SHORT-TERM VARIABILITY DURING AN ANCHOR STATION STUDY IN THE SOUTHERN BENGUELA UPWELLING SYSTEM - ABUNDANCE, DISTRIBUTION AND ESTIMATED PRODUCTION OF MESOZOOPLANKTON WITH SPECIAL REFERENCE TO CALANOIDES-CARINATUS (KROYER, 1849)

Daytime zooplankton samples collected at a fixed station position in St Helena Bay during a 27-day time series in March-April 1987 were used to describe the variability in the vertical distribution, abundance and population age structure of Calanoides carinatus in response to upwelling-related processes, and to provide an estimate of their production. The vertical distribution of C. carinatus was characterised by ontogenetic layering of copepodites and adults. The thermocline delimited the maximum depth of young copepodites (CI-CIII). Older stages avoided low-oxygen (< 1 ml O2l-1) bottom water. C. carinatus was usually not associated with the chlorophyll a maximum. The spatial segregation of young and older stages is briefly discussed in relation to differential feeding habits and diel vertical migration behaviour. The demographic structure of the C. carinatus population showed evidence of a stable age distribution of copepodites and adults during the first of two upwelling cycles observed during the study. Overall mean abundance was 360 animals m-3. However, during the second upwelling cycle their mean abundance was reduced to 183 animals m-3 and adults dominated the population by 54%. These changes in abundance and age structure are discussed in relation to upwelling-induced advective processes. Surface-dwelling young copepodites (CI-CIII) are thought to be transported away from the study site, while re-seeding of the reduced nearshore population probably took place through advection of diapausal pre-adults (CV) in the upwelled water. Daily production of juvenile C. carinatus was estimated at 0.7mg C m-3d-1 which, combined with an egg production of 1.0mg C m-3d-1 by adult females, is equivalent to 2% of the observed daily primary production. The mean P:B ratio for the copepodite stages was 0.167d-1. The role of this dominant copepod and the mesozooplankton in the carbon budget in St Helena Bay is discussed. Consumption by mesozooplankters was estimated at 22% of the daily primary production indicating that there was a considerable imbalance between primary producers and production by these heterotrophs.