Physical controls on the distribution of phytoplankton round the Crozet Plateau, Southern Ocean

The spatial and temporal distribution of phytoplankton around the Crozet Plateau,Southern Ocean, is studied through a variety of satellite datasets and in situ cruise andArgo float measurements. Parts of the study area regularly contain phytoplanktonconcentrations significantly enhanced over the other parts, and over most of the rest ofthe Southern Ocean, due to natural iron fertilization from the Crozet Islands. Study ismade of these contrasts in order to further understand the processes that limitphytoplankton over the mostly High Nutrient Low Chlorophyll (HNLC) Southern Ocean.The longer timescales, larger areas and regularity of naturally iron-fertilizedphytoplankton blooms make them preferable to study compared to studies of the effectsof iron released from a ship. Data from these experiments, and from other islands in theSouthern Ocean, is investigated in the context of the results found around the CrozetPlateau.The circulation patterns around the Crozet Plateau are studied in detail, with particularemphasis on understanding where surface water that has passed close to the CrozetIslands flows, as this is a known source of iron to the surface water. The surface mixedlayer depth and incoming irradiance are also studied so that the variation in lightavailability with time and latitude can be understood. The distribution of phytoplankton isstudied through satellite chlorophyll-a images, with values adjusted to match thoseobserved in situ during the research cruises.The initial development of the bloom is controlled by variations in light availability butthe advection and lateral mixing of iron is sufficient to explain the spatial extent of thebloom and the variations in peak chlorophyll-a concentrations. The variability of thestrength of the bloom is driven mostly by variations in the iron utilization byphytoplankton during the preceding winter and wind-driven variations in the circulation

Circulation, retention, and mixing of waters within the Weddell-Scotia Confluence, Southern Ocean:The role of stratified Taylor columns

The waters of the Weddell-Scotia Confluence (WSC) lie above the rugged topography of the South Scotia Ridge in the Southern Ocean. Meridional exchanges across the WSC transfer water and tracers between the Antarctic Circumpolar Current (ACC) to the north and the subpolar Weddell Gyre to the south. Here, we examine the role of topographic interactions in mediating these exchanges, and in modifying the waters transferred. A case study is presented using data from a free-drifting, intermediate-depth float, which circulated anticyclonically over Discovery Bank on the South Scotia Ridge for close to 4 years. Dimensional analysis indicates that the local conditions are conducive to the formation of Taylor columns. Contemporaneous ship-derived transient tracer data enable estimation of the rate of isopycnal mixing associated with this column, with values of O(1000 m2/s) obtained. Although necessarily coarse, this is of the same order as the rate of isopycnal mixing induced by transient mesoscale eddies within the ACC. A picture emerges of the Taylor column acting as a slow, steady blender, retaining the waters in the vicinity of the WSC for lengthy periods during which they can be subject to significant modification. A full regional float data set, bathymetric data, and a Southern Ocean state estimate are used to identify other potential sites for Taylor column formation. We find that they are likely to be sufficiently widespread to exert a significant influence on water mass modification and meridional fluxes across the southern edge of the ACC in this sector of the Southern Ocean

The Role of Eddies and Topography in the Export of Shelf Waters From the West Antarctic Peninsula Shelf

Oceanic heat strongly influences the glaciers and ice shelves along West Antarctica. Prior studies show that the subsurface onshore heat flux from the Southern Ocean on the shelf occurs through deep, glacially carved channels. The mechanisms enabling the export of colder shelf waters to the open ocean, however, have not been determined. Here, we use ocean glider measurements collected near the mouth of Marguerite Trough (MT), west Antarctic Peninsula, to reveal shelf‐modified cold waters on the slope over a deep (2,700 m) offshore topographic bank. The shelf hydrographic sections show subsurface cold features (θ \u3c=1.5 °C), and associated potential vorticity fields suggest a significant instability‐driven eddy field. Output from a high‐resolution numerical model reveals offshore export modulated by small (6 km), cold‐cored, cyclonic eddies preferentially generated along the slope and at the mouth of MT. While baroclinic and barotropic instabilities appear active in the surrounding open ocean, the former is suppressed along the steep shelf slopes, while the latter appears enhanced. Altimetry and model output reveal the mean slope flow splitting to form an offshore branch over the bank, which eventually forms a large (116 km wide) persistent lee eddy, and an onshore branch in MT. The offshore flow forms a pathway for the small cold‐cored eddies to move offshore, where they contribute significantly to cooling over the bank, including the large lee eddy. These results suggest eddy fluxes, and topographically modulated flows are key mechanisms for shelf water export along this shelf, just as they are for the shoreward warm water transport

Modification of deep waters in Marguerite Bay, western Antarctic Peninsula, caused by topographic overflows

Circumpolar Deep Water (CDW) intrudes from the mid-layers of the Antarctic Circumpolar Current onto the shelf of the western Antarctic Peninsula, providing a source of heat and nutrients to the regional ocean. It is well known that CDW is modified as it flows across the shelf, but the mechanisms responsible for this are not fully known. Here, data from underwater gliders with high spatial resolution are used to demonstrate the importance of detailed bathymetry in inducing multiple local mixing events. Clear evidence for overflows is observed in the glider data as water flows along a deep channel with multiple transverse ridges. The ridges block the densest waters, with overflowing water descending several hundred metres to fill subsequent basins. This vertical flow leads to entrainment of overlying colder and fresher water in localised mixing events. Initially this process leads to an increase in bottom temperatures due to the temperature maximum waters descending to greater depths. After several ridges, however, the mixing is sufficient to remove the temperature maximum completely and the entrainment of colder thermocline waters to depth reduces the bottom temperature, to approximately the same as in the source region of Marguerite Trough. Similarly, it is shown that deep waters of Palmer Deep are warmer than at the same depth at the shelf break. The exact details of the transformations observed are heavily dependent on the local bathymetry and water column structure, but glacially-carved troughs and shallow sills are a common feature of the bathymetry of polar shelves, and these types of processes may be a factor in determining the hydrographic conditions close to the coast across a wider area

Antiplatelet treatment compared with anticoagulation treatment for cervical artery dissection (CADISS): a randomised trial.

BACKGROUND: Extracranial carotid and vertebral artery dissection is an important cause of stroke, especially in young people. In some observational studies it has been associated with a high risk of recurrent stroke. Both antiplatelet drugs and anticoagulant drugs are used to reduce risk of stroke but whether one treatment strategy is more effective than the other is unknown. We compared their efficacy in the Cervical Artery Dissection in Stroke Study (CADISS), with the additional aim of establishing the true risk of recurrent stroke. METHODS: We did this randomised trial at hospitals with specialised stroke or neurology services (39 in the UK and seven in Australia). We included patients with extracranial carotid and vertebral dissection with onset of symptoms within the past 7 days. Patients were randomly assigned (1:1) by an automated telephone randomisation service to receive antiplatelet drugs or anticoagulant drugs (specific treatment decided by the local clinician) for 3 months. Patients and clinicians were not masked to allocation, but investigators assessing endpoints were. The primary endpoint was ipsilateral stroke or death in the intention-to-treat population. The trial was registered with EUDract (2006-002827-18) and ISRN (CTN44555237). FINDINGS: We enrolled 250 participants (118 carotid, 132 vertebral). Mean time to randomisation was 3·65 days (SD 1·91). The major presenting symptoms were stroke or transient ischaemic attack (n=224) and local symptoms (headache, neck pain, or Horner's syndrome; n=26). 126 participants were assigned to antiplatelet treatment versus 124 to anticoagulant treatment. Overall, four (2%) of 250 patients had stroke recurrence (all ipsilateral). Stroke or death occurred in three (2%) of 126 patients versus one (1%) of 124 (odds ratio [OR] 0·335, 95% CI 0·006-4·233; p=0·63). There were no deaths, but one major bleeding (subarachnoid haemorrhage) in the anticoagulant group. Central review of imaging failed to confirm dissection in 52 patients. Preplanned per-protocol analysis excluding these patients showed stroke or death in three (3%) of 101 patients in the antiplatelet group versus one (1%) of 96 patients in the anticoagulant group (OR 0·346, 95% CI 0·006-4·390; p=0·66). INTERPRETATION: We found no difference in efficacy of antiplatelet and anticoagulant drugs at preventing stroke and death in patients with symptomatic carotid and vertebral artery dissection but stroke was rare in both groups, and much rarer than reported in some observational studies. Diagnosis of dissection was not confirmed after review in many cases, suggesting that radiographic criteria are not always correctly applied in routine clinical practice. FUNDING: Stroke Association.Hugh S Markus is supported by an NIHR Senior Investigator award and his work is supported by the Cambridge University Hospital Comprehensive Biomedical Research Centre. Adina Feldman was supported by a project grant from the British Heart Foundation (PG/13/30/30005).This is the final published version. It first appeared at http://www.thelancet.com/journals/laneur/article/PIIS1474-4422%2815%2970018-9/abstract

The seasonal cycle of ocean-atmosphere CO2 Flux in Ryder Bay, West Antarctic Peninsula

Approximately 15 million km2 of the Southern Ocean is seasonally ice covered, yet the processes affecting carbon cycling and gas exchange in this climatically important region remain inadequately understood. Here, 3 years of dissolved inorganic carbon (DIC) measurements and carbon dioxide (CO2) fluxes from Ryder Bay on the west Antarctic Peninsula (WAP) are presented. During spring and summer, primary production in the surface ocean promotes atmospheric CO2 uptake. In winter, higher DIC, caused by net heterotrophy and vertical mixing with Circumpolar Deep Water, results in outgassing of CO2 from the ocean. Ryder Bay is found to be a net sink of atmospheric CO2 of 0.59–0.94 mol C m−2 yr−1 (average of 3 years). Seasonal sea ice cover increases the net annual CO2 uptake, but its effect on gas exchange remains poorly constrained. A reduction in sea ice on the WAP shelf may reduce the strength of the oceanic CO2 sink in this region

Plasticity in dormancy behaviour of Calanoides acutus in Antarctic coastal waters

Copepods that enter dormancy, such as Calanoides acutus, are key primary consumers in Southern Ocean food webs where they convert a portion of the seasonal phytoplankton biomass into a longer-term energetic and physiological resource as wax ester (WE) reserves. We studied the seasonal abundance and lipid profiles of pre-adult and adult C. acutus in relation to phytoplankton dynamics on the Western Antarctic Peninsula. Initiation of dormancy occurred when WE unsaturation was relatively high, and chlorophyll a (Chl a) concentrations, predominantly attributable to diatoms, were reducing. Declines in WE unsaturation during the winter may act as a dormancy timing mechanism with increased Chl a concentrations likely to promote sedimentation that results in a teleconnection between the surface and deep water inducing ascent. A late summer diatom bloom was linked to early dormancy termination of females and a second spawning event. The frequency and duration of high biomass phytoplankton blooms may have consequences for the lifespan of the iteroparous C. acutus females (either 1 or 2 years) if limited by a total of two main spawning events. Late summer recruits, generated by a second spawning event, likely benefitted from lower predation and high phytoplankton food availability. The flexibility of copepods to modulate their life-cycle strategy in response to bottom-up and top-down conditions enables individuals to optimize their probability of reproductive success in the very variable environment prevalent in the Southern Ocean