Impact of water regime and fish predation on zooplankton resting egg production and emergence

The emergence of zooplankton from ephemeral areas upon re-wetting has been suggested as a significant contributor to billabong productivity. Sixteen e xperimental billabongs were designed to test two hypotheses (i) that changi ng the pattern of flooding alters zooplankton abundance within billabongs, and (ii) the presence of small planktivorous fish alters the zooplankton co mmunity structure and diversity within billabongs. Results indicated that f looding caused microcrustaceans to increase in abundance but not rotifers. This response was not modified by changing the time of flooding. The presen ce of the planktivorous fish had only a small influence on the structure of rotifer assemblages, whereas microcrustacean assemblages became dominated by juveniles. Emergence of rotifers and microcrustaceans from sediment take n from the experimental billabongs and incubated under controlled condition s indicated the existence of a large reservoir of resting stages. The numbe r of resting stages in the sediment had not been modified by changing the s eason of flooding in the experimental billabongs, but it was modified by pe rmanent inundation, which may have removed environmental cues. The presence of planktivorous fish also appeared to influence the number of resting sta ges that accumulated in the sediment. Emergence under stable laboratory con ditions was rapid. This was not the case under more variable natural condit ions.