Showing items 1 - 9 of 9

Physical and biological coupling in eddies in the lee of the South-West Indian Ridge

  • Authors: Ansorge, Isabelle J , Pakhomov, Evgeny A , Kaehler, Sven , Lutjeharms, Johan R E , Durgadoo, Jonathan V
  • Date: 2010
  • Language: English
  • Type: Article
  • Identifier: vital:6493 , http://hdl.handle.net/10962/d1004477
  • Description: Eddies have some decisive functions in the dynamics of the Southern Ocean ecosystems. This is particularly true in the Indian sector of the Southern Ocean, where a region of unusually high-mesoscale variability has been observed in the vicinity of the South-West Indian Ridge. In April 2003, three eddies were studied: eddy A, a recently spawned anticyclone south of the Antarctic Polar Front (APF),; eddy B, an anticyclone north of lying between the Subantarctic Front and the APF; and eddy C, a cyclone north of the APF west of the ridge. Elevated concentrations of total Chl-a coincided with the edges of the cyclonic eddy, whereas both anticyclonic eddies A and B were characterised by low total Chl-a concentrations. Biologically, the two anticyclonic eddies A and B were distinctly different in their biogeographic origin. The zooplankton community in the larger anticyclonic eddy A was similar in composition to the Antarctic Polar Frontal Zone (APFZ) community with an addition of some Antarctic species suggesting an origin just north of the APF. In contrast, the species composition within the second anticyclonic eddy B appeared to be more typical of the transitional nature of the APFZ, comprising species of both subantarctic and subtropical origin and thus influenced by intrusions of water masses from both north and south of the Subantarctic Front. Back-tracking of these features shows that the biological composition clearly demarcates the hydrographic origin of these features.
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  • Date Issued: 2010

Structure and distribution of the slope fish community in the vicinity of the sub-Antarctic Prince Edward Archipelago

  • Authors: Pakhomov, Evgeny A , Bushula, T , Kaehler, Sven , Watkins, B P , Leslie, Rob W
  • Date: 2006
  • Language: English
  • Type: Article
  • Identifier: vital:6958 , http://hdl.handle.net/10962/d1012000
  • Description: Demersal fish community structure, distribution and trophic relationships on the slope (depth range 200–1500 m) of the sub-Antarctic Prince Edward Islands and surrounding sea rises were investigated during a pilot survey conducted in April 2001 onboard fishing vessel MV Iris. A total of 56 fish taxa were collected during the survey, of which 44 were identified to the species level, seven to the genus level and five to the family level. Among the identified taxa, 36 constituted new records for the area investigated. Total catch per unit effort (cpue) during the survey ranged from 1•1 to 241•2 individuals h 1. Both average fish diversity and total cpue positively correlated with trawling depth. Overall, mean sampling depth and near-bottom temperature explained 56% of total fish cpue. Hierarchal cluster analysis identified three distinct fish assemblages with pronounced dominant species. Major shifts in fish community composition occurred at 500–600 m and 800–900 m depth strata and could probably be a result of physical and biological vertical zonation. Analysis of the diet of selected fish species showed that they were generalist feeders, consuming predominantly pelagic, including epipelagic, meso- and benthopelagic, prey. Diets of six species and nitrogen stable isotope signatures of 22 species revealed that with a few exceptions most fishes occupied the fourth trophic level and were tertiary consumers. Wide variability in carbon isotopic signatures is discussed with respect to alternative, e.g. possible importance of high Antarctic and chemoautotrophic v. photoautotrophic sub-Antarctic primary production, organic matter sources at the base of deep-sea food webs.
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  • Date Issued: 2006

Trophic importance of kelp-derived suspended particulate matter in a through-flow sub-Antarctic system

  • Authors: Kaehler, Sven , Pakhomov, Evgeny A , Kalin, R M , Davis, S
  • Date: 2006
  • Language: English
  • Type: Article
  • Identifier: vital:6876 , http://hdl.handle.net/10962/d1011624
  • Description: Spatial dynamics of surface chlorophyll concentrations, diatom abundance and carbon and nitrogen stable isotope signatures of surface suspended particulate matter (SPM) were investigated during a bloom event observed in March 2003 in the vicinity of the sub-Antarctic Prince Edward Islands. The surface water composition was studied to estimate the overall importance and spatial extent of kelp derived SPM in the water column of the islands. It was observed that high chlorophyll concentrations (up to 2 mg m–3) between and downstream of the islands could not be explained by the development of the diatom bloom. Instead, microscopic and stable isotope analyses suggested that the chlorophyll signal was largely derived from the residual chlorophyll in fresh and decaying particles of small fragments of the kelp Macrocystis laevis, an endemic kelp species abundant along the shoreline of the islands. The findings of this study suggest that the dietary subsidy of kelp-derived carbon and nitrogen to benthic communities and possibly the plankton is not limited to the vicinity of kelp beds, but rather is a widespread phenomenon between the islands. Due to the dominating unidirectional Antarctic Circumpolar Current, large quantities of kelp-derived SPM may be transported and utilised tens of kilometres downstream of the islands.
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  • Date Issued: 2006

Diet and daily ration of two nototheniid fish on the shelf of the sub-Antarctic Prince Edward Islands

  • Authors: Bushula, T , Pakhomov, Evgeny A , Kaehler, Sven , Davis, S , Kalin, R M
  • Date: 2005
  • Language: English
  • Type: Article
  • Identifier: vital:6948 , http://hdl.handle.net/10962/d1011977
  • Description: The seasonal dietary composition and estimates of daily consumption rate of Lepidonotothen larseni and Gobionotothen marionensis juveniles were obtained for the first time using fish collected near sub-Antarctic Prince Edward Islands during April 1999–2003. The diet of L. larseni consisted mainly of pelagic prey, with copepods and arrow worms making up the most significant prey groups and accounting for 46% and 40% of prey mass, respectively. The diet of G. marionensis was more diverse than that of L. larseni and was composed mainly of benthic prey, including bottom-dwelling decapods (Nauticaris marionis) and sedentary polychaetes, which accounted for 54% and 30% of prey mass, respectively. During the present study, dietary overlap between juveniles of L. larseni and G. marionensis was very low (<5%) indicating that competition for food resources between them was negligible. They not only relied on different prey species, both also exhibited different diel feeding regimes. Daily consumption rate of L. larseni and G. marionensis juveniles was estimated to be 4.5% and 5.2% of body dry mass, respectively. Stomach contents and stable isotope analyses suggested, that both L. larseni and G. marionensis occupy the forth-trophic level of the sub-Antarctic food web but depend mainly on allochthonous and autochthonous (kelp derived) organic matter, respectively.
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  • Date Issued: 2005

Variability in pH, fCO2, oxygen and flux of CO2 in the surface water along a transect in the Atlantic sector of the Southern Ocean

  • Authors: Chierici, M , Fransson, A , Turner, D R , Pakhomov, Evgeny A , Froneman, P William
  • Date: 2004
  • Language: English
  • Type: Article
  • Identifier: vital:6847 , http://hdl.handle.net/10962/d1011105
  • Description: Underway sampling and measurements of pH, fCO2, oxygen and Chlorophyll a (Chl a) were performed in the surface waters from Cape Town (South Africa) to Queen Maud Land (Antarctica) in the Atlantic sector of the Southern Ocean during the austral summer 1997/1998. From direct measurements of these parameters and from calculated fCO2 the oceanic carbon dioxide system was studied and related to hydrological and biological parameters. fCO2 was in general undersaturated relative to the atmosphere and showed a large variability with values ranging from 313 to 377 μatm with a mean value of 346±13 μatm. The undersaturation was more pronounced in areas associated with fronts where high Chl a and high pH in situ values were observed. Using shipboard wind speed data, estimates of the CO2 flux were made along the transect and during three mesoscale surveys on the northward return transect in the area of the Spring Ice Edge (SIE), the Winter Ice Edge (WIE) and in the Antarctic Polar Front (APF). The undersaturation observed during the transect caused the ocean to act as a sink for CO2 with a mean sea–air flux for the entire transect of −3±5 mmol m−2 d−1 with a large variability between −20 mmol m−2 d−1 (oceanic uptake) to 1.3 mmol m−2 d−1 (oceanic source). The lowest fCO2 values (largest oceanic uptake of CO2) were found at the southern boundary of the APF at 53°S, which coincided with a supersaturation in oxygen and high pH values. Oxygen concentrations were measured from 50°S to 63°S and varied between 324 and 359 μmol kg−1 with a mean value of 347±9 μmol kg−1. In general only small deviations from equilibrium oxygen saturation were observed (mean value=99±2%). However, in the SIE oxygen was clearly undersaturated, probably an effect of upwelling of oxygen poor deep water which had not yet been compensated for by biological production. Three weeks later, the ice edge had retreated in the SIE region and the Chl a concentration had increased three-fold, suggesting the start of a phytoplankton bloom. This was also seen in the oxygen concentration which had increased and showed supersaturation. This coincided with an increased oceanic uptake of CO2 in the SIE during the mesoscale survey.
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  • Date Issued: 2004

Studying the impact of ocean eddies on the ecosystem of the Prince Edward Islands: DEIMEC ll

Diet of two Antarctic dragonfish (Pisces: Bathydraconidae) from the Indian sector of the Southern Ocean

  • Authors: Pakhomov, Evgeny A
  • Date: 1998
  • Subjects: Antarctica Bathydraconidae
  • Language: English
  • Type: Article
  • Identifier: vital:6943 , http://hdl.handle.net/10962/d1011970 , http://dx.doi.org/10.1017/S095410209800008X
  • Description: The diet of Cygnodraco mawsoni Waite 1916 and Gymnodraco acuticeps Boulenger 1902 (Pisces: Bathydraconidae) was studied in the Cooperation and Cosmonaut seas in the depth range 200–400 m during the summer 1988. Stomach content analysis showed that both species are piscivorous predators but with different feeding habits. Cygnodraco mawsoni fed mostly on young notothenioid fish, regularly complementing these with pelagic, Euphausia superba, and benthic crustaceans, such as amphipod gammarids and mysids. In contrast, G. acuticeps relied mostly on mesopelagic fish of the family Myctophidae. Although C. mawsoni and G. acuticeps occupy similar depths in the Cosmonaut Sea, different feeding habits appear to limit the probability of interspecific competition for food.
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  • Date Issued: 1998

Prince Edward Islands' offshore oceanographic study: report of research cruise April-May 1997

  • Authors: Pakhomov, Evgeny A , Froneman, P William , Ansorge, Isabelle J
  • Date: 1998
  • Language: English
  • Type: text , article
  • Identifier: vital:6939 , http://hdl.handle.net/10520/AJA00382353_9017 , https://journals.co.za/content/sajsci/94/4/AJA00382353_9017 , http://hdl.handle.net/10962/d1011962
  • Description: preprint , Focuses on the preliminary results of the second cruise of the Marion Island Oceanographic Study designed to provide oceanological observations off Prince Edward Islands from April 25 to May 28, 1997. Physical and chemical oceanographic results; Surface seawater temperature; Total chlorophyll-a distribution.
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  • Date Issued: 1998

Surface distribution of microphytoplankton of the south-west Indian Ocean along a repeat transect between Cape Town and the Prince Edward Islands

  • Authors: Froneman, P William , Pakhomov, Evgeny A , Meaton, V
  • Date: 1998
  • Language: English
  • Type: text , Article
  • Identifier: vital:6933 , http://hdl.handle.net/10962/d1011953
  • Description: Surface chlorophyll-a (chl-a) concentrations, microphytoplankton (>20 μm) species composition and distribution along a repeat transect between Cape Town and the Prince Edward Islands were investigated in early austral autumn (April/May) 1996. Samples were collected at approximately 30 nautical mile intervals for the analysis of size-fractionated chl-a and the identification and enumeration of microphytoplankton species. Peaks in total chl-a (>1 μg 1 [superscript -1]) were recorded at the Subtropical Convergence (STC), at the Sub-Antarctic Front (SAF) and in the waters surrounding the Prince Edward Islands. In addition, a minor peak in chl-a concentration was recorded in the continental shelf waters. At stations where elevated chl-a concentrations were recorded, microphytoplankton generally formed a substantial contribution (-10%) to total chlorophyll. Outside these regions, total chlorophyll concentrations were lower (<0.9 μg 1 [superscript -1]) and almost entirely dominated by nano- and picophytoplankton, which contributed >95% of the total. Microphytoplankton species composition along both transects were dominated by chain-forming species of the genera Chaetoceros (mainly C. neglectum, C. peruvianus and C. constrictus), Nitzschia spp. and Pseudoeunotia doliolus. Cluster and ordination analysis based on species composition identified five distinct microphytoplankton assemblages, which were closely associated with the different water masses in the region between Cape Town and the Prince Edward Islands. The microphytoplankton species composition and biogeographic zones identified during this investigation are in general agreement with similar studies conducted in the south-west Indian Ocean during the austral summer, which suggests that there are little seasonal trends in both the microphytoplankton species composition and biogeographic zonation.
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  • Date Issued: 1998
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