Seasonal variations in distribution and population structure of Microcalanus pygmaeus and Ctenocalanus citer (Copepoda: Calanoida) in the eastern Weddell Sea, Antarctica

Abstract

The abundance, vertical distribution and population structure of two important small calanoid copepod species, Microcalanus pygmaeus (G. O. Sars) and Ctenocalanus citer Heron and Bowman, were studied in the eastern Weddell Sea in summer (January/February 1985), in late winter/early spring (October/November 1986) and in autumn (April/May 1992). The population of Microcalanus pygmaeus consisted mainly of copepodite stages CII and CIII in late winter/early spring and were concentrated between 500 and 200 m depth. In summer, stage CIV was the modal stage and the bulk of the population had ascended above 300 m. In autumn the population structure was bimodal with CI and CV dominating. Most of the population was concentrated between 300 and 200 m. In all investigation periods M. pygmaeus had their maximal concentrations in the thermo-pycnocline. The developmental stages CIII to CV of Ctenocalanus citer formed the bulk of the population in late winter/early spring. In October all developmental stages had their main distribution between 500 and 200 m, except females, which were concentrated in the upper 50 m. In November most of the population occurred between 200 and 50 m. The summer population was concentrated in the upper 50 m, and numbers increased dramatically as the new cohort hatched. Copepodite stages CII and CIII dominated the population at the end of January, while CIV dominated 2 wk later. In autumn, CV was the modal stage. The majority of the population was concentrated in the upper 100 m, but there was an increase in abundance below 300 m compared to summer. Age structure changed with depth with a younger surface population and an older one in deeper water layers. The seasonal change in number of M. pygmaeus is much smaller than that of C. citer; the summer:winter:autumn ratio of the former being about one, whereas the winter:summer/autumn of the latter was about nine. Early copepodite stages and adults of M. pygmaeus occurred throughout all investigation periods. The large proportion of early copepodite stages in April and in mid-October suggests autumn and early to midwinter breeding. Apparently, M. pygmaeus may reproduce and grow year-round or perhaps has a 2-yr life-cycle. In contrast, the dramatic increase in abundance of early copepodite stages of C. citer in summer suggests springtime reproduction.