Interannual variation in the composition of the assemblages of medusae and ctenophores in St. Helena Bay, Southern Benguela Ecosystem

Abstract

The assemblages of medusae and ctenophores were examined from samples collected each winter from St Helena Bay, over the 10-year period 1988-1997. A total of 50 hydromedusae, 1 scyphozoan and 2 ctenophore species were identified from 243 samples. Although the data set is characterised by great interannual variability, two main assemblages could be identified each year. These were characterised by either holoplanktonic medusae (e.g. Liriope tetraphylla, Aglaura hemistoma) or meroplanktonic medusae (e.g. Mitrocomella millardae, Chrysaora hysoscella) and ctenophores (e.g. Pleurobrachia pileus). The holoplanktonic medusae were typical of samples at the southern edge of the Bay, and were positively associated with both depth and temperature. Their abundances tended to increase during warm years (1992, 1993 and 1997) as warm surface water flooded the Bay. The meroplanktonic medusae and ctenophores were typical of samples collected within the Bay, and the density of species tended to be negatively correlated with temperature and depth. In spite 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 important base line information on the structure of regional pelagic assemblages.