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South African Journal of Science

versión On-line ISSN 1996-7489
versión impresa ISSN 0038-2353

S. Afr. j. sci. vol.103 no.5-6 Pretoria may./jun. 2007




Physical and biological processes at the Subtropical Convergence in the South-west Indian Ocean



P.W. FronemanI; I.J. AnsorgeII; N. RichouxI; J. BlakeI; R. DalyI; J. SterleyI; B. MostertI; E. HeynsI; J. SheppardI; B. KuyperII; N. HartII; C. GeorgeII; J. HowardII; E. MustafaII; F. PeyII; J.R.E. LutjeharmsII

ISouthern Ocean Group, Department of Zoology and Entomology, Rhodes University, Grahamstown 6140, South Africa
IIDepartment of Oceanography, University of Cape Town, Private Bag, Rondebosch 7701, South Africa




A detailed hydrographic and biological survey was conducted in the region of the Subtropical Convergence in the Indian sector of the Southern Ocean in April 2007. Hydrographic data revealed that the subsurface expression of the Subtropical Convergence (at 200 m), marked by the 10°C isotherm, appeared to meander considerably between 41°S and 42°15'S. Total surface chlorophyll-a concentration was low and ranged from 0.03 to 0.42 μg l-1 and was always dominated by the pico- (<2 μm) and nano-(2-120 μm) size classes, which contributed between 81% and 93% of the total pigment. The total chlorophyll-a integrated over the top 150 m of the water column showed no distinct spatial trends, and ranged from 12.8 to 40.1 mg chl-a m2. There were no significant correlations between the total integrated chlorophyll-a concentration and temperature and salinity (P > 0.05). The Zooplankton community was dominated, numerically and by biomass, by mesozooplankton comprising mainly copepods of the genera, Oithona, Paraeuchaeta, Pleuromamma, Calanus and Clausocalanus. An exception was recorded at those stations in the region of the front where the tunicate, Salpa thompsoni, dominated the total Zooplankton biomass.



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