Regional variation in the life cycle of Rhincalanus gigas (Copepoda: Calanoida) in the Atlantic Sector of the Southern Ocean–re-examination of existing data (1928 to 1993)

A regional comparison of the life cycle of Rhincalanus gigas was undertaken on the basis of selected net haul data collected in the Atlantic sector of the Southern Ocean between the years 1928 and 1993. Data were pooled into 3 regions, a northern region (NR) extending from the Scotia Sea to the sub-Antarctic Front and incorporating the Polar Front and Polar Frontal Zone, the Weddell-Scotia Confluence (WSC) and the Eastern Weddell Sea (EWS). There are distinct regional contrasts in the physical environment, the NR being largely ice-free throughout the year compared to the 9 mo ice cover found in the EWS. R. gigas was more abundant in the NR, particularly at the Polar Front, than in either the WSC or EWS and seasonal fluctuations in abundance in all regions were low. A clear seasonal vertical migration pattern was found in the NR and EWS, with the population commencing its descent in February in both; however, arrival back in the surface waters was up to 2-3 mo earlier in the NR and recruitment commenced correspondingly sooner. In the WSC no clear seasonal pattern could be resolved, which probably reflects this region's dynamic nature. Clear differences in overwintering stage structure were also seen between the NR and EWS. In the former, the new generation overwinters mainly as CIII, compared to CII in the EWS. A winter decline in the abundance of CIV to CVI in the NR and their clear recruitment from October onwards suggests that a proportion of the population has a 1 yr life cycle. In contrast in the EWS a strong overwintering presence of CIV to CVI suggests that 2 yr is more likely. In both regions females were present throughout the year indicating their ability to overwinter and possibly spawn again. Low mortality rates (<0.004 d-1) in both regions during the autumn and winter offer further support for a 2 yr life cycle.

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