Digenean Trematoda infection of inshore fish at South Georgia

first_imgA sample of 111 fish of eight species caught in two fjords at South Georgia were examined for digenean trematode parasites. The alimentary tracts of all specimens were infected with digeneans. The dominant species was Elytrophalloides oatesi (Leiper & Atkinson) which was found in all fish, with a maximum number of 1961 specimens per fish. Other common species were; Macvicaria pennelli (Leiper & Atkinson), Lepidapedon garrardi (Leiper & Atkinson), Lecithaster macrocotyle Szidat & Graefe, Genolinea bowersi (Leiper & Atkinson) and Postmonorchis variabilis Prudhoe & Bray. Three further species, Neolebouria antarctica (Szidat & Graefe), Discoverytrema markowskii Gibson and Gonocerca phycidis Manter, were rare. Infection of the most commonly caught fish at South Georgia, Notothenia rossii Richardson, is compared with that of N. rossii at Admiralty Bay, South Shetland Islands. The species composition of common parasites was similar in both areas but conspicuous differences in the frequency of individual digenean species were found.last_img read more

Reassessment of net surface mass balance in Antarctica

first_imgRecent in situ measurements of surface mass balance and improved calculation techniques are used to produce an updated assessment of net surface mass balance over Antarctica. A new elevation model of Antarctica derived from ERS-1 satellite altimetry supplemented with conventional data was used to delineate the ice flow drainage basins across Antarctica. The areas of these basins were calculated using the recent digital descriptions of coastlines and grounding lines. The delineation of drainage basins was achieved using an automatic procedure, which gave similar results to earlier hand-drawn catchment basins. More than 1800 published and unpublished in situ measurements of net surface mass balance from Antarctica were collated and then interpolated. A net surface mass balance map was derived from passive microwave satellite data, being employed as a forcing field to control the interpolation of the sparse in situ observations. Basinwide integrals of net surface mass balance were calculated using tools available within a geographic information system. It is found that the integrated net surface mass balance over the conterminous grounded ice sheet is 1811 Gton yr−1 (149 kg m−2 yr−1), and over the entire continent (including ice shelves and their embedded ice rises) it is 2288 Gton yr−1 (166 kg m−2 yr−1). These values are around 18% and 7% higher than the estimates widely adopted at present. The uncertainty in these values is hard to estimate from the methodology alone, but the progression of estimates from early studies to the present suggests that around ±5% uncertainty remains in the overall values. The results serve to confirm the great uncertainty in the overall contribution of the Antarctic Ice Sheet to recent and future global sea level rise even without a substantial collapse of the West Antarctic Ice Sheet.last_img read more

Poleward‐moving HF radar flow bursts in the cusp: Transient changes in flow speed or direction?

first_imgPoleward‐moving line‐of‐sight velocity “flow bursts” have been observed in the cusp by two southern hemisphere SuperDARN HF radars with overlapping fields‐of‐view. This has allowed the estimation of unambiguous two‐dimensional velocity vectors in the vicinity of the “flow bursts”. Rather than showing enhancements in the flow magnitude, the velocity vectors suggest that the line‐of‐sight velocity enhancements are a result of a change in the direction of the flow associated with latitudinal motion of the convection reversal boundary. These observations may have important implications for understanding the ionospheric footprint of flux transfer events, and also illustrate that caution is needed when interpreting line‐of‐sight velocity data from single radars.last_img read more

Effects of a viscoelastic lithosphere on the isostatic bedrock response

first_imgThe lithosphere responds to loading by elastic flexure which is followed by viscous relaxation, the amount of which depends on the stress duration. This study compares results of an Earth model in which the lithosphere is modelled as a purely elastic layer and as a more general viscoelastic solid overlain by a rigid crust. It shows the emergence of a noticeable difference in the short wavelength (1×102–5×102 km) component of the bedrock deformation after loading durations of the order of 105–106 yr assuming a lower lithosphere viscosity of the order of 1023–1024 Pa.s. In particular, for the long-term loading hypothesised to be imposed by the East Antarctic ice sheet, we find that aviscoelasticlithosphere yields a more local deformation pattern to which ice sheet dynamics are highly sensitive. It confirms that modelling of the Antarctic long-term evolution would benefit from a fully coupled ice/bedrock approach in which the lithosphere would be represented by aviscoelastic solid.last_img read more

The surface climatology of an ordinary katabatic wind regime in Coats Land, Antarctica

first_imgThe surface climatology of Coats Land, Antarctica, is described through observations from automatic weather stations, from Halley station, from upper air soundings and from satellite remote sensing. Coats Land consists of the Brunt Ice Shelf and the adjoining continent to the South. The topography of this region is typical of much of the Antarctic coastal fringes: a modest slope (5% at most) and relative uniformity across the slope. A basic climatology broken into site and season is presented. In winter, and to an extent in the equinoctial seasons, the region clearly divides into two dynamical regimes. Over the ice shelf winds are usually from the east or occasionally from the west, whereas over the continental slopes winds are from the east to south quadrant. Over the ice shelf the surface layer is about 10 K colder, in terms of potential temperature, than on the continent, and is also more stable than on the steeper parts of the slope. Motivated by case studies. three criteria are developed to select a subset of the data that are katabatic in the sense that the flow is believed to be primarily due to a downslope buoyancy forcing. On the continental slope, the Criteria pick out a coherent Subset of the data that are tightly clustered in wind speed and wind direction. Typical katabatic winds are from 10degrees to the east of the fall line and 7.5 ms(-1) at the steepest part of the slope (5.1 ms(-1) higher up). They are rarely more than 15 ms(-1) in this region; hence their description as ordinary, in contrast with those extraordinary katabatic regimes that have been the focus of previous studies. The katabatic flow remains close to adiabatic as it moves down the slope, and is relatively dry near the slope foot. We estimate the flow to be primarily katabatic at most 40-50% of the time, although it may appear to be katabatic, from wind speed and wind direction characteristics, some 60-70% of the time. There is no coherent katabatic-flow signature on the ice shelf.last_img read more

Weight loss during breeding is adaptive for female macaroni penguins, Eudyptes chrysolophus

first_imgQuestion: How does the female macaroni penguin balance her own needs with those of her chick during breeding?Features of the model: We model the behaviour of female macaroni penguins during a sensitive life-history stage as a function of the availability of their main prey species, Antarctic krill (Euphausia superba), using stochastic dynamic programming. In the model, females maximize accumulated delivery to the chick, accounting for metabolic losses. Chick fullness is included as a state in the model. Range of key variables: We test three scenarios for krill availability, which changes with distance from the nest. In the first, krill abundance increases with distance from the nest, with no variability in the reward at each distance. In the second, variability increases proportionally with the increasing amount of krill available at each distance from the nest. In the third, the abundance of krill at each distance from the nest is constant, but variability decreases further from the nest. Conclusions: Natural selection should produce females that sacrifice their own condition to meet the increasing demands of their chicks. We predict a weight loss of 10-20%, which is comparable to the empirical average of 14%. We also predict that females will endure the cost of travelling further from the nest to obtain a more predictable meal of krill, even if the mean reward does not change with distance from the nest.last_img read more

Feeding ecology of myctophid fishes in the northern Scotia Sea

first_imgThe diets of 9 species of myctophid fishes, Electrona carlsbergi, E. antarctica, Gymnoscopelus fraseri, G. nicholsi, G. braueri, Protomyctophum bolini, P. choriodon, Krefftichthys anderssoni and Nannobrachium achirus, were investigated during austral autumn in the northern ScotiaSea. Based on the percent index of relative importance (%IRI), the data suggest dietary specialisation in some species, which may permit resource partitioning. Hierarchical agglomerative cluster analysis of Bray-Curtis similarity (60% threshold) separated the myctophid community into distinct feeding guilds. One group (G. braueri and E. antarctica) fed principally on Themisto gaudichaudii, another (P. choriodon and G. fraseri) primarily on copepods (Metridia spp. and Rhincalanus gigas),and a third group (G. nicholsi and P. bolini) consumed copepods and euphausids (mostly Metridia spp. and Euphausia frigida). The diets of E. carlsbergi and K. anderssoni differed from the other species, with E. carlsbergi being the only species that consumed salps. There was a general switch in diet from copepods to euphausiids and amphipods as the myctophid predator size increased. Dietary specialisation is likely the result of a combination of predator size, gape size, filtering capacity of the gillrakers and the vertical distribution of predators and prey. Antarctic krill were only consumed by the larger myctophids, which represented a numerically minor part of the myctophid community, supporting the concept that myctophids can provide a krill-independent link between secondary production and higher trophic levels. However, the northern Scotia Sea is dominated by adult krill, whichare only suitable as prey for larger fish. In the northern Scotia Sea, myctophid predation had a very small impact on copepod production but a higher impact on macrozooplankton, with a best-estimate of 4% of the daily production of Themisto gaudichaudii and 6% of that of Euphausia superba being consumed.last_img read more

Strong population genetic structure in a broadcast-spawning Antarctic marine invertebrate

first_imgAlthough studies of population genetic structure are commonplace, a strong bias exists toward species from low latitudes and with relatively poor dispersal capabilities. Consequently, we used 280 amplified fragment length polymorphism bands to explore patterns of genetic differentiation among 8 populations of a high latitude broadcast-spawning marine mollusc, the Antarctic limpet Nacella concinna. Over 300 individuals were sampled along a latitudinal gradient spanning the Antarctic Peninsula from Adelaide Island to King George Island (67 degrees-62 degrees S), then to Signy Island (60 degrees S) and South Georgia (54 degrees S). Populations from the Antarctic Peninsula exhibited little genetic structure but were themselves strongly differentiated from both Signy and South Georgia. This finding was analytically highly robust and implies the presence of significant oceanographic barriers to gene flow in a species long regarded as a classic example of a widely dispersing broadcast spawner.last_img read more