Zooplankton biomass variability off Angola and Namibia investigated by a lowered ADCP and net sampling

Variability in horizontal and vertical zooplankton biomass distribution was investigated down to a maximum depth of 1250 m off Angola/Namibia in August/September 2000 using a combination of net samples and backscatter profiles. The latter were measured by a 300-kHz ADCP mounted on the CTD frame. Thus, we obtained a near constant signal to noise ratio over the entire water column. This approach allowed both, a large spatial coverage and a relative high resolution in horizontal and vertical plane. The volume-backscattering coefficient was calibrated against biomass concentration of size fractionated net samples. In this way differences in shape and scatter properties of organisms were considered in a first approximation. Potential occasional larger signals were smoothed by averaging backscatter intensity over net sampling layers prior to the statistical comparison. The agreement in biomass concentration ranges and in horizontal distribution patterns derived by both methods was used as an additional quality factor. The biomass-rich zone spread deeper in the regions south of the Angola Benguela Frontal Zone (ABFZ) than north of it as a response to coastal upwelling. The diel vertical migration (DVM) caused “centres of the populations” (weighted mean depths) in 122 m (night), 303 m (day), 135 m (dawn), and 154 m (dusk). The average biomass at night within the upper 200 m exceeded those at day by a factor of 3.5. The phenomenon was more pronounced north and seaward of the ABFZ than south and shoreward of it. Minimum oxygen concentrations of about 0.2 ml/l were no barrier for migrating organisms passing the intermediate oxygen minimum zone (OMZ), which is a characteristic feature in the region.