Spawning locations of mackerel icefish at South Georgia

first_imgHistorical information on the distribution of spawning and larval mackerel icefish (Champsocephalus gunnari) within CCAMLR Subarea 48.3 (South Georgia and Shag Rocks)is assessed. This is considered alongside new data from commercial fisheries and research surveys. It is concluded that there is strong evidence of inshore spawning at SouthGeorgia during April within and close to the bays on the north side of the island. Some spawning almost certainly occurs over much of the island shelf, althougl~th is appears to be at a very much lower intensity than inshore. There is some evidence of spawning at Shag Rocks. There are also indications of a possible second spawning season in January, although the evidence is weak. Concentrations of larval icefish within Cumberland Bay are an order of magnitude higher than in adjacent coastal waters and their density declines exponentially offshore. All this evidence indicates that the most important spawning locations are within the bays.last_img read more

Plankton community structure and variability in the Scotia Sea: austral summer 2003

first_imgPlankton community structure in the Scotia Sea was investigated during January/earlyFebruary 2003 based on phytoplankton cell counts from 20 m depth and mesozooplankton countsfrom 0 to 400 m net hauls. Cluster analysis and multi-dimensional scaling revealed 4 major groups ofstations within each ordination that broadly corresponded geographically. A grouping of stations tothe east of the Antarctic Peninsula was characterised by low phytoplankton cell counts. The correspondinggrouping of stations in the mesozooplankton data were characterised by low abundance,overwintered state of many species, low egg production rates, and low carbon mass of copepodinstars. In contrast, groupings of stations in the northern part of the Scotia Sea were characterised aschlorophyll and mesozooplankton rich, and the summer generation was well advanced. Latitude wasmost strongly correlated with mesozooplankton community pattern (rank correlation ρ = 0.608),whereas surface chlorophyll a was a weaker correlate (ρ = 0.344) but along with measures of sizefractionedchlorophyll contributed towards explaining variation in species stages carbon mass andegg production rates. Additional hauls to 1000 m with an LHPR indicated copepod populations werebroadly in an overwintered state in the south of the region, whereas to the north of South Georgiarecruitment had been completed and some species were undergoing a seasonal descent. A comparisonwith January/February 2000 revealed higher abundances of krill larvae throughout the ScotiaSea in 2000 as well as a more advanced generation of the copepod Calanoides acutus. Ice cover duringthe 2 years differed considerably; in 2000 the position of the summer ice edge broadly accordedwith the 25 yr average, whereas in 2003 the ice edge lay much further north than usual. We suggestthat the timing of ice retreat influenced the timing of reproduction with the late retreat in 2003causing delayed reproduction and reduced population sizes.last_img read more

Late Neogene interglacial events in the James Ross Island region, northern Antarctic Peninsula, dated by Ar/Ar and Sr-isotope stratigraphy

first_imgNew outcrops of Late Neogene sedimentary deposits discovered on James Ross and Vega islands, northern Antarctic Peninsula, are fossiliferous and contain mainly fragmented pectinids amongst other as yet unstudied biota. The sedimentary deposits are interbedded with fresh volcanic units, thus providing the unusual opportunity to investigate the ages of the sedimentary rocks using two independent isotopic systems (40Ar/39Ar and 87Sr/86Sr). The fossils record past periods of warmer conditions (interglacials) like today. However, our results demonstrate unequivocally that some of the macrofossils present in the sedimentary rocks are reworked and the host sediments are, in these cases, much younger than the ages of their included fossils would suggest. Nevertheless, the Sr ages date these records of interglacial events in the region independently of the age of the strata in which they occur. Conversely, the volcanic rocks show abundant field evidences that they are coeval with the underlying sedimentary deposits, and hence their Ar ages reliably date the timing of their deposition. Our results indicate that at least three generally warmer periods are represented in the James Ross Island region: at 6.5–5.9, 5.03–4.22 and< 0.88 Ma, although our data do not have the resolution for identifying Milankovitch-scale cyclicity. However, the Early Pliocene warm period is particularly well represented and the James Ross Island data may well be capturing a higher proportion of the longer-lasting interglacials that characterised that period. We also present evidence for an interglacial in the Late Pliocene (at 2.54 + 0.86/− 0.36 Ma). Our data suggest that both Antarctic Peninsula and the East Antarctic ice sheets responded in a similar way to long-term regional shifts in climate, but the Antarctic Peninsula is more sensitive to short-term warming, as occurs today.last_img read more

Spatial distribution of average vorticity in the high-latitude ionosphere and its variation with interplanetary magnetic field direction and season

first_imgWe present a technique to measure the magnetic field-aligned vorticity of meso-scale plasma flows in the F-region ionosphere using line-of-sight velocity measurements made by the Super Dual Auroral Radar Network (SuperDARN).Vorticity is often used as a proxy for magnetic field-aligned current (FAC) intensity in the ionosphere but also provides information about turbulent processes in the ionosphere and magnetosphere.Using 6 years (2000-2005 inclusive) of vorticity measurements made by 6 SuperDARN radars in the northern hemisphere we have compiled, for the first time, maps of average vorticity across the northern polar ionosphere.These maps have been subdivided according to different seasonal and interplanetary magnetic field (IMF) conditions.The variations in the morphology of the vorticity maps with IMF direction match very closely those seen in maps of average FAC intensity (determined using different methods and instrumentation), suggesting that vorticity is a good proxy for FAC in an averaged sense.The variations in the morphology of the vorticity maps with season show differences from those seen in the FAC maps, illustrating that ionospheric conductance plays a major role in determining the differences between measurements of vorticity and FAC.last_img read more

Geographical variation in thermal tolerance within Southern Ocean marine ectotherms

first_imgLatitudinal comparisons of the Southern Ocean limpet, Nacella concinna, and clam, Laternula elliptica,acclimated to 0.0 °C, were used to assess differences in thermal response to two regimes, 0.0, 5.1 to 10.0 °Cand 2.5, 7.5 to 12.5 °C, raised at 5.0 °C per week. At each temperature, tissue energy status was measuredthrough a combination of O2 consumption, intracellular pH, cCO2, citrate synthase (CS) activity, organic acids(succinate, acetate, propionate), adenylates (ATP, ADP, AMP, ITP, PLA (phospho-L-arginine)) and heart rate.L. elliptica from Signy (60°S) and Rothera (67°S), which experience a similar thermal regime (−2 to +1 °C)had the same lethal (7.5–10.0 °C), critical (5.1–7.5 °C) and pejus (b5.1 °C;=getting worse) limits with onlysmall differences in biochemical response. N. concinna, which experiences a wider thermal regime (−2 to+15.8 °C), had higher lethal limits (10.0–12.5 °C). However, at their Northern geographic limit N. concinna,which live in a warmer environment (South Georgia, 54°S), had a lower critical limit (5.1–10.0 °C; O2, PLAand organic acids) than Rothera and Signy N. concinna (10.0–12.5 °C). This lower limit indicates that SouthGeorgia N. concinna have different biochemical responses to temperatures close to their thermal limit, whichmay make them more vulnerable to future warming trendslast_img read more

Pliocene Model Intercomparison Project (PlioMIP) : experimental design and boundary conditions (Experiment 1)

first_imgIn 2008 the temporal focus of the Palaeoclimate Modelling Intercomparison Project was expanded to include a model intercomparison for the mid-Pliocene warm period (3.29–2.97 million years ago). This project is referred to as PlioMIP (Pliocene Model Intercomparison Project). Two experiments have been agreed upon and comprise phase 1 of PlioMIP. The first (Experiment 1) will be performed with atmosphere-only climate models. The second (Experiment 2) will utilise fully coupled ocean-atmosphere climate models. The aim of this paper is to provide a detailed model intercomparison project description which documents the experimental design in a more detailed way than has previously been done in the literature. Specifically, this paper describes the experimental design and boundary conditions that will beutilised for Experiment 1 of PlioMIP.last_img read more

Global statistical evidence for chorus as the embryonic source of plasmaspheric hiss

first_imgThe origin of plasmaspheric hiss, the electromagnetic emission responsible for the gap between the inner and outer radiation belts, has been debated for over four decades. Recently, a new theory proposed that chorus, which is excited in the equatorial region outside the plasmapause, can propagate to low altitudes on the dayside and evolve into plasmaspheric hiss. Here we combine data from six satellites and show that chorus extends along the Earth’s magnetic field to high latitudes in the prenoon sector, and, in the equatorial region, there is a clear gap of the order of 1–2 Earth radii between plasmaspheric hiss at L∗<4 and chorus further out, consistent with ray tracing modeling from a chorus source. Our observations confirm two of the key predictions of the new theory and provide the first statistical evidence for chorus as the embryonic source of plasmaspheric hiss.last_img read more

Population genetics of three sympatric springtail species (Hexapoda: Collembola) from the South Shetland Islands: evidence for a common biogeographic pattern

first_imgThree sympatric springtail species, from the South Shetland Islands archipelago in the maritime Antarctic, are analysed here in a common biogeographic and evolutionary framework. This study was designed to compare their population genetic structure using the same molecular marker. Haplotype data for the mitochondrial cox1 gene have been obtained for seven populations of Folsomotoma octooculata and are compared with the data obtained, in previous studies and the current one, for the sympatric species Cryptopygus antarcticus antarcticus, and Friesea grisea. Molecular data obtained are consistent with the hypothesis that all species have been present in the archipelago since well before the last glacial maximum (around 20 000 ybp) and that their early diversifications appear to be linked with known interglacial periods in the region. These springtails may have survived the last glacial cycle in local refugia, from which they dispersed subsequently to ice-free ground re-exposed during the current interglacial period. The populations of the different species diversified at different times, although all of them are within the Pleistocene epoch. We propose that the earliest diversification of haplotypes in all three springtails in this archipelago occurred from local refugia in Livingston I., with subsequent spread of some haplotypes throughout the South Shetland Islands.last_img read more

Projected distributions of Southern Ocean albatrosses, petrels and fisheries as a consequence of climatic change

first_imgGiven the major ongoing influence of environmental change on the oceans, there is a need to understand and predict the future distributions of marine species in order to plan appropriate mitigation to conserve vulnerable species and ecosystems. In this study we use tracking data from seven large seabird species of the Southern Ocean (black-browed albatross Thalassarche melanophris, grey-headed albatross T. chrysostoma, northern giant petrel Macronectes halli, southern giant petrel M. giganteus, Tristan albatross Diomedea dabbenena, wandering albatross D. exulans and white-chinned petrel Procellaria aequinoctialis, and on fishing effort in two types of fisheries (characterised by low or high-bycatch rates), to model the associations with environmental variables (bathymetry, chlorophyll-a concentration, sea surface temperature and wind speed) through ensemble species distribution models. We then projected these distributions according to four climate change scenarios built by the Intergovernmental Panel for Climate Change for 2050 and 2100. The resulting projections were consistent across scenarios, indicating that there is a strong likelihood of poleward shifts in distribution of seabirds, and several range contractions (resulting from a shift in the northern, but no change in the southern limit of the range in four species). Current trends for southerly shifts in fisheries distributions are also set to continue under these climate change scenarios at least until 2100; some of these may reflect habitat loss for target species that are already over-fished. It is of particular concern that a shift in the distribution of several highly threatened seabird species would increase their overlap with fisheries where there is a high-bycatch risk. Under such scenarios, the associated shifts in distribution of seabirds and increases in bycatch risk will require much-improved fisheries management in these sensitive areas to minimise impacts on populations in decline.last_img read more

An analytical derivation of ice-shelf basal melt based on the dynamics of meltwater plumes

first_imgThe interaction between ice shelves and the ocean is an important process for the development of marine ice sheets. However, it is difficult to model in full detail due to the high computational cost of coupled ice-ocean simulations, so that simplified basal-melt parameterizations are required. In this work, a new analytical expression for basal melt is derived from the theory of buoyant meltwater plumes moving upward under the ice shelf and driving the overturning circulation within the ice-shelf cavity. The governing equations are nondimensionalized in the case of an ice shelf with constant basal slope and uniform ambient ocean conditions. An asymptotic analysis of these equations in terms of small slopes and small thermal driving, assumed typical for Antarctic ice shelves, leads to an equation that can be solved analytically for the dimensionless melt rate. This analytical expression describes a universal melt-rate curve onto which the scaled results of the original plume model collapse. Its key features are a positive melt peak close to the grounding line and a transition to refreezing further away. Comparing the analytical expression with numerical solutions of the plume model generally shows a close agreement between the two, even for more general cases than the idealized geometry considered in the derivation. The results show how the melt rates adapt naturally to changes in the geometry and ambient ocean temperature. The new expression can readily be used for improving ice-sheet models that currently still lack a sufficiently realistic description of basal melt.last_img read more