Southern Benguela frontal region. I. Hydrology, phytoplankton and bacterioplankton

Abstract

Physical and biological features of the shelf-break oceanic frontof the southern Benguela upwelling region off the Cape Peninsula,South Africa, were studied in March 1983. The convergence front wassituated between 90 and 110 km offshore, delineated by strong temperatureand salinity gradients. Further inshore (45 km offshore), salinitysections revealed a shallow frontal feature between water massesof different upwelling origin. Recently upwelled water close inshorewas slightly aged as indicated by some sun-warming to ca 15 degreeC, the development of high chlorophyll a (up to 9 mg m super(-3))and reduced nitrate concentrations (< 11 mu g-at N l super(-1)).These frontal regions were associated with elevated chlorophyll aconcentrations and bacterial numbers. Although recently upwelledwater was characterised by high chlorophyll a concentrations (5 to8 mg m super(-3)), bacterial biomass was relatively low (< 20 mgC m super(-3)) while the converse was generally true for older offshorewaters (< 2 mg chl a m super(-3); 20 to 40 mg bacterial C m super(-3))