Relationships between hydrography, phytoplankton production, biomass, cell size and species composition, and copepod production in the southern Benguela upwelling system in April 1988

Abstract

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 μg chl a·ℓ−1. Midshelf, c. 80 per cent of the chlorophyll a was in cells > 10 μm, but nearshore 90 per cent was in cells < 10 μm. 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 μm size fraction and 15 and 48 per cent in the fraction <2 μm 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 μg 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.