Zooplankton are an important aquatic community component of dryland river because they provide a trophic link from primary producers to higher consumers. The zooplankton community that develops following flooding in dryland rivers is heavily reliant on hatching from a persistent sediment egg bank. Studies have shown that flood frequency influences the zooplankton communities that emerge from floodplain sediments, with sediments not flooded for several years yielding less abundant and diverse zooplankton communities than more frequently flooded areas. However, less is known about variation at the more frequently flooded end of the gradient (from multiple floods per year to once every few years). Variation at this end of the scale is potentially important because communities hatching from more frequently flooded areas are a key resource for short-lived fish species that cannot rely on 1 in 5 year floods to recruit. Similarly, the effects of flood duration on egg bank composition or hatching communities have not been tested. Thus, this study examines spatial variation in hatching zooplankton at higher flood frequencies and in relation to inundation duration.
Surface sediment samples were collected from dry anabranches subject to flood frequencies from ~5 to 0.5 per year on the Macintyre River floodplain. Anabranches were selected randomly from predefined flood frequency classes based on a previous study’s classification. Within anabranches, samples were collected from deep and shallow locations, with depth assumed to be a proxy for the duration (deep sites experiencing relatively longer periods of inundation). Sediment samples were inundated in mesocosms and hatched zooplankton sampled weekly for 6 weeks.
Zooplankton abundance and community patterns varied by flood frequency, relative depth and week. Species richness also varied by flood frequency. Ilyocypris Cypretta and juvenile ostracods (Ostracoda) and Moinidae (Cladocera) contributed most to variation in emergence patterns.