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

Abstract

SUMMARY: 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.

Observations on the Diel Vertical Distribution of Hydromedusae in the Southern Benguela

“…Although the overall diversity and the abundance of the medusae were both lower than previously reported in the region (Buecher and Gibbons 2000), this reflects the small number of samples collected, as well as their restricted geographic coverage. For Survey 1, it might also reflect station location, because areas at or close to upwelling regions are believed to have a depauperate (and certainly different) zooplankton assemblage relative to areas downstream (Gibbons and Hutchings 1996).…”

“…All the species under study have a global distribution, except in the polar areas (Kramp 1961), and have been collected regularly during previous surveys in the Benguela (Pagès et al 1992, Buecher andGibbons 2000). Although the overall diversity and the abundance of the medusae were both lower than previously reported in the region (Buecher and Gibbons 2000), this reflects the small number of samples collected, as well as their restricted geographic coverage.…”

Jellyfication of Marine Ecosystems as a Likely Consequence of Overfishing Small Pelagic Fishes: Lessons from the Benguela

“…Both A. forskalea and C. fulgida are also part of the jellyfish assemblages of the SB (Pagès et al 1992, Buecher andGibbons 2000) but have not been observed at high densities, and no indication is apparent of a temporal trend in abundance like that in the NB. No jellyfish species has been exploited commercially in the Benguela region.…”

Observations on the Ecology ofPleurobrachia Pileus(Ctenophora) in the Southern Benguela Ecosystem

“…However, it is not higher than has been observed in shallow estuaries (e.g. Wang et al 1995, Attrill andThomas 1996), or previously in the southern Benguela ecosystem (Buecher and Gibbons 2000). It is suggested that the high numbers observed here might reflect the local circulation in St Helena Bay, which is retentive owing to the presence of cyclonic eddies and coastally trapped gyres (Holden 1985).…”