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, Southern Benguela region, during a period of active upwelling in late June 1993. This transect crossed five hydrological domains that were identified from the vertical sections of temperature and salinity, namely the upwelling zone, the upwelling front, the intermediate domain, the offshore divergence, and the oceanic region. Total coccolithophore standing crops ranged from less than 10 x 10(3) cells 1(-1) to a maximum of 278 x 10(3) cells 1(-1). Highest values were recorded in the top 25 m in the upweIling zone and at the offshore edge of the upwelling front. Fifty different taxa were identified with Emiliania huxleyi and Syracosphaera epigrosa being the dominant forms in the standing stock. The offshore divergence and upwelling front effectively isolated assemblages with distinct species compositions and diversity trends. The most successful assemblage in terms of both species diversity and standing crop was found in the intermediate domain where oceanic and upwelled water mix. Equally important was the control of the water-masses motion on the vertical distribution of coccolithophore communities. Shallow-dwelling communities thrived in the inshore upwelling and offshore divergence areas, whereas deep-dwelling communities were found in the intermediate domain, where surface waters downwell. No clear relationship was found between the concentrations in major nutrients, and the species diversity and total standing crop values. S. epigrosa was the only recorded species whose absolute abundance showed a positive relationship with the nitrate and phosphate surface-water concentrations.