9 May
2021

Microbial colonization processes in Antarctic fellfield soil – an experimental overview

first_imgThe process of colonization of Antarctic fellfield soils by micro- organisms has two phases: Firstly, the immigration, survival and establishment of microbial propagules thenlselves, and secondly the stabilization of the soil for subsequent colonization and establishment by mosses, lichens and invertebrates. Dominant amongst primary microbial colonizers are the phototrophic cyano- bacteria and algae. Not only do they introduce organic nutrients into the micro- habitat but they also have a structural function. Filaments of several dimensions frequently form a mesh over the surface of the soil. This mesh often has a canopy structure of fine filaments closely admixed with the mineral soil grains, overlain by a layer of broader, longer filaments. This structure may provide a rich grazing zone for micro-invertebrates such as protozoa and nematodes. Microbial filaments, clusters and unicells frequently have mucilaginous sheaths or capsules which may cement mineral grains together and improve soil crust stability. The combination of filaments and mucigel promotes the formation of microbial “rafts” which are dispersed by wind or water. The diverse microbiota of these compound propa- gules is likely to improve their chances of successful colonization of unpopulated soil surfaces. Phototrophic microbes can be distinguished in mixed natural com- munities of undisturbed soil crusts by selective filtration of their autofluorescence spectra. Heterotrophs can be distinguished after staining. The population can be selectively quantified by television image analysis (TVIA) using criteria of pig- mentation, size and morphology.last_img read more

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9 May
2021

Digenean, monogenean and cestode infection of inshore fish at the South Orkney Islands

first_imgNotothenioid fish of 3 species – 23 immature Notothenia coriiceps, 4 Gobionotothen gibberifrons and 11 Chaenocephalus aceratus, from the coastal zone at the South Orkney Islands were examined to determine their parasite infection. Five digenean species from the intestine, one monogenean species from the skin, two tetraphyllidean larval forms (cercoids) from the intestine, and diphyllobothriid plerocercoids from the stomach’s wall, liver and body cavity are reported. Infections are compared with those of fish from adjacent island groups along the Scotia Arc. Parasite infections of the fish from the South Orkney Islands were more similar to those at the South Shetland Islands area than at South Georgia. The infection of N. coriiceps and G. gibberifrons is similar to that of fjord fish at South Georgia and the South Shetland Islands, whereas the parasite burden of Ch. aceratus was more similar to that of this fish species inhabiting the open sea.last_img read more

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9 May
2021

On the SuperDARN autocorrelation function observed in the ionospheric cusp

first_imgThe SuperDARN radars are proving to be a very powerful experimental tool for exploring solar wind-magnetosphere-ionosphere interactions. They measure the autocorrelation function (ACF) of the transmitted signal backscattered from the ionospheric irregularities, and derive parameters such as the Doppler velocity and the spectral width. The associated spectra have a specific behaviour inside the cusp, a strong temporal and spatial evolution of the velocity and spectral width, and a high value of the spectral width. Although no studies have explained these characteristics, they are routinely used to detect the cusp in the radar data and estimate the open/closed field line boundary. Both satellite and magnetometer data show in the same region broadband wave activity in the Pc1, Pc2 frequency band. In this study, we evaluate how such wave activity modifies the radar’s ACF, and conclude that it explains the spectra seen in the cusp.last_img read more

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9 May
2021

Numerical simulations of the ice flow dynamics of the Brunt Ice Shelf – Stancomb Wills Ice Tongue System

first_imgIce shelves play an important role in determining regional ocean properties and in modulating ice flux fromland to sea. Their dynamics are complex, however, and localised rifts and zones of weakness can have asignificant but poorly understood effect on flow and ultimately on the integrity of the shelf.The Brunt Ice Shelf (BIS)- Stancomb Wills Ice Tongue (SWIT) System, situated on the Caird Coast, OatesLand, Antarctica, is characterised as a thin, unbounded ice shelf with a highly heterogeneous structure. Incontrast to most ice shelves, icebergs calve along much of the grounding line but are trapped and subsequentlybound together by sea ice. This calf-ice / sea-ice aggregate makes up a large part of the Brunt Ice Shelf inparticular, and this heterogeneity makes the BIS-SWIT a good test case for investigating the importance ofweak zones in shelf dynamics.We applied a diagnostic, dynamic/thermodynamic ice-shelf model to simulate the present flow of the iceshelf that results from the ice-thickness distribution, the influx at the grounding line and the surface andbottom temperature. We then compared the model results with flow velocities measured by SyntheticAperture Radar feature tracking. We found that our simulations were clearly improved by the use of ahigh- resolution ice thickness distribution on the heterogeneous ice shelf calculated from ICESat surfaceelevation data using an assumption of hydrostatic equilibrium. We then assessed the model’s sensitivity toice thickness, inflow velocities and a flow enhancement factor that parameterises the role of sea ice, whosemechanical properties are known to be significantly different from those of meteoric ice.We found that the numerical simulations were improved by incorporating the detailed variations in shelfstructure. Simulated flow velocities on either side of rifts in the ice shelf became decoupled as we softenedthe sea ice within the rifts. On a larger scale, we found that soft sea ice can lead to a decoupling of themovement of the Stancomb-Wills Ice Tongue and the Brunt Ice Shelf. When we simulated a regime wheresea ice was absent, ice shelf flow speeds increased along the western edge of the SWIT ice front, in generalagreement with observations made in just such a sea- ice-free dynamic regime that occurred inlast_img read more

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9 May
2021

Multi-year observations on the gametogenic ecology of the Antarctic seastar Odontaster validus

first_imgThis study reports the first multi-year observations on the reproductive patterns for an Antarctic predator/scavenger, Odontaster validus (Koehler 1912). Seastars were collected monthly from a shallow site (15–20 m depth) near the British Antarctic Survey (BAS) Rothera Research Station (Adelaide Island, 67°34′S 68°08′W) from July 1997 to January 2001. Reproductive condition, oocyte size frequencies and spermatogenesis were examined in at least ten seastars each month using histological and image analysis techniques. Gonad indices (GI) and pyloric caeca indices (PI) were also examined in the same samples. Female and male GIs varied seasonally, in parallel with a reduction in the proportion of large oocytes and mature sperm in the gonad in August to mid-October following winter spawning. Despite there being remarkable consistency in the timing of spawning from year to year, differences in the reproductive condition of individuals were apparent. Patterns in the digestive tissues also varied with season, peaking in December and reaching a minimum in February in two of the three study years. This weaker annual pattern may partly reflect the varied diet of this predator/scavenger species, which is not directly dependant on the timing and magnitude of the annual phytoplankton bloom. Pooled oocyte size distributions and residual analysis suggested that oogenesis progressed over 18–24 months, with the largest of the two size classes (maximum diameter = 183 μm) being spawned annually. This pattern of oocyte growth and spawning was previously reported in the early 1960s for an O. validus population from McMurdo Sound, which lies south of Rothera by 10° latitude. The extremely catholic diet of this predator/scavenger suggests the reproductive patterns of the seastar will be less susceptible to changes in food supply compared to polar suspension feeders or deposit feeders.last_img read more

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9 May
2021

Avalanching systems under intermediate driving rate

first_imgThe paradigm of self-organized criticality (SOC) has found application in understanding scaling and bursty transport in driven, dissipative plasmas. SOC is, however, a limiting process that occurs as the ratio of driving rate to dissipation rate is taken to zero. We consider the more realistic scenario of finite driving rate. Similarity analysis reveals that there is a control parameter R-A which is analogous to the Reynolds number R-E of turbulence in that it relates to the number of excited degrees of freedom, that is, the range of spatio-temporal scales over which one finds scaling behaviour. However for avalanching systems the number of excited degrees of freedom is maximal at the zero driving rate, SOC limit, in the opposite sense to fluid turbulence. Practically, at finite R-E or R-A one observes scaling over a finite range which for turbulence, increases with R-E and for SOC, decreases with increasing R-A, suggesting an observable trend to distinguish them. We use the BTW sandpile model to explore this idea and find that whilst avalanche distributions can, depending on the details of the driving, reflect this behaviour, power spectra do not and thus are not clear discriminators of an SOC state.last_img read more

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9 May
2021

Use of POES SEM-2 observations to examine radiation belt dynamics and energetic electron precipitation into the atmosphere

first_imgThe coupling of the Van Allen radiation belts to the Earth’s atmosphere through precipitating particles is an area of intense scientific interest. Currently, there are significant uncertainties surrounding the precipitating characteristics of medium energy electrons (> 20 keV), and even more uncertainties for relativistic electrons. In this paper we examine roughly 10 years of measurements of trapped and precipitating electrons available from the Polar Orbiting Environmental Satellites (POES)/Space Environment Monitor (SEM-2), which has provided long-term global data in this energy range. We show that the POES SEM-2 detectors suffer from some contamination issues that complicate the understanding of the measurements, but that the observations provide insight into the precipitation of energetic electrons from the radiation belts, and may be developed into a useful climatology for medium energy electrons. Electron contamination also allows POES/SEM-2 to provide unintended observations of > 700 keV relativistic electrons. Finally, there is an energy-dependent time delay observed in the POES/SEM-2 observations, with the relativistic electron enhancement (electrons > 800 keV) delayed by approximately one week relative to the > 30 keV electron enhancement, probably due to the timescales of the acceleration processes. Observations of trapped relativistic electron fluxes near the geomagnetic equator by GOES show similar delays, indicating a “coherency” to the radiation belts at high and low orbits, and also a strong link between trapped and precipitating particle fluxes. Such large delays should have consequences for the timing of the atmospheric impact of geomagnetic storms.last_img read more

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9 May
2021

Widespread low rates of Antarctic glacial isostatic adjustment revealed by GPS observations

first_imgBedrock uplift in Antarctica is dominated by a combination of glacial isostatic adjustment (GIA) and elastic response to contemporary mass change. Here, we present spatially extensive GPS observations of Antarctic bedrock uplift, using 52% more stations than previous studies, giving enhanced coverage, and with improved precision. We observe rapid elastic uplift in the northern Antarctic Peninsula. After considering elastic rebound, the GPS data suggests that modeled or empirical GIA uplift signals are often over-estimated, particularly the magnitudes of the signal maxima. Our observation that GIA uplift is misrepresented by modeling (weighted root-mean-squares of observation-model differences: 4.9–5.0 mm/yr) suggests that, apart from a few regions where large ice mass loss is occurring, the spatial pattern of secular ice mass change derived from Gravity Recovery and Climate Experiment (GRACE) data and GIA models may be unreliable, and that several recent secular Antarctic ice mass loss estimates are systematically biased, mainly too highlast_img read more

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9 May
2021

Images as proximity sensors: the incidence of conspecific foraging in Antarctic fur seals

first_imgBackgroundAlthough there have been recent advances in the development of animal-attached ‘proximity’ tags to remotely record the interactions of multiple individuals, the efficacy of these devices depends on the instrumentation of sufficient animals that subsequently have spatial interactions. Among densely colonial mammals such as fur seals, this remains logistically difficult, and interactions between animals during foraging have not previously been recorded. ResultsWe collected data on conspecific interactions during diving at sea using still image and video cameras deployed on 23 Antarctic fur seals. Animals carried cameras for a total of 152 days, collecting a total of 38,098 images and 369 movies (total time 7.35 h). Other fur seals were detected in 74 % of deployments, with a maximum of five seals detected in a single image (n = 122 images, 28 videos). No predators other than conspecifics were detected. Detection was primarily limited by light conditions, since conspecifics were usually further from each other than the 1-m range illuminated by camera flash under low light levels. Other seals were recorded at a range of depths (average 27 ± 14.3 m, max 66 m). Linear mixed models suggested a relationship between conspecific observations per dive and the number of krill images recorded per dive. In terms of bouts of dives, other seals were recorded in five single dives (of 330) and 28 bouts of dives <2 min apart (of 187). Using light conditions as a proxy for detectability, other seals were more likely to be observed at the bottom of dives than during descent or ascent. Seals were also more likely to be closer to each other and oriented either perpendicular or opposing each other at the bottom of dives, and in the same or opposite direction to each other during ascent. ConclusionsThese results are contrary to animal-attached camera observations of penguin foraging, suggesting differing group-foraging tactics for these marine predators. Group foraging could have consequences for models linking predator behaviour to prey field densities since this relationship may be affected by the presence of multiple predators at the same patch.last_img read more

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9 May
2021

Using the aa index over the last 14 solar cycles to characterize extreme geomagnetic activity

first_imgGeomagnetic indices are routinely used to characterize space weather event intensity. The DST index is well resolved, but is only available over 5 solar cycles. The aa index extends over 14 cycles but is highly discretized with poorly resolved extremes. We parameterize extreme aa activity by the annual averaged top few % of observed values, show these are exponentially distributed and they track annual DST index minima. This gives a 14 cycle average of ~ 4% chance of at least one great (DST<‐500nT) storm and ~ 28% chance of at least one severe (DST<‐250nT) storm per year. At least one DST=‐809 [‐663,‐955]nT event in a given year would be a 1:151 year event. Carrington event estimate DST~ ‐850nT is within the same distribution as other extreme activity seen in aa since 1868 so that its likelihood can be deduced from that of more moderate events. Events with DST ≲ ‐1000 nT are in a distinct class, requiring special conditions.last_img read more

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