Comparison of mesozooplankton communities from a pair of warm- and cold-core eddies off the coast of Western Australia

We examined the abundance and biomass, size, trophic and community structure of the mesozooplankton within a pair of warm- and cold-core eddies off the west coast of Australia in October 2003, about five months after their initial formation. Zooplankton assemblages were highly diverse in both eddies, with little dominance by any taxa. Assemblages were significantly different in the two eddies, and night assemblages differed from day. Contrary to initial expectation, mesozooplankton abundance and biomass were twice as high in the warm-core eddy as in the cold-core eddy. This was consistent with the higher phytoplankton and microzooplankton biomass and higher primary production in the warm-core eddy. The source water for the warm-core eddy was Leeuwin Current water with about a 10% admixture of shelf water. The plankton biomass and productivity of these water masses are substantially higher than of subtropical Indian Ocean water, the source water for the mixed layer of the cold-core eddy. Five months after eddy formation, the abundance and biomass of zooplankton in the eddies continued to reflect the values generally observed in their parent water masses. The trophic structure of the zooplankton in both eddies was dominated by carnivores (mostly chaetognaths). Although this apparent trophic structure may be an artefact of sampling oligotrophic waters with coarse-meshed (335 μm) nets, it points to a ‘lumpiness’ in the Sheldon particle spectrum.