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

Abstract

Physical and biological features of the shelf-break oceanic front of the southern Benguela upwelling region off the Cape Peninsula, South Africa, were studied in March 1983. The convergence front was situated between 90 and 110 km offshore, delineated by strong temperature and salinity gradients. Further inshore (45 km offshore), salinity sections revealed a shallow frontal feature between water masses of different upwelling origin. Recently upwelled water close inshore was slightly aged as indicated by some sun-warming to ca lSC, the development of high chlorophyll a (up to 9 mg m-3) and reduced nitrate concentrations (< 11 pg-at N 1-l). These frontal regions were associated with elevated chlorophyll a concentrations and bacterial numbers. Although recently upwelled water was characterised by high chlorophyll a concentrations (5 to 8 mg m-3), bacterial biomass was relatively low (< 20 mg C m-3) w M e the converse was generally true for older offshore waters (< 2 mg chl a m-3; 20 to 40 mg bacterial C m-3). The overall positive relationship between chlorophyll a concentration and bacterial biomass was best described by a power curve as at high chlorophyll concentrations, bacterial blomass was proportionally reduced, probably due to predation. Close to the sediment water interface there was some evidence of nitrification. Indirect estimates of NHZ-N regeneration by the microplankton (< 200 pm), based on bacterial biomass, suggested that physical mixing processes at the shelf-break front ensure that 'new' nitrogen (NOT-N) can be entrained into the euphotic zone, while well-stratified oceanic waters rely mainly (65 %) on regenerated nitrogen.