SPATIAL DYNAMICS OF COCCOLITHOPHORE COMMUNITIES DURING AN UPWELLING EVENT IN THE SOUTHERN BENGUELA SYSTEM

The spatial dynamics of coccolithophore communities were investigated in the top 100 m of a longitudinal transect off Hondeklip Bay, Souther n Benguela region, during a period of active upwelling in late June 19 93. This transect crossed five hydrological domains that were identifi ed from the vertical sections of temperature and salinity, namely the upwelling zone, the upwelling front, the intermediate domain, the offs hore divergence, and the oceanic region. Total coccolithophore standin g crops ranged from less than 10 x 10(3) cells 1(-1) to a maximum of 2 78 x 10(3) cells 1(-1). Highest values were recorded in the top 25 m i n the upweIling zone and at the offshore edge of the upwelling front. Fifty different taxa were identified with Emiliania huxleyi and Syraco sphaera epigrosa being the dominant forms in the standing stock. The o ffshore divergence and upwelling front effectively isolated assemblage s with distinct species compositions and diversity trends. The most su ccessful assemblage in terms of both species diversity and standing cr op was found in the intermediate domain where oceanic and upwelled wat er mix. Equally important was the control of the water-masses motion o n the vertical distribution of coccolithophore communities. Shallow-dw elling communities thrived in the inshore upwelling and offshore diver gence areas, whereas deep-dwelling communities were found in the inter mediate domain, where surface waters downwell. No clear relationship w as found between the concentrations in major nutrients, and the specie s diversity and total standing crop values. S. epigrosa was the only r ecorded species whose absolute abundance showed a positive relationshi p with the nitrate and phosphate surface-water concentrations.