Interannual variation in the composition of the assemblages of medusae andctenophores in St Helena Bay, Southern Benguela Ecosystem

The assemblages of medusae and ctenophores were examined from samples colle cted each winter from St Helena Bay, over the 10-year period 1988-1997. A t otal of 50 hydromedusae, 1 scyphozoan and 2 ctenophore species were identif ied from 243 samples. Although the data set is characterised by great inter annual variability, two main assemblages could be identified each year. The se were characterised by either holoplanktonic medusae (e.g. Liriope tetrap hylla, Aglaura hemistoma) or meroplanktonic medusae (e.g. Mitrocomella mill ardae, Chrysaora hysoscella) and ctenophores (e.g. Pleurobrachia pileus). T he holoplanktonic medusae were typical of samples at the southern edge of t he Bay, and were positively associated with both depth and temperature. The ir abundances tended to increase during warm years (1992, 1993 and 1997) as warm surface water flooded the Bay. The meroplanktonic medusae and ctenoph ores were typical of samples collected within the Bay, and the density of s pecies tended to be negatively correlated with temperature and depth. In sp ite of the eurythermal nature of the meroplanktonic species, they were more common during cold years (1990 and 1995). This paper represents the first Bay wide, interannual study of any zooplankton group, and contributes impor tant base line information on the structure of regional pelagic assemblages .