Seasonal and spatial variability of dominant copepods along a transect off Walvis Bay (23°S), Namibia

The study provides the first quantitative analysis of changes in zooplankton abundance and community structure in the Walvis Bay area, off Namibia, over a full seasonal cycle (February-December 2000). Zooplankton was collected monthly along a 90-nautical-mile transect. Calanoid copepods were numerically the most abundant mesozooplankton, Metridia lucens, Calanoides carinatus, Rhincalanus nasutus and Centropages brachiatus being the most common species, with each exhibiting a specific distribution pattern. Mean calanoid copepod abundance was lower during autumn, when upwelling was minimal, than during other times of the year. However, abundances were exceptionally low in September, when upwelling was most intense. Offshore transport is believed to be responsible for copepod losses from the area during that period. Abundances of C. carinatus and R. nasutus were high inshore between June and August, at the onset of the upwelling season, and peaked during October-December, towards the end of the upwelling season. These cool-water species were closely associated with upwelled water near the coast. The results concur with previous distribution patterns of zooplankton and their association with upwelling in Namibian waters. Findings suggest a possible decade-scale shift in the zooplankton community structure since the 1960s.