RESEARCH IN ACTION

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

P.W. Froneman^I; I.J. Ansorge^II; N. Richoux^I; J. Blake^I; R. Daly^I; J. Sterley^I; B. Mostert^I; E. Heyns^I; J. Sheppard^I; B. Kuyper^II; N. Hart^II; C. George^II; J. Howard^II; E. Mustafa^II; F. Pey^II; J.R.E. Lutjeharms^II

^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

ABSTRACT

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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References

1. Longhurst A. (1998). Ecological Geography of the Sea. Elsevier, New York.         [ Links ]

2. Belkin I.M. and Gordon A.L. (1996). Southern Ocean fronts from the Greenwich meridian to Tasmania. J. Geophys. Res. 101, 3675-13696.         [ Links ]

3. Smythe-Wright D., Chapman P., Duncombe Rae C., Shannon L.V and Boswell S.M. (1998). Characteristics of the South Atlantic Subtropical Frontal Zone between 15°W and 5°E. Deep-Sea Res. I 45, 167-1192.         [ Links ]

4. Lutjeharms J.R.E., Valentine H.R. and van Ballegooyen R.C. (1993). The Subtropical Convergence in the South Atlantic Ocean. S. Afr. J. Sci. 89, 552-1559.         [ Links ]

5. Lutjeharms J.R.E. and Ansorge I.J. (2001). The structure and transport of the Agulhas Return Current between South Africa and 70°E. J. Mar. Syst 30, 115-1138.         [ Links ]

6. Lutjeharms J.R.E. (1985). Location of frontal systems between Africa and Antarctica: some preliminary results. Deep-Sea Res. 32, 1499-11509.         [ Links ]

7. Weeks S.J., and Shillington F.A. (1996). Phyto-plankton pigment distribution and frontal structure in the subtropical convergence region south of Africa. Deep-Sea Res. I 43, 739-1741.         [ Links ]

8. Metzl N., Tilbrook B. and Poisson A. (1999). The annual f_co2 cycle and the air sea CO₂ flux in the sub-Antarctic Ocean. Tellus 51, 849-1861.         [ Links ]

9. Allanson B.R., Hart R.C. and Lutjeharms J.R.E. (1981). Observations on the nutrients,chlorophyll and primary production of the Southern Ocean south of Africa. S. Afr. J. Antarc. Res. 10/11, 3-114.         [ Links ]

10. Lutjeharms J.R.E., Walters N.M. and Allanson B.R. (1985). Oceanic frontal systems and biological enhancement. In Antarctic Nutrient Cycles and Food Webs, eds W.R. Siegfried, P.R. Condy and R.M. Laws, pp. 11-121. Springer-Verlag, Heidelberg.         [ Links ]

11. Llido J., Garçon V., Lutjeharms J.R.E. and Sudre J. (2005). Event-scale blooms drive enhanced primary productivity at the Subtropical Convergence. Geophys. Res. Lett. 32, L5611, doi. 10.1029/2005GL022880.         [ Links ]

12. Bernard K.S. and Froneman PW. (2005). Tropho-dynamics of selected mesozooplankton in the Polar Frontal Zone, Southern Ocean, during austral autumn 2004. PolarBiol. 28, 594-1606.         [ Links ]

*Author for correspondence. E-mail: w.froneman@ru.ac.za