Prydz Bay Region Research Articles

Age groups of antarctic krill,Euphausia superba dana, in the Prydz Bay region

Age groups of Antarctic krill (Euphausia superba Dana) in the Prydz Bay region were studied by distribution mixture analysis based on length/frequency data collected by R/VJidi during the 1989/1990 and 1990/1991 austral summer. Five age groups were determined, i.e. 1+, 2+, 3+, 4+, and 5+, or six age groups in all, if the 0+ larvae were included. The mean body length of 1+ to 5+ age groups was 25.70 mm, 40.47 mm, 45.52 mm, 50.52 mm and 54.52 mm respectively. Supposing the difference in body length between successive age groups is a reflection of the early growth, the maximum growth rate occurred during the period from 1+ juveniles to 2+ subadults (14.77 mm/a). From 2+ subadults to 3+ adults the growth rate dropped steeply (5.05 mm/a) because at this stage, increase of body length was substituted, to a great extent, by the growth of sexual products. From 3+ onwards the growth rate was maintained at a relatively low level and decreased slowly with age. The relative abundance of age groups 1+ and 2+, in our sample must be much lower than that in the real population owing to both the large mesh size we used and the distribution difference between juveniles and adults. If we left aside 1+ and 2+ age groups and just looked at the relative abundance of adults, we found that age group 3+ dominated the adult population and that the relative abundance decreased sharply with increasing age. If this situation is normal, one can expect an extremely high mortality rate in adults, 82.6% from 3+ to 4+ and 94.0% from 4+ to 5+. This is reasonably expectable for the Prydz Bay region.

Demography and life cycle of Antarctic krill, <i>Euphausia superba</i>, in the Indian sector of the Southern Ocean: long-term comparison between coastal and open-ocean regions

Size/age composition and reproductive status of Antarctic krill, Euphausia superba, in the central part of the Indian sector of the Southern Ocean, e.g., the Cooperation Sea (Prydz Bay region) and the Cosmonaut Sea, during austral summers 1977-1990 were summarized to estimate growth rates, longevity, reproduction, recruitment, life span, and mortality rates. The life span of Antarctic krill exceeds 5 years in both the Cosmonaut and Cooperation seas. The age composition of the southern and northern groupings differs markedly, with substantial reduction in numbers of early age groups in the northern grouping. Long-term observations of spawning success, recruitment, and age composition suggest that a self-sustained grouping of krill persists in the Cooperation Sea south of the Antarctic Divergence. However, periodic gene flow via recruits from surrounding regions most probably accounts for the lack of spatial genetic differences between the Cooperation Sea and adjacent areas, thus preventing the establishment of an isolated subpopulation in the region investigated. The major factor responsible for the substantial interannual variability in krill dynamics appears to be macroscale oceanographic and atmospheric circulations, which determine a level of environmental isolation of the Cooperation Sea from adjacent waters.

The thermal history of the Eastern Ghats Belt (India) as revealed by U–Pb and [formula omitted] dating of metamorphic and magmatic minerals: implications for the SWEAT correlation

The Eastern Ghats belt is a large high-grade terrane exposed along the east coast of India. The late Proterozoic orogen consists of N–S trending charnockite and metasedimentary belts. The younger E–W trending Godavari Rift divides the orogen into a northern and a southern segment. Textural and structural relationships indicate a complex thermo-tectonic evolution involving several episodes of metamorphism and deformation. This orogenic belt represents an important part of the reconstructed global Southwest-United-States–East-Antarctica (SWEAT) orogen. The exact timing of metamorphism and tectonism in this large orogenic belt plays a key role in this global reconstruction. Textural and structural relationships indicate a complex thermo-tectonic evolution involving several episodes of metamorphism and deformation. In order to reconstruct parts of the regional thermal history of the Eastern Ghats belt, U–Pb ages were determined on monazite, allanite, zircon and sphene and 40Ar/ 39Ar ages on hornblende. Monazite, allanite and some sphenes provide evidence that in the central and eastern tectonic units north of the Godavari Rift the last high-grade metamorphism occurred at ca. 960 Ma. In the most western unit, the Western charnockite zone, allanite and monazite from late pegmatites indicate a major thermal event around 1.6 Ga. This is the first indication that there may be pronounced temporal differences in the thermal evolution of different tectonic or ltihological units in the Eastern Ghats belt. The majority of the sphenes from the Eastern Ghats belt are discordant and lie along a reference line extending from ca. 935 Ma to 504 Ma. This discordance is interpreted to be due to a thermal disturbance during a Pan-African deformation phase, which is dated by the emplacement of apatite–magnetite veins that contain zircon with a concordant U–Pb age of 516±1 Ma. All hornblende 40Ar/ 39Ar ages from the central units of the belt provide Pan-African plateau ages. The partially reset sphene and completely reset hornblende ages indicate that the Pan-African metamorphism reached at least middle-amphibolite facies conditions in parts of the Eastern Ghats belt. This thermal overprint was not strong enough to cause retrogression of the high-grade mineral assemblages, but has so far only been recognized in the reset mineral ages and some late magmatic rocks. A hornblende 40Ar/ 39Ar age of ca. 1110 Ma from an amphibolite south of the Godavari rift indicates that the Pan-African thermal overprint was weaker in the Western charnockite zone, if it affected this area at all. The new ages presented for the Eastern Ghats belt are similar to published ages for the Rayner Complex and the Prydz Bay region of Eastern Antarctica. This similarity in ages is strong support for these areas in East Antarctica and the Eastern Ghats belt being complementary parts of an extensive orogenic belt formed during the Grenvillian Orogeny around 1 Ga. Both continental fragments also show evidence for a Pan-African overprint, however, this event is much more pronounced in Antarctica than in the Eastern Ghats belt.

Lithofacies distribution in relation to the geomorphic provinces of Prydz Bay, East Antarctica

Over the past 15 years, Japanese, Australian and Russian expeditions to Prydz Bay have collected about 30 000 km of bathymetric data, 6000 km of sidescan sonar data and more than 250 sediment grab and core samples. These data were used in the present study to compile surficial sediment, bathymetric, and geomorphological maps of the Prydz Bay region. Lithofacies distribution was determined by surficial sediment data analysis using sample matrix (Q-mode) and cluster analysis techniques based on data from 206 sites. Data included percentage biogenic silica (opal), calcium carbonate, gravel, mud, and relative abundance of two diatom species (Fragilariopsis curta and F. kerguelensis). Five lithofacies are identified from the available data: (1) slightly gravelly sandy mud (g)sM lithofacies, (2) siliceous mud and diatom ooze (SMO) lithofacies, (3) F. kerguelensis pelagic ooze lithofacies, (4) F. curta gravelly muddy sand gmS lithofacies and (5) calcareous gravel lithofacies. In many areas the lithofacies correlate to geomorphological provinces as defined by previous investigators using 3.5 kHz and sidescan sonar data. In some cases, Holocene SMO sediments are seen to drape over iceberg plough marks, implying that these are relict features. These five lithofacies are likely to dominate most of the East Antarctic shelf region and may be helpful in defining sedimentary successions resulting from ice-sheet advance and retreat over glacial-interglacial cycles.

Microfossil and stable-isotope evidence for changes in Late Holocene palaeoproductivity and palaeoceanographic conditions in the Prydz Bay region of Antarctica

Microfaunal and microfloral data from two gravity cores, and stable-isotope results from one of these cores taken on Fram Bank near Prydz Bay, Antarctica indicate changes in sea-ice patterns and oceanographic conditions which may have been linked to regional and global climate changes that occurred over the past 8000 yr. Modern diatom assemblages indicative of ice free conditions 1–2 months of the year, and dominated by Nitzschia species became prevalent some time around 2000–2700 yr B.P. The occurrence of Chaetoceros spp. diatom spore-dominated assemblages, and increased abundances of benthic and planktonic foraminifera, ostracods, and diatoms, coupled with carbon- and oxygen-isotope changes around 2700–3400 yr B.P. strongly suggests that this area of the shelf experienced conditions conducive to increased productivity during this time period. Based on diatom assemblage associations recognized in modern environments, the upper water column of Fram Bank shelf waters was probably stratified during that time due to the presence of low-salinity melt water and a very shallow mixed layer, protected from storms, with sea-ice cover for less than 10 months per year. The close correspondence of δ13C and δ18O values from N. pachyderma (r2 = 0.74) throughout one core suggests that whatever oceanographic conditions influenced δ18O also influenced δ13C. Changes in Prydz Bay microfossil abundances and isotopes may result from alterations in circulation patterns, upwelling conditions, or sea-ice patterns which significantly affected benthic and planktonic productivity in the area. The environmental conditions indicated by older sediments in the cores are less clear, but an ice tongue may have been in place 3200–3800 yr B.P., and minor fluctuations in productivity are indicated around 6000 and 7000–7500 yr B.P. The lower 100 cm of both cores may not represent in situ deposition, but if the corrected AMS dates reflect the actual age of the sediments analyzed, microfossil and isotope data indicate that productivity, and oceanographic conditions were very variable around 8500-8600 yr B.P. The chronology adopted herein must be regarded as tentative until further study of the area has been undertaken.

Characteristics of atmospheric oscillation in the Prydz Bay region during the ozone hole period in 1993

IN ref.[1],a preliminary analysis was reported on the change characteristics of total ozoneduring the Antarctic'ozone hole'period in 1993,revealing that there is a very obvious dailychange of total ozone over Zhongshan Station and suggesting that this change would be relatedto the position of Zhongshan Station on the rim of the ozone hole influenced by the polar vortexvacillation and the expanding and contracting processes in Antarctic stratosphere.In this note,

Reproductive trade-off in male Antarctic krill, Euphausia superba

Freshly caught male and female Euphausia superba from the same swarm exhibited different rates of mortality subsequent to capture. Mortality was significantly higher for reproductive males (100%, n=68) than for females (3%, n=186) within the first 3 d of capture. Total lipid and triacylglycerol levels in male, female and juvenile Euphausia superba were analysed and compared. All reproductive male krill analysed from this swarm had low lipid levels (1 to 3% dry weight) with negligible triacylglycerol stores (0 to 2% of total lipid). Somatic lipid stores in female and juvenile krill ranged from 8 to 30% of which up to 40% was triacylglycerol. The levels of algal sterols in the digestive gland of males, females and juveniles indicate that all krill had been feeding recently. An analysis of the sex ratio of krill catches derived from data collected over seven summers from the Prydz Bay region showed a decrease in the proportion of males with increasing size. There was a sharp decline in numbers of male krill once they attained a length of 51 to 55 mm. Low lipid levels in redroductive male krill may be due to reproductive costs. The resulting low storage-lipid levels are accompanied by high mortality in male krill.

Physical oceanography of the Prydz Bay region of Antarctic waters

Hydrographic data from four major Australian cruises to the Prydz Bay region are interpreted to show features of the large-scale circulation that remain consistent across a number of years. A large cyclonic gyre, extending from within the Bay to the Antarctic Divergence north-west of the Bay, is recognised in all of the data. It is associated with a relatively narrow coastal current that runs from the southern limits of Prydz Bay past the Amery Ice Shelf, and continues westward after leaving the Bay (becoming very strong, with currents exceeding 1 m s −1) along the MacRobertson Land shelf. Part of the current flows offshore near 63°E, while another (unknown) fraction continues westward. Data from a number of current-meter moorings deployed within the Bay and on the MacRobertson Land shelf lend support to the interpretation of gross features of the circulation. Coherences between different vertical levels show that the shelf waters respond barotropically to variations of a few days period or more. Little coherence with winds measured at Davis and Mawson stations was evident. Diurnal variations are strong near the shelf edge, and are associated with substantial encroachment of (modified) Circumpolar Deep Water over the shelf break. Dense water production within Prydz Bay appears to mimic processes occurring within the Weddell Sea, involving the intrusion of modified Circumpolar Deep Water over the shelf, and its mixing with High Salinity Shelf Water (and Ice Shelf Water) promoted by marginal or unstable conditions below the intrusions. No observations of dense plumes leaving Prydz Bay have been obtained. Instead, this paper tests the hypothesis, dynamically, that locally formed Antarctic Bottom Water observed near the foot of the continental slope at 62°E in 1974, had its origin in Prydz Bay. An integral model of the descent of a dense plume over a sloping boundary, also incorporating the fully non-linear equation of state of seawater, implies an origin either within Prydz Bay or to the west of the Bay.

Antarctic neritic krill Euphausia crystallorophias: spatio-temporal distribution, growth and grazing rates

Dynamics of distribution, growth, life span and feeding were studied in the endemic Antarctic euphausiid Euphausia crystallorophias in the central part of the Indian sector of the Southern Ocean during the austral summer, from 1977 to 1990, and in the Lazarev Sea during the summer 1990/91. Both larvae and adult E. crystallorophias were found in abundance in shelf waters of 100–500 m depth. Maximum abundances of larvae (up to 23,969 ind. 1000 m −3) and adults (up to 12,670 ind. 1000 m −3) were found in the Prydz Bay region. Size frequency analysis indicated that the generation time was ≈ 3 years in the Indian sector of the Southern Ocean. Total life span of E. crystallorophias may reach 5 years in the Prydz Bay region and 4 years in the Cosmonaut Sea. Analysis of size distribution by sex suggests that male E. crystallorophias may have a shorter life span than females. Growth for only 180 days per year is assumed, mean growth rates range from 0.070–0.075 to 0.019–0.022 mm per day during the first and the fourth year, respectively. Von Bertalanffy growth curves calculated for different areas were similar to those obtained in the Antarctic Peninsula region. In the Prydz Bay and the Cosmonaut Sea, spawning of E. crystallorophias appears to peak from the end of November to early December and may extend to the beginning of January. Most larvae were in the stage of metanauplius/calyptopis I at the beginning of January and calyptopis III/furcilia I during February. The year-1 cohort was identified in January-February and exhibited a mean length in the range 10.8–15.8 mm. In situ feeding rates were estimated in the Lazarev Sea using the gut fluorescence method. Ingestion rates during austral summer 1990/91 ranged from 52 to 471 ng (pigm) ind −1 h −1 in adults and from 2.5 to 25.2 ng (pigm) ind −1 h −1 in calyptopis II larvae. Total population impact on the phytoplankton stock varied between 160–2860 and 215–652 μg (C) m −1 day −1 for adults and larvae, respectively. This is equivalent to 0.06–1.12% and 0.02–0.07% of total daily production. In areas with dense E. crystallorophias swarms, however, daily consumption rates may attain levels as high as 13.6–96.5% of daily primary production. The long-term monitoring of different populations of E. crystallorophias allowed the identification of similar patterns between its abundance and spawning success and the formation of coastal polynyas, especially in the Prydz Bay region.

The Pliocene environment of Antarctica

Debate continues about the environment of Antarctica during intervals in the Pliocene (5.2-1.8 Ma), particularly during the Late Pliocene (3.5-1.8 Ma). Two schools of thought - dynamic versus stable ice sheet - have developed. One hypothesis calls for the Antarctic ice sheet to vary dramatically, for Antarctica at times to be vegetated and for circum-antarctic waters to be significandywarmer than at present. The other calls for a stable ice sheet, no vegetation and water temperatures much less varied. During 1995, the two sides converged by recognising that the change to conditions suitable for vegetation may not have been as marked as originally thought, and that oxygen isotope data may contain scope for more flexibility in interpretation than the stabilist view has supported. New data and new participants have emerged, but there is a long way to go in resolving the issue. The key issue remains the age of the Sirius Group in the Transantarctic Mountains and the source of its contained diatoms. Diatoms of Pliocene age have been recovered from the Sirius Group but also from modern ice, and this raises the question of whether the Sirius Group diatoms are actually in situ. New data and re-evaluation of existing data are necessary. Ocean Drilling Program activity, particularly in the Prydz Bay region, and analysis of samples from the Prince Charles Mountains can be expected to contribute significantly to a solution, by providing more information on past temperatures and evidence of possible terrestrial vegetation.

Acoustic detection of ice crystals in Antarctic waters

During the voyage of the RSV Aurora Australis to the region of Prydz Bay, Antarctica in January–March 1991, ice crystals were encountered at depths from the surface to 125‐m in the western area of the bay. On two occasions, crystals were retrieved by netting, and echo sounder records have been used to infer additional regions of occurrence. Acoustic target strength estimates made on the ice crystal assemblies encountered show significant spatial variation, which may relate to crystal size and/or aggregation. Data from a suite of conductivity‐temperature‐depth casts have been used to map regions of the study area where in situ water temperatures fell below the computed freezing point. Such regions correlate well with those selected on the basis of echogram type and imply that ice crystals occurred at depth over large areas of the bay during the cruise period. The ice crystal distribution described is consistent with that expected from a plume of supercooled water emerging from under the Amery Ice Shelf and forming part of the general circulation of the bay. The magnitude of the supercooled water plume is greater than those reported previously in the Prydz Bay region. If misinterpreted as biota on echo sounder records, ice crystals could significantly bias biomass estimates based on echo integration in this and potentially other areas.

Contribution to the biology of squid in the Prydz Bay region, Antarctica

The teuthoid fauna of the Prydz Bay region of the Southern Ocean (Indian Ocean sector) has been studied based on the material collected from 1981–1991 using a rectangular midwater trawl (RMT-8), pelagic trawl (IYGPT), and bottom trawl. Eight species of squid have been recognized: Brachioteuthis sp., Kondakovia longimana, Bathyteuthis abyssicola, Psychroteuthis glacialis, Alluroteuthis antarcticus, Mastigoteuthis psychrophila, Mesonychoteuthis hamiltoni and Galiteuthis glacialis. Size frequency distribution, geographical and vertical distributions of each species as well as diets of common species are analysed. There is no evidence of a diel vertical migration but ontogenetic descent appears to occur in P. glacialis and G. glacialis. Antarctic krill, Euphausia superba and the Antarctic silverfish, Pleuragramma antarcticum are important prey for most species with cannibalism occurring in P. glacialis, A. antarcticus and M. hamiltoni. Equations for calculating total weight from mantle length, and mantle length and total weight from upper and lower rostral length are provided for B. abyssicola, P. glacialis, A. antarcticus, M. psychrophila, and G. glacialis.

New Information and Locality Records for the Antarctic Amphipod Clarencia Chelata K. H. Barnard, 1931, and a Reappraisal of the Family Clarenciidae J. L. Barnard & Karaman, 1987 (Amphipoda, Gammaridea)

An examination of fresh material of Clarencia chelata K. H. Barnard, 1931, collected from the Prydz Bay region, Antarctica, has revealed a number of characters not recognised previously or diagnosed incorrectly. New morphological information is provided and the systematic position of Clarenciidae is re-evaluated and a new superfamily Clarencioidea is proposed. A new diagnosis for Clarenciidae is also provided. The species seems to be associated with the white sponge Asbestopluma belgicae (Topsent, 1901) (Cladorhizidae).

A comparison of Antarctic petrel (Thalassoica antarctica) diets with net samples of Antarctic krill (Euphausia superba) taken from the Prydz Bay region

Samples of the stomach contents of Antarctic petrels (Thalassoica antarctica) were obtained on board ship in the Prydz Bay region of Antarctica from birds which had spontaneously regurgitated. The weight of food and the species composition of the stomach contents were measured. Antarctic krill,Euphausia superba, the sole prey item taken, were compared to krill obtained by nets in the same region as part of a large-scale krill survey. Krill from petrel stomach samples were larger in mean size than those sampled by nets. This may be attributed to selection of the larger sized krill by the petrels, it may be caused by the nets sampling different populations of krill or it may be due to net avoidance by the larger krill.

Distribution and abundance of euphausiid larvae in the Prydz Bay region, Antarctica

In January 1985 a net sampling survey was carried out on the distribution and abundance of euphausiid larvae in the Prydz Bay region. Euphausia superba occurred in low abundance, probably due to sampling preceding the main spawning period. Thysanoessa macrura occurred throughout the study area in consistently high abundance. Euphausia crystallorophias was marginally more abundant within its restricted range. Distinct north-south variations in larval age and developmental stages of T. macrura were observed indicating regional differences in spawning. Euphausia frigida was mainly confined to the upper 200 m of the Antarctic Circumpolar Current. E. superba larvae produced north of the shelf break, between 70°–83°E, moved north-east into the Antarctic Circumpolar Current. Larvae originating on the shelf moved rapidly west in the East Wind drift. E. crystallorophias had the same westward dispersion, but some larvae appeared to return eastward via the Prydz Bay Gyre and remain in the region. The data indicate that most E. superba larvae, providing they survive injuries cold temperature an food deprivation, will leave the area, suggests that Prydz Bay krill may not be a self maintaining stock.

The distribution and abundance of aerial seabirds in relation to Antarctic krill in the Prydz Bay region, Antarctica, during late summer

The distribution patterns of aerial seabirds are analysed from counts made in the Prydz Bay region, Antarctica, during the African legs of SIBEX I and II in late summer (end of February to April), and compared with those made farther west at the same time of year during FIBEX. Species composition and abundances were similar in all three surveys, with sooty shearwaters Puffinus griseus contributing approximately half of the total aerial bird energy demand. Differences between surveys are explained in terms of longitudinal or seasonal differences in sampling areas and periods. Correlations between bird distribution patterns and environmental parameters are used to infer the scale-dependent factors affecting bird dispersion at sea. Two macro-scale bird assemblages, identified by physical parameters, were separated along latitudinal gradients (temperature and salinity) associated with the Antarctic Divergence. These assemblages are consistent with the Intermediate and Southern High Latitude Groups identified during FIBEX. At smaller spatial scales, almost all species were correlated with the abundance of Antarctic krill Euphausia superba, both across the entire SIBEX I grid, and within the areas north and south of the Antarctic Divergence. Similarly, during SIBEX II, seabird densities were six times greater when krill was abundant than when krill was scarce. Sooty shearwaters, which appeared to be moving through the area, were the only abundant bird species not correlated with krill abundance. Possible reasons why previous studies have not detected correlations between seabird and krill abundances are discussed.

Biomass and population structure of a large aggregation of krill near Prydz Bay, Antarctica

Biomass and population structure of a large aggregation of krill near Prydz Bay, Antarctica

Pack-Ice Drift off East Antarctica and some Implications

Three satellite-tracked data buoys were deployed between 70° and 80°E south of 65°S in February-March 1985. These buoys were subsequently trapped within the expanding seasonal sea ice and drifted with the ice. The buoys measured air temperature and pressure, and water temperatures to 100 m depth. Data from the buoys are used to describe the ice drift and environment within the winter sea-ice zone in this region, both north and south of the Antarctic Divergence. Additional preliminary data from a further six buoys deployed in the same area in March 1987 are also presented. Besides providing information on the broad-scale drift of the ice in the Prydz Bay region, data from the buoys have shown: (i) the important role that ice drift plays in determining the autumn and winter expansion of Antarctic sea ice; (ii) the highly mobile nature of the ice, even hundreds of kilometres from the ice edge; (iii) the role that ocean-bottom topography has in determining ice drift over the continental shelf; and (iv) the modifying influence that an ice cover has on regional climate. A qualitative assessment is made of the relative importance of the major forces driving the ice, although the data are insufficient for a detailed study of the ice dynamics.

Distribution, Abundance and population structure of the Antarctic krill (Euphausia superba Dana) in the Prydz Bay region, Antarctica

This paper presents results of net sampling carried out on four marine science cruises, between 1981 and 1985, in the Prydz Bay region of Antarctica by the Australian Antarctic Division. Krill exhibited a patchy distribution and overall low abundance. The majority of sampling sites in January 1985 returned no post-larval krill or densities of <1 individual 1000 m-3. The estimated mean abundance of E. superba in January 1985 was 6 individuals or 2 g (wet wt.) 1000 m-3 integrated for the upper 200 m of the water column which represented 3.4% of the total zooplankton biomass. No more than five years-groups, including the larvae, were observed in Prydz Bay, with mean lengths of groups 1+, 2+, 3+ and 4+ being 24, 38, 46 and 53 mm (standard 1), respectively in the middle of January. A high proportion of naupliar stages observed in January 1985 indicated that spawning in Prydz Bay begins in January and examination of adult maturation showed that the spawning continues at least to March.

Genetic variation and population structure of krill (Euphausia superba dana) from the Prydz Bay region of Antarctic waters

Samples of Antarctic krill E. superba from six locations near Prydz Bay were analysed electrophoretically to detect genetically-based protein variation. Analyses of allele distributions at four polymorphic loci indicate no evidence of significant heterogeneity, a result consistent with the hypothesis that all samples were derived from a single breeding population of krill. The results of this study agree closely with genetic data from other studies on krill in the Atlantic sector of the Southern Ocean, suggesting that krill stocks over at least 6000 km of Antarctic waters are derived from a single interbreeding population.