Inundation timing, more than duration, affects the community structure of California vernal pool mesocosms

The hydroregime (duration, timing, and frequency of inundation) of temporary aquatic ecosys- tems are well known to affect relative abundance, species richness, and community composition. The effects of hydroperiod (inundation duration) have been well studied, but this is not the case with inundation timing. I conducted an experiment in mesocosms that were inoculated with California vernal pool soil. Seven replicates of three inundation duration and timing treatments were implemented: long hydroperiod (16 weeks), short-early (first 8 weeks), and short-late (last 8 weeks). Sampling consisted of abiotic variables (dissolved oxygen, conductivity, turbidity, nitrates, and orthophosphates) and biotic variables (invertebrate species densities, including active and passive dispersal strategies, and species richness). Inundation length had little effect on abiotic and biotic variables. Species richness and total density significantly increased in late inundation timing treatments. Active dispersers densi- ties increased in late inundation treatments, especially in short-inundation treatments. Species composition changed in response to both inundation timing and succession. These results suggest that inundation timing needs to be considered in temporary aquatic ecosystems. Indeed, understanding how this variation affects communities may provide a framework for predicting ecosystem responses to climate change.