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 experimental billabongs were designed to test two hypotheses (i) that changing the pattern of flooding alters zooplankton abundance within billabongs, and (ii) the presence of small planktivorous fish alters the zooplankton community structure and diversity within billabongs. Results indicated that flooding caused microcrustaceans to increase in abundance but not rotifers. This response was not modified by changing the time of flooding. The presence 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 taken from the experimental billabongs and incubated under controlled conditions indicated the existence of a large reservoir of resting stages. The number of resting stages in the sediment had not been modified by changing the season of flooding in the experimental billabongs, but it was modified by permanent inundation, which may have removed environmental cues. The presence of planktivorous fish also appeared to influence the number of resting stages that accumulated in the sediment. Emergence under stable laboratory conditions was rapid. This was not the case under more variable natural conditions.