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 cycle 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 ~3 to 4°C higher but summer chlorophyll a concentrations generally lower than the EWS, which is ice-covered for three-quarters of the year. C. acutus was rarer in the vicinity of the WSC than in the southern part of the SS or in the EWS. In all 3 regions seasonal vertical migration characterised the populations, but their appearance in the surface waters of the SS was 1 to 2 mo earlier 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/summer development rates were similar in all 3 regions, with moulting from copepodid CI to CIV taking approx 1.5 mo. Population mortality in autumn/winter was 0.0070 d-1 in the SS and 0.0059 d-1 in the EWS; highly variable abundance in the WSC area precluded mortality estimation. Based on summer moulting rates, winter stage structure and mortality rates, the life cycle appears to be completed normally within 1 yr in all 3 regions, but it is suggested that the shorter summer growth season in the EWS results in a small proportion of the population taking 2 yr to reach adulthood.

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