REGIONAL DIFFERENCES IN THE LIFE-CYCLE OF CALANOIDES ACUTUS (COPEPODA, CALANOIDA) WITHIN THE ATLANTIC SECTOR OF THE SOUTHERN-OCEAN

All suitable data from the Atlantic sector of the Southern Ocean were compiled to elucidate regional differences in the abundance and life c ycle of Calanoides acutus. Data from 205 stations (1928 to 1992) were pooled into 3 regions, namely the Scotia Sea (SS), the Weddell-Scotia Confluence/northern Weddell Sea area (WSC) and the Eastern Weddell Sea (EWS). The regions contrast sharply: the ice-free SS has summer mixed layer temperatures similar to 3 to 4 degrees C higher but summer chlo rophyll a concentrations generally lower than the EWS, which is ice-co vered for three-quarters of the year. C. acutus was rarer in the vicin ity of the WSC than in the southern part of the SS or in the EWS. In a ll 3 regions seasonal vertical migration characterised the populations , but their appearance in the surface waters of the SS was 1 to 2 mo e arlier than in the EWS, and was of longer duration. Because C. acutus is herbivorous, this is presumably a response to the timing of summer primary production. Despite the contrasting environments, spring/summe r development rates were similar in all 3 regions, with moulting from copepodid CI to CIV taking approx 1.5 mo. Population mortality in autu mn/winter was 0.0070 d(-1) in the SS and 0.0059 d(-1) in the EWS; high ly variable abundance in the WSC area precluded mortality estimation. Based on summer moulting rates, winter stage structure and mortality r ates, the life cycle appears to be completed normally within 1 yr in a ll 3 regions, but it is suggested that the shorter summer growth seaso n in the EWS results in a small proportion of the population taking 2 yr to reach adulthood.