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.
Highlights
Coastal upwelling areas are of ecological interest, as well as economic importance, because of their enhanced primary productivity, which is generated by the vertical transport of nutrients to surface waters
The present paper looks at interannual variability in the composition, abundance and distribution of medusae and ctenophores over a part of the same time period in an effort to improve our understanding of long term dynamics of ecosystems
This study describes the relationship between medusae and ctenophores with the environment, and so might enable us to make more direct links with fisheries in future
Summary
Coastal upwelling areas are of ecological interest, as well as economic importance, because of their enhanced primary productivity, which is generated by the vertical transport of nutrients to surface waters. The region is bounded by two warm currents, the Angola Current in the north and the Agulhas Current in the south This highly hydrodynamic area can be considered an unstable environment (Hart and Currie, 1960), where physical events can be evoked to explain the variability in species abundance and distribution (McGowan, 1974). Cnidarians and ctenophores are thought to be good indicators of water mass movement (Colebrook, 1977; Raymont, 1983). Studies of their spatial distribution in and around the Benguela system have revealed the presence of distinct assemblages associated with both latitudinal and longitudinal gradients (Pagès and Gili, 1991a, 1991b; Pagès et al, 1991; Pagès, 1992; Fearon et al, 1992)
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