Regulation of zooplankton composition and distribution by a zooplanktivorous fish in a shallow, eutrophic floodplain lake in south east Australia

The diet of Australian smelt (Retropinna semoni) and the patterns of vertical distribution and diel vertical migration of fish and zooplankton were studied over a single Austral spring/summer in a shallow, eutrophic, floodplain lake. The diet of smelt was dominated by cladocera despite the frequently low abundances of such prey in the water column. Calanoid copepods were abundant in the zooplankton community at all times yet formed only a minor component of smelt diet. Patterns of vertical distribution and diel vertical migration exhibited by the various zooplankton taxa appeared to be related to the intensity of predation by smelt. The most heavily predated cladoceran taxa tended to remain in deeper water, whilst lightly predated calanoid copepods were abundant throughout the water column at all times. Neither taxon exhibited marked diel vertical migration. In contrast, cladoceran taxa suffering intermediate levels of predation exhibited marked diel vertical migration tending to avoid surface waters during the day. Smelt also appeared to exhibit diel vertical migration with few fish being caught in the water column during the day. The patterns of predation on zooplankton and the distribution and abundance of zooplankton suggest that smelt may play a significant role in regulating zooplankton community structure in the lowland habitats of south east Australia.