SHORT-TERM VARIABILITY DURING AN ANCHOR STATION STUDY IN THE SOUTHERN BENGUELA UPWELLING SYSTEM - CHEMICAL AND PHYSICAL OCEANOGRAPHY

During March and April 1987, an anchor station experiment was conducted in 45m water depth in St Helena Bay (32-degrees 30'S) on the west coast of South Africa, "downstream" of a major upwelling centre at Cape Columbine. Temperature, salinity, oxygen, chlorophyll and nutrient concentrations were measured at seven depths down to 43m at four-hourly intervals over a 30 day period. These measurements were accompanied by current metering and weather station observations. Additional biological measurements of primary and secondary productivity were also carried out (see other papers in this suite). The period of the anchor station covered one major upwelling/decay cycle during the first fortnight of the experiment, followed by a second upwelling-favourable period. Throughout the experiment, stratification was observed in all parameters. The physical oceanography data suggested that the gyral circulation in the area tends to trap organic matter on the shelf as part of a two-layer system Coastally-trapped internal waves and tidal periodicity control the relative importance of surface and bottom layers, which show considerable shear across the interface, because of the difference in cross-shelf circulation in the two layers. Despite sudden changes in bottom current velocities, little change was found in the distribution of chemical parameters below the thermocline, suggesting that relatively little net advection occurs into die bay, in contrast to the steeper slope regime. Chemical parameters showed consistent variability over the inertial period (22.5h). The inverse relationship between nitrate, phosphate or silicate and dissolved oxygen confirmed the importance of nutrient regeneration in the bottom mixed layer (BML), although considerable differences existed between nitrate and the other two nutrients. Rates of uptake during phytoplankton growth and replenishment during upwelling suggests that minimum upwelling rates in the Cape Columbine area were of the order of 0.5-1.0 md-1 during the anchor station experiment. A considerable flux of ammonia from the sediment to the BML was also recorded. Data obtained during this study are compared to those obtained from similar anchor stations off Walvis Bay (22-degrees-S). Considerable differences in chemical parameters are related to changes in the oceanographic regimes at the two sites, together with changes in source water masses.