RELATIONSHIPS BETWEEN HYDROGRAPHY, PHYTOPLANKTON PRODUCTION, BIOMASS, CELL-SIZE AND SPECIES COMPOSITION, AND COPEPOD PRODUCTION IN THE SOUTHERN BENGUELA UPWELLING SYSTEM IN APRIL 1988

Hydrography and plankton were sampled along a cross-shelf transect off the Olifants River. South Africa, in April 1988, during a relaxation phase of upwelling. Chlorophyll concentrations were similar across the shelf, 3-5-mu-g chl a . l-1. Midshelf, c. 80 per cent of the chlorophyll a was in cells > 10-mu-m, but nearshore 90 per cent was in cells < 10-mu-m. Large cells (diatoms) dominated midshelf waters and small cells (flagellates) nearshore waters. Smaller copepods (Paracalanus parvus, Ctenocalanus vanus, Centropages brachiatus and Oithona spp.) were most abundant shorewards of the 130 m isobath; larger ones (Calanus australis and Rhincalanus nasutus) were most abundant farther offshore. Calanoides carinatus was equally abundant at all stations. Primary production averaged 2,4 g C . m-2 . day-1 at the large-cell stations and 1,5 g C . m-2 . day-1 at the small-cell stations. In all, 69 and 29 per cent of the production was in the > 10-mu-m size fraction and 15 and 48 per cent in the fraction < 2-mu-m at the large- and small-cell stations respectively. Rates of egg production of female C. australis, C. carinatus, C. brachiatus and P. parvus averaged 43 per cent of maximum at the large-cell stations but only 8 per cent of maximum at the small-cell stations. Growth rates of small juvenile copepods (< 5 - 10-mu-g dry mass) were at their maximum, but those of larger individuals were less than maximum, suggesting food-limitation of growth. Rates of copepod production were similar in both water types, 4,1 and 3,6 Mg C . m-3 . day-1. Food-chain efficiency (FCE), the ratio of secondary to primary production, ranged from 3,5 to 12,5 per cent. Copepods consumed 12-42 per cent of the primary production. The FCE and the absolute proportion of phytoplankton consumed were dependent upon which sizes of phytoplankton were assumed eaten by the grazers.