Ontogeny of the lymphoid organs in an antarctic teleost, Harpagifer antarcticus (Notothenioidei: Perciformes)

first_imgThe effect of an evolutionary adaptation to low environmental temperature on the development of lymphoid organs was examined in Harpagifer antarcticus from Signy Island (South Orkney Islands; 60° 43′S, 45° 38′W). Thymus, pronephric kidney and spleen were typical, both in position and structural development, of those observed in warmer-water teleosts. The pronephric kidney was the first organ to be infiltrated by leucocytes, at 1 h post-hatch, though the infiltration of the thymic epithelia and the development of the splenic anlage were not observed until 4 weeks post-hatch. Full development of the lymphoid organs was not achieved until the juvenile stage. Although an increased infiltration of the thymus, by subepithelial connective tissues and epithelial mucous cells, occurred in the juvenile and adult stages, there was no evidence of an advanced stage of thymic regression or involution in the adult Harpagifer. Thus, a suppressive influence of the low temperature environment, on the onset and degree of lymphoid organ development and thymic involution, was indicated in this species.last_img read more

Oribatid mites (Acari: Oribatida) of the Falkland Islands, South Atlantic and their zoogeographical relationships

first_imgA total of 21 species of oribatid mites (Acari: Oribatida) are recorded from heat-extracted samples of soil and plant material collected in the Falkland Is., South Atlantic. Previous records of a further 11 species provide a total of 32 species. The distribution of each non-endemic species recorded from the Falkland Is. is figured. The oribatid mite fauna of these islands belongs to the Neotropical Region with strong sub-Antarctic elements and some similarities with New Zealand.last_img

Temporal plankton dynamics in a maritime Antarctic Lake

first_imgChanges in abundance, diversity and productivity of plankton in a maritime Antarctic lake were studied between December 1994 and February 1996. There were large intra- and inter-annual fluctuations in population densities, which were related to changing physical and chemical parameters. The plankton included an abundant protozoan population, comprising at least 66 taxa. This is amongst the highest diversities so far reported for Antarctic lakes. Heterotrophic nanoflagellates (HNF) were the most common, with densities between 4 × 104 and 1.5 × 107 l-1, and were the largest contributors to total protozoan carbon biomass for most of the year. Ciliate abundance varied from 4.0 × 100 to 1.4 × 104 l-1 and included taxa from several trophic levels. Numbers of naked amoebae were usually low but occasional patches of high density occurred. An anoxic sump developed in bottom waters at the end of the winter and this contained a distinct population of anaerobic HNF. There was evidence that the excrement of increasing fur seal numbers in the lake catchment over the last 15 years is having an impact on the lake ecosystem. The Chl-a maximum of 49 μg l-1 and primary productivity of up to 40 mgC m-3 h-1 were significantly higher than those reported by previous studies of this lake and continental Antarctic lakes. Increased summer bacterial abundance and productivity, together with higher winter nutrient concentrations, were also noted.last_img read more

The effect of Forbush decreases on tropospheric parameters over South Pole

first_imgEgorova et al. (2000) conclude that Forbush decreases (FDs) in galactic cosmic rays have a significant effect on the atmosphere at Antarctic station Vostok (78.5°S, 106.9°E) via the mechanism of electrofreezing. We present the results of a similar study at South Pole (90.0°S), located not, vert, similar1000 km from Vostok, which has been conducted in order to examine the spatial extent of this phenomenon. Both Vostok and South Pole are ideal locations at which to search for such an effect since they are subject to relatively stable weather regimes. We find no observable effect of FDs on the atmosphere above South Pole. We discuss possible reasons for the disagreement between the results of the two studies and conclude that it is due to differences in methodology.last_img read more

Tracer-derived freshwater composition of the Siberian continental shelf and slope following the extreme Arctic summer of 2007

first_imgWe investigate the freshwater composition of the shelf and slope of the Arctic Ocean north of the New Siberian Islands using geochemical tracer data (δ18O, Ba, and PO4*) collected following the extreme summer of 2007. We find that the anomalous wind patterns that partly explained the sea ice minimum at this time also led to significant quantities of Pacific‐derived surface water in the westernmost part of the Makarov Basin. We also find larger quantities of meteoric water near Lomonosov Ridge than were found in 1995. Dissolved barium is depleted in the upper layers in one region of our study area, probably as a result of biological activity in open waters. Increasingly ice‐free conditions compromise the quantitative use of barium as a tracer of river water in the Arctic Ocean.last_img read more

Breaking the routine: individual Cory’s shearwaters shift winter destinations between hemispheres and across ocean basins

first_imgThere is growing evidence that migratory species are particularly vulnerable to rapid environmental changes arising from human activity. Species are expected to vary in their capacity to respond to these changes: long-distance migrants and those lacking variability in migratory traits are probably at considerable disadvantage. The few studies that have assessed the degree of plasticity in behaviour of marine animals suggest that fidelity to non-breeding destinations is usually high. In the present study, we evaluated individual flexibility in migration strategy of a highly pelagic seabird, the Cory’s shearwater Calonectris diomedea. Geolocation data from 72 different migrations, including 14 birds that were tracked for more than one non-breeding season, showed a remarkable capacity to change winter destinations between years. Although some birds exhibited high site fidelity, others shifted from the South to North Atlantic, from the western to eastern South Atlantic, and from the Atlantic to Indian Ocean. Individuals also showed flexibility in stopover behaviour and migratory schedule. Although their K-selected life-history strategy has the disadvantage that the chances of microevolution are slight if circumstances alter rapidly, these results suggest that Cory’s shearwaters may be in a better position than many other long-distance migrants to face the consequences of a changing environment.last_img read more

Modeling the mass and surface heat budgets in a coastal blue ice area of Adelie Land, Antarctica

first_imgMeteorological data recorded from December 12, 2008 to June 30, 2010 were analyzed to assess the Surface Energy Balance (SEB) in a blue ice area of Cap Prudhomme, Adelie Land (66°41’S, 139°55’E). The SEB was computed with a newly developed model forced by direct measurements and with a voluntarily limited number of parameters to better assess model sensitivity. Incoming shortwave radiation was corrected for the slope and orientation of the local terrain assuming direct and diffuse radiation components. Turbulent heat fluxes were assessed using the bulk aerodynamic approach. Heat conduction in the ice was computed by solving the thermal diffusion equation. Snow accumulation was modeled using ERA-interim total precipitation and a one-dimension erosion model. The surface heat budget and accumulation/erosion model accurately reproduced field observations. The occurrence of blue ice is linked with higher rates of erosion than in the surrounding snow covered areas, which may be caused by local flow divergence or snow not being redistributed from higher elevations. Melting occurs between December and February when incoming shortwave radiation is high. However, the SEB was closely linked to air temperature through the incoming longwave radiation and the turbulent sensible heat flux. Several warm events caused by cyclones intruding into the continent led to significant warming of the ice and highmelting rates. Intruding cyclones were also associated with high precipitation that led to significant accumulation. Except in blue ice areas, modeling suggests that expected higher precipitation in a warmer climate will result in more accumulation.last_img read more

The links between large igneous provinces, continental break-up and environmental change: evidence reviewed from Antarctica

first_imgEarth history is punctuated by events during which large volumes of predominantly mafic magmas were generated and emplaced by processes that are generally accepted as being, unrelated to ‘normal’ sea-floor spreading and subduction processes. These events form large igneous provinces (LIPs) which are best preserved in the Mesozoic and Cenozoic where they occur as continental and ocean basin flood basalts, giant radiating dyke swarms, volcanic rifted margins, oceanic plateaus, submarine ridges, and seamount chains. The Mesozoic history of Antarctica is no exception in that a number of different igneous provinces were emplaced during the initial break-up and continued disintegration of Gondwana, leading to the isolation of Antarctica in a polar position. The link between the emplacement of the igneous rocks and continental break-up processes remains controversial. The environmental impact of large igneous province formation on the Earth System is equally debated. Large igneous province eruptions are coeval with, and may drive environmental and climatic effects including global warming, oceanic anoxia and/or increased oceanic fertilisation, calcification crises, mass extinction and release of gas hydrates. This review explores the links between the emplacement of large igneous provinces in Antarctica, the isolation of Antarctica from other Gondwana continents, and possibly related environmental and climatic changes during the Mesozoic and Cenozoic.last_img read more

From strategic ambiguity to technical reference points in the Antarctic krill fishery: the worst journey in the world

first_imgThe goals of ecosystem based management (EBM) are strategically ambiguous, meaning that they require interpretation to identify objectives for ecosystem state. Ecosystem states that are useful for achieving such objectives are known as reference points. Soft reference points specify both a state and a probability of the ecosystem being in that state. They are used with simulation models to identify management measures for which the risk of the ecosystem entering an undesirable state is below a specified level. The Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) is responsible for the EBM of Antarctic krill fisheries. CCAMLR used soft reference points for the krill stock in the Scotia Sea and southern Drake Passage to set a regional catch limit. However, this catch limit needs spatial subdivision to protect predators from localized depletion. Model-based evaluations of different options for subdividing the catch limit used illustrative reference points to assess the depletion risk to multiple predators. This study demonstrates that the apparent risk is sensitive to the choice of reference point and method for aggregating modelled predators. EBM practitioners and stakeholders need to be aware that these factors could therefore bias comparisons of management measures. Nonetheless, qualitative distinctions between different spatial subdivision options are relatively consistent except at high levels of aggregation and extreme reference points. This study also demonstrates a lack of generality in the relationship between current and future ecosystem state. Thus, the EBM goal of maintaining ecosystem resilience implies different reference points for the current state of different ecosystem components. Despite early progress in defining soft reference points for the krill stock, CCAMLR has not yet defined reference points for krill predators. Structured dialogue aimed at identifying collective objectives might be necessary to achieve further progress in CCAMLR and other EBM organizations.last_img read more

Surface melt and ponding on Larsen C Ice shelf and the impact of foehn winds

first_imgA common precursor to ice shelf disintegration, most notably that of Larsen B Ice Shelf, isunusually intense or prolonged surface melt and the presence of surface standing water. However, therehas been little research into detailed patterns of melt on ice shelves or the nature of summer melt ponds.We investigated surface melt on Larsen C Ice Shelf at high resolution using Envisat advanced syntheticaperture radar (ASAR) data and explored melt ponds in a range of satellite images. The improvedspatial resolution of SAR over alternative approaches revealed anomalously long melt duration inwestern inlets. Meteorological modelling explained this pattern by föhn winds which were common inthis region.Melt ponds are difficult to detect using optical imagery because cloud-free conditions are rarein this region and ponds quickly freeze over, but can be monitored using SAR in all weather conditions.Melt ponds up to tens of kilometres in length were common in Cabinet Inlet, where melt duration wasmost prolonged. The pattern of melt explains the previously observed distribution of ice shelfdensification, which in parts had reached levels that preceded the collapse of Larsen B Ice Shelf,suggesting a potential role for föhn winds in promoting unstable conditions on ice shelves.last_img read more