Scotia Sea sediments, which excludes dust as a main source of the magnetic signal. Opal content can be used to distinguish between cold and warm intervals for the past 300 thousand years. Abundance fluctuation patterns of diatom species Fragilariopsis kerguelensis and Eucampia antarctica are useful stratigraphic tools for periods back to Marine Isotope Stage (MIS) 6. The Mono Lake geomagnetic excursion is identified in Scotia Sea sediments for the first time. Possible correlations of ash layers are suggested between Scotia Sea sediments and East Antarctic ice cores. They have potential to serve as additional age markers for further studies in this area.

The contribution of zooplankton faecal pellets to deep-carbon transport in the Scotia Sea (Southern Ocean)

Abstract. The northern Scotia Sea contains the largest seasonal uptake of atmospheric carbon dioxide yet measured in the Southern Ocean. This study examines one of the main routes by which this carbon fluxes to the deep ocean: through the production of faecal pellets (FPs) by the zooplankton community. Deep sediment traps were deployed at two sites with contrasting ocean productivity regimes (P3, naturally iron-fertilized, and P2, iron-limited) within the same water mass. The magnitude and seasonal pattern of particulate organic carbon (POC) and FPs in the traps was markedly different between the two sites. Maximum fluxes at P3 (22.91 mg C m−2 d−1; 2534 FP m−2 d1) were 1 order of magnitude higher than at P2 (4.01 mg C m−2 d−1; 915 FP m−2 d1, with flux at P3 exhibiting a double seasonal peak, compared to a single flatter peak at P2. The maximum contribution of FP carbon to the total amount of POC was twice as high at P3 (91%) compared to P2 (40%). The dominant FP category at P3 varied between round, ovoidal, cylindrical and tabular over the course of the year, while, at P2, ovoidal FPs were consistently dominant, always making up more than 60% of the FP assemblage. There was also a difference in the FP state between the two sites, with FPs being relatively intact at P3, while FPs were often fragmented with broken peritrophic membranes at P2. The exception was ovoidal FPs, which were relatively intact at both sites. Our observations suggest that there was a community shift from a herbivorous to an omnivorous diet from spring through to autumn at P3, while detritivores had a higher relative importance over the year at P2. Furthermore, the flux was mainly a product of the vertically migrating zooplankton community at P3, while the FP flux was more likely to be generated by deeper-dwelling zooplankton feeding on recycled material at P2. The results demonstrate that the feeding behaviour and vertical distribution of the zooplankton community plays a critical role in controlling the magnitude of carbon export to the deep ocean in this region.

Insights about the structure and evolution of the Scotia Arc from a new magnetic data compilation

Analysis of a new regional compilation of magnetic anomalies from marine, aeromagnetic and satellite data reveals the main structural/tectonic elements of the Scotia Arc. The most relevant magnetic anomaly in the continental crust, the Pacific Margin Anomaly (PMA), is related to composite magmatic arc batholiths. It was emplaced by subduction processes along the Pacific continental margin of the Antarctic Peninsula and can be followed within the continental blocks of the South Scotia Ridge and South America. Four representative magnetic profiles also show the structure in depth, and allow us to characterize the main crustal elements of the region. The new compilation and models improve our knowledge of the Scotia Arc's development. The PMA is seen to have a roughly W–E orientation, decreasing in intensity eastwards from the Pacific Margin of the Antarctic Peninsula, and extending towards the South Scotia Ridge to Discovery Bank and even to Herdman Bank. However, the identification of the PMA in the North Scotia Ridge is uncertain, since the magnetic anomalies and the modeled profiles do not support the presence of an important batholithic body. This setting can be attributed to the kinematics of subduction, almost orthogonal to the Pacific margin of the Antarctic Peninsula and oblique along the South American margin. Based on the new magnetic anomaly map, magnetic modeling, and the continuity of the PMA along the Antarctic Peninsula and South Scotia Ridge, we propose a reconstruction of the initial distribution of the main continental blocks in the initial stages during the Cretaceous. The anomalies identified in the northern Scotia Sea are probably related to local basic and/or intermediate igneous rocks intruded in pull-apart basins that developed in the South America–Antarctica plate boundary deformation zone during the initial stages of South Atlantic Ocean and Weddell Sea spreading.

Food web structure and bioregions in the Scotia Sea: A seasonal synthesis

Bioregionalisation, the partitioning of large ecosystems into functionally distinct sub-units, facilitates ecosystem modelling, management and conservation. A variety of schemes have been used to partition the Southern Ocean, based variously on frontal positions, sea ice, productivity, water depth and nutrient concentrations. We have tested the utility and robustness of ecosystem bioregionalisation for the Scotia Sea, by classifying spring, summer and autumn stations on the basis of nutrient concentrations, phytoplankton taxa, meso- and macrozooplankton, fish catches and acoustic data. Despite sampling across different seasons and years, at different spatial scales and taxonomic resolutions, cluster analysis indicated basically consistent spatial divisions across this wide range of trophic levels. Stations could be classified into two main groups, lying broadly to the north and south of the Southern Antarctic Circumpolar Current Front (SACCF). In some aspects the 2 station groups were similar, with both having variable but often high phytoplankton biomass as well as similar biomass of fish. However, the colder water southern group, most of which was covered by seasonal sea ice, had a fundamentally different food web structure to that in the northern Scotia Sea. The cold water community had a depleted, cold-adapted fauna characterised by low zooplankton biomass, Euphausia superba and the fish Electrona antarctica and Gymnoscopelus braueri. In contrast the northern group was richer with higher mesozooplankton biomass and a fauna of warmer or more cosmopolitan species such as Themisto gaudichaudii, Euphausia triacantha and the fish Protomyctophum bolini, Kreffticthys anderssoni and Gymnoscopelus fraseri. The position of the food web transition, broadly consistent with the position of the SACCF, supports a recent circumpolar-scale bioregionalisation. However, there is little evidence that this relatively weak frontal transition represents a significant barrier either here or elsewhere in the Southern Ocean. We suggest that broader-scale factors, namely temperature and possibly the extent of the seasonal ice-zone, within which most of the southern stations lay, were more likely to influence biological zonation.

Food web dynamics in the Scotia Sea in summer: A stable isotope study

The pelagic food web of the Scotia Sea was studied by analysing natural abundances of nitrogen and carbon stable isotopes of primary producers and pelagic consumers, sampled from the seasonal ice edge in the south to the Antarctic Polar Front in the north. The analysis covered, within a single mid-summer period, particulate organic matter (POM) and 38 taxa, ranging from suspension feeding copepods and salps to omnivorous euphausiids, pelagic fish and higher, land-based predators including fur seals, penguins and flying birds. Spatial variation in δ 15N of POM correlated well with nutrient availability and primary productivity. Latitudinal differences in δ 13C of POM were closely linked to variations in temperature, nutrients and productivity depending on the frontal region sampled. This translated to equivalent (although smaller) regional δ 13C differences among higher trophic levels. The trophic positions of species based on isotope values broadly agreed with previously published dietary data with three important exceptions. First, the carnivorous amphipod Themisto gaudichaudii had anomalously low δ 15N values. Second, Euphausia superba had δ 15N values that were also surprisingly low, considering the abundant literature suggesting its omnivory. Third, the copepod Rhincalanus gigas, considered a suspension feeder, had unexpectedly high δ 15N values rather more in keeping with omnivorous feeding. The consumer δ 15N values ranged from 1.2‰ (min.) measured in Salpa thompsoni (designated here as trophic level (TL) 2 across all regions) to 15.2‰ (max.) measured in white-chinned petrels ( Procellaria aequinoctialis, calculated as TL5 relative to the TL2 of salps). Excluding seabirds, the resulting food chain length of 3.7 TL (above POM at TL1) was lower than in most other Southern Ocean and temperate marine pelagic ecosystems. The majority (60%) of vertebrate predators occupied only 1–1.5 trophic levels above the herbivorous suspension feeders such as krill. This indicates the existence of the classic short food chain of POM–suspension feeder–vertebrate predator. However the presence of trophic levels 4 and above indicates the existence of alternative trophic pathways, for example involving myctophid fish or carrion, and that some wide-ranging predators which breed at South Georgia also feed outside the region. This conclusion is supported first by the continuum of δ 15N values between krill, suspension feeding copepods and myctophid fish, and secondly by higher trophic levels in several of the myctophid species in the low-krill region of the northern Scotia Sea, suggesting latitudinal differences in food web structure and food chain length.

Substantial primary production in the land-remote region of the central and northern Scotia Sea

The Scotia Sea area has high productivity relative to the Southern Ocean as a whole, but this displays strong latitudinal and longitudinal gradients. Elucidating the extent of these from a single cruise is problematic, given the high variability of bloom timing and location in this region. Therefore, this study used data from transects across the central Scotia Sea in spring, summer and autumn of 2006, 2008 and 2009, combined with satellite data, to obtain a larger-scale appreciation of the latitudinal contrasts in phytoplankton standing stock and primary production across the region. Concentrations of nitrate, phosphate and particularly silicic acid increased towards the south of the transect with the latter showing a step change at the Southern Antarctic Circumpolar Current Front (SACCF). Changes in seasonal nutrient concentrations indicated increasing phytoplankton uptake north of ∼57°S that peaked at ∼53°S in the Georgia Basin. Based on seasonal depletions of nitrate relative to phosphate, the highest relative nitrate uptake occurred northwest of South Georgia on the periphery of the Georgia Basin, indicating efficient nitrate use here due to iron-replete conditions. An integrative approach to examine these gradients was with the use of 10-year satellite climatology data. These showed that the lowest mean chlorophyll a (chl- a) values were in the central/northern Scotia Sea, but these were still substantial values, 67% of values within the Georgia Basin bloom. Cruise data on chl- a and on microplankton biomass from cell counts support this finding of substantial biomass in the central Scotia Sea; since these averaged half of values in the iron-fertilised bloom of the Georgia Basin downstream of South Georgia. Given that our transect was nearly 1000 km long and in parts was land remote with low iron concentrations, the relatively high production in the central and northern Scotia Sea is surprising. Iron levels may be maintained here by efficient recycling and irregular injections, possibly for example from dust deposition or shelf-derived inputs from the south. The moderate chl- a concentration across the mid-Scotia Sea and southwest of South Georgia reflect periodic, non-ice-associated blooms that occur in some years and not others. These may provide a connection between the large populations of krill inhabiting the northern and southern fringes of the Scotia Sea.

Antarctic bottom water in the Scotia Sea and the Drake Passage

The quantitative features and circulation of the Antarctic bottom water (AABW) in the Scotia Sea are investigated using an original procedure for the determination of the boundaries between the water masses. It is shown that the AABW is effectively transferred across the Antarctic Circumpolar Current (ACC) from the regions on the south flank of this current where the AABW penetrates into the Scotia Sea. This transfer results in the abyssal water cooling and freshening in the Yaghan Basin of the north Scotia Sea. Some rises and depressions in the bottom relief of the western and northern Scotia Sea are important features that impact the AABW transfer. It is shown that there is an additional path of the AABW transit transport to the North Atlantic passing through the western Scotia Sea. The existence of the semienclosed cyclonic abyssal water circulation in the South Shetland Trench and the westward transport of the Atlantic AABW along the Antarctic slope foot into the Pacific are proved.