Upwelling in a nearshore marine ecosystem and its biological implications

Abstract

The flow of water into and out of a kelp-bed on the west coast of southern Africa has been monitored over a 24-day period, spanning three upwelling cycles. Currents, measured by SCUBA divers at five stations around the study site reached 40 cm s−1 at the surface, but were slower and more variable in mid-water (4–11 m) and just above the bottom (8–22 m). Surface currents correlated with wind, but not tide, offshore winds causing offshore surface drift and onshore currents below, while onshore winds reversed these flow patterns. Water transport budgets indicate that the volume of water at the study site (3·75×106m3) turned over between three and seven times per day during upwelling. Nutrients (NO3, P and Si) peaked during active upwelling, when temperatures fell below 12 °C, and were minimal under downwelling conditions, NO3 failling to 1 μg at l−1. Phytoplankton density was lowest during upwelling and peaked during onshore winds. The concentration of suspended particulate organic matter varied between 0·7 and 5·5 mg l−1 dry mass, the highest values occurring during heavy wave action. The number of suspended bacteria and ranged from 0·7×105 to 36·2×105 cells ml−1, with peaks lagging 1 day behind those of POM. Biological implications of these measurements are discussed.