Zooplankton biomass variability off Angola and Namibia investigated by a lowered ADCP and net sampling Lutz Postel a, ⁎ , António José da Silva b , Volker Mohrholz a , Hans-Ulrich Lass a a Baltic Sea Research Institute, D-18119 Rostock, Warnemünde, Germany b BENEFIT Programme, PO Box 912 Swakopmund, Namibia Received 18 May 2006; received in revised form 14 November 2006; accepted 20 November 2006 Available online 10 January 2007 Abstract 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. © 2006 Elsevier B.V. All rights reserved. Keywords: Zooplankton biomass; Multinet; Acoustics; Lowered ADCP; Diel vertical migration; Oxygen minimum zone; Angola–Benguela Front; Upwelling 1. Introduction Zooplankton net sampling and particularly the sample analysis is time consuming. Ship time restric- tions often limit the vertical coverage during deep-sea studies. This may result in sampling effects due to insufficient measuring length and/or measuring inter- vals. For example, short-term influences, like the diel vertical migration, may produce false large-scale horizontal distribution patterns if sampling continues round the clock (cf., Platt and Denman, 1975). These particular limitations were evident when examining the zooplankton biomass patterns by net sampling near the Angola–Benguela Frontal Zone (ABFZ) during the interdisciplinary r/v “Meteor” Cruise 48-3, in August/ September 2000. We tried to overcome this problem by complementing the data with the backscatter signal of an Available online at www.sciencedirect.com Journal of Marine Systems 68 (2007) 143 – 166 www.elsevier.com/locate/jmarsys ⁎ Corresponding author. E-mail address: lutz.postel@io-warnemuende.de (L. Postel). 0924-7963/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2006.11.005