Browsing Biodiversity & Conservation Biology by Subject "Benguela upwelling ecosystem"
Now showing items 1-7 of 7
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Gibbons, Mark J.; Stuart, V (NISC and Taylor & Francis, 1994)[more][less]
Abstract: The feeding biology and the vertical migration of Sagitta friderici were examined over 24 h at two stations in the southern Benguela during spring (October) 1987. Together with studies conducted during summer (February 1991) and winter (May 1984), they serve to allow valuable generalizations of the biology and ecology of this abundant chaetognath. Populations migrate vertically and feed nocturnally, although the timing and the extent of migration vary between studies. S.friderici exhibits ontogenetic layering and the cross-shelf distribution of maturity stages differs, suggesting that it is able to take advantage of cross-shelf water movement in order to maintain populations in the nearshore waters of the West Coast. S.friderici prey almost exclusively on copepodg (cannibalism is rare), and there is a positive relationship between the lengths of predator and prey that is influenced by the size structure of the prey environment. This casts doubt on the validity of a chaetognath species-specific relationship between predator and prey size. S. friderici selects its prey on the basis of size, and not species. Daily ration is related linearly to prey density, so reflecting the low density of prey and providing support for theoretical predictions regarding ingestion rates under oceanic conditions. The impact of S. friderici predation on the copepod assemblage is generally less than 3% of the standing stock, although it could be much higher under conditions of low copepod biomass and poor secondary production. URI: http://hdl.handle.net/10566/306 Files in this item: 1
GibbonsFeeding1994.pdf (866.4Kb) -
Sparks, Conrad, A.J.; Gibbons, Mark J. (Taylor & Francis and NISC, 2003)[more][less]
Abstract: A total of 242 zooplankton samples from the upper 100 m of the water column was collected discontinuously from March 1997 to January 1999 off the Orange River mouth on the west coast of southern Africa. Six species of hydromedusae were recovered at generally low abundance, of which Euphysa aurata, Leuckartia octona and Proboscidactyla menoni were dominant. E. aurata and L. octona showed evidence of seasonality in abundance. The low diversity of the fauna was remarkable and it is hypothesized that this might be attributable in part to sedimentation from the Orange River, and in part to locally weak circulation and the wide extent of the continental shelf. URI: http://hdl.handle.net/10566/380 Files in this item: 1
SparksHydromedusaeOrangeRiver2003.pdf (57.50Kb) -
Buecher, Emmanuelle; Gibbons, Mark J. (NISC (Pty) Ltd and Taylor& Francis, 2003)[more][less]
Abstract: Diel vertical migration (DVM) of six common species of hydromedusae was investigated during two drogue studies conducted in St Helena Bay on the west coast of South Africa in February 1995. Clytia spp., Obelia spp. and Bougainvillia macloviana, were largely confined to surface waters and did not appear to display any DVM. By contrast, Leuckartiara octona and Euphysa aurata displayed clear DVM and, like Proboscidactyla menoni, were found at greater depths than the other species. The depth distribution and amplitude of migration varied between surveys for most species, so definitive interpretations of patterns could not be made. URI: http://hdl.handle.net/10566/377 Files in this item: 1
BuecherDielVerticalMigration2003.pdf (121.3Kb) -
Gibbons, Mark J.; Buecher, Emmanuelle; Thibault-Botha, Delphine (NISC (Pty) Ltd and Taylor& Francis, 2003)[more][less]
Abstract: The vertical distribution, abundance and size composition of a population of Pleurobrachia pileus was studied during a six-day drogue study conducted off the west coast of South Africa in February 1995. The population was centred in deep water, possibly because of the presence of a potential predator, Beroe sp., at shallower depths. The population of P. pileus failed to display clear patterns of migration, although nocturnal recruitment from deeper water resulted in a deepening of the depth of occupation then. The population was dominated by individuals <4 mm polar diameter, and large animals were confined to the upper water layers. These results are discussed with reference to possible spawning activities. Using published equations relating size to clearance rates, it was estimated that the population could remove up to 27% of integrated mesozooplankton standing stocks, and in excess of 100% (293%) at some depths. URI: http://hdl.handle.net/10566/373 Files in this item: 1
GibbonsPleurobrachia2003.pdf (270.1Kb) -
Barange, M; Macpherson, E; Gibbons, Mark J. (NISC and Taylor & Francis, 1994)[more][less]
Abstract: Information on vertical and horizontal distribution patterns, abundance and morphology of the decapod Pasiphaea semispinosa in the Benguela upwelling system is presented. P. semispinosa is the dominant pelagic decapod in the system, occurring between 19 and 32°S along the mid and inner shelf in concentrations of 10-100•m-2• Although it is more abundant during abated than during active upwelling in the northern Benguela, it would appear to be equally abundant during both upwelling scenarios in the southern Benguela. In the southern Benguela, reproductive females are present during winter and juveniles dominate during active upwelling, but juveniles appear to be more common during abated upwelling in the northern Benguela. The estimates of abundance of P. semispinosa presented in this study are subject to sampling biases attributable to their vertical migratory behaviour and net avoidance capabilities. URI: http://hdl.handle.net/10566/307 Files in this item: 1
GibbonsPasiphaea1994.pdf (692.3Kb) -
Gibbons, Mark J.; Stuart, V; Verheye, H M (NISC and Taylor & Francis, 1992)[more][less]
Abstract: Carnivorous zooplankton in the Benguela system have tended to be ignored by all but early taxonomists and a handful of recent researchers. An attempt is made here to address the importance of carnivores in this system but, because the database is poor, it was necessary to apply material from outside the region. The taxa covered include ctenophores, cnidarians (Scyphozoa and Hydrozoa, including Siphonophora), chaetognaths and hyperiid amphipods as well as such minor groups as pelagic gastropods and decapods. Most groups of carnivores within the Benguela are represented by only one or two "common" species. These tend to be epiplanktonic and generally concentrated inshore of the shelf-break, where their distributions are influenced by changes in local hydrography. Their densities frequently exceed those of herbivores. The dominant carnivores share a number of attributes: wide-ranging diets, high assimilation and growth efficiencies and mostly a Type I functional response to food concentration. They fall into two categories. The first is the gelatinous species, which have short lifespans and exhibit rapid growth and often massive reproductive response to high concentrations of food. They are patchily distributed and some appear to have behavioural traits that enable them to stay within localized patches of prey on which they may be dependent because of high metabolic costs. In the second category are crustacean predators and chaetognaths which have slower response times to the food environment but may have longer lifespans, lower daily maintenance needs and an ability to store lipids. They are less dependent on high densities of food organisms and can survive throughout winter, when gelatinous predators are rare. The effect of predation on abundance of copepods is estimated on the basis of both published and unpublished data. Should swarms of predators coincide with a critical gateway of zooplankton or larvae, then there may be total depletion. This applies especially to the nearshore zone of the West Coast and to semi-enclosed bays. The role of carnivorous zooplankton in the diets of commercially important fish species is stressed, but their importance in the regeneration of nutrients may be insignificant. URI: http://hdl.handle.net/10566/305 Files in this item: 1
GibbonsTrophicEcology1992.pdf (1.199Mb) -
Gibbons, Mark J.; Sulaiman, Amina; Hissman, K.; Schauer, J.; Wickens, Patti; McMillan, Ian (NISC and Taylor & Francis, 2000)[more][less]
Abstract: A semi-quantitative assessment is made of the animals observed in archived videotapes taken from the research submersible Jago, during diamond mining and exploratory surveys off the mouth of the Orange River on the west coast of southern Africa (28° IS'S, 29°11 'S) in November 1996. The seabed environment is described and nekton associations with substratum features are identified. The area is characterized by heterogeneity to its physical and biological struture. The variety of observed nekton is low, and communities are dominated by goby Sufflogobius bibarbatus, juvenile hake Merluccius spp. and cuttlefish Sepia spp. (on soft substrata), as well as false jacopever Sebastes capen sis and kingklip Genypterus capensis (on rocky substrata). URI: http://hdl.handle.net/10566/419 Files in this item: 1
GibbonsDemersalNekton2000.pdf (501.8Kb)
Now showing items 1-7 of 7