Effects of the Leeuwin Current on the Distribution of Carnivorous Macrozooplankton in the Shelf Waters off Southern Western Australia

Plankton samples were collected using 500-μm nets in the surface waters (up to a depth of 70 m) of the continental shelf in three regions (Albany, Bremer Bay and Esperance) off southern Western Australia in July 1992 (winter) and January 1993 (summer). The carnivorous zooplankton of these samples were characterized by low species richness and high variability in abundance. Abundances of most of the major taxa collected differed significantly between seasons and many also differed between regions. Siphonophores and chaetognaths dominated the carnivorous plankton, with lower abundances of hydromedusae and raptorial copepods. The most abundant siphonophores were Chelophyes appendiculata and Eudoxoides spiralis in summer and winter, respectively. The most abundant chaetognaths were Sagitta minima in summer, with Pterosagitta draco and Sagitta enflata dominant during winter. Overall, the numbers of species of both siphonophores and chaetognaths were highest during winter. At this time, there was also a trend for decreasing numbers of species in an easterly direction between Albany and Esperance, which was probably due to the presence of subtropical species entrained within the warm Leeuwin Current, which was flowing east along the continental shelf during winter. During summer, when the current was not present in this region, there was an even spread of fewer species along the coast.