DIEL VARIATIONS IN GUT PIGMENT CONTENT, DIEL VERTICAL MIGRATION AND ESTIMATES OF GRAZING IMPACT FOR COPEPODS IN THE SOUTHERN BENGUELA UPWELLING REGION IN OCTOBER 1987

The gut fluorescence technique was used to estimate ingestion and filtration rates of the adult female copepods Paracalanus parvus, Cenlropages brachiatus and Calanus austrails, and copepodite stages 3, 4 and 5 of C.australis in the southern Benguela upwelling region. During the study period chlorophyll concentrations within the upper 20 m of the water column were high, 5 μg I-1 in mid-shelf waters and 15-30 μg I-1 in inner shelf waters. Copepod gut pigment content was low and constant during the day then increased sharply during the first 2 h after sunset. Gut pigment content was 2-6 times higher during the night compared with daytime values. Small non-migrating copepods (Paracalanus parvus) showed the smallest diel difference in gut pigment content and large migrating copepods (Centropages brachiatus and Calanus australis) the largest difference. Egg production rates were 20 and 50% of maximum at the mid-shelf and inner shelf stations respectively, suggesting food-limitation. Comparison of ingestion rates calculated from egg production data with ingestion rates calculated from gut pigment data suggested that the copepods were feeding omnivorously at the inner shelf stations but herbivorously at the mid-shelf stations. Assuming that all of the phytoplankton was available as food, the nearshore copepod assemblage grazed ̃1% of the standing crop each day, and the mid-shelf assemblage grazed 5% day-1. Because of errors and uncertainties associated with the gut fluorescence technique, the feeding impact could be underestimated by 2-4-fold. We discuss several approaches which could lead to more precise estimates of feeding rates. © 1990 Oxford University Press.