How do temporal and spatial dispersal interact to form zooplankton communities

Background/Question/Methods

Although many have investigated how spatially dispersing organisms affect community structure, less work has been done on how temporal dispersal may interact with spatial dispersal to structure communities.  This has important implications for metacommunity theory, since temporal community dynamics might be more important than traditionally studied spatial dispersal.  Dormant eggs of zooplankton that hatch from the egg bank may need certain cues to initiate hatching, but if adult zooplankton are passively dispersed they may be able to immediately reproduce.  We hypothesized temporal dispersal from dormant egg banks would be more important in structuring the zooplankton community than spatial dispersal.  Mesocosms were used to manipulate spatial dispersal and temporal dispersal was manipulated via sediment bags.  Treatments consisted of 2 levels of temporal dispersal, absence or presence, and 3 levels of spatial dispersal: high, low or none.  Dispersal distances were 3m, 50m and 55m respectively from a natural pond.  The no spatial dispersal control mesocosms were covered with light, clear plastic covers.  Zooplankton were sampled every 7 days using a PVC tube sampler, where 1L was removed per mesocosm.  Samples were filtered with fine mesh (64 um) and preserved in 70% ethanol for later identification. 

Results/Conclusions

A total of 16 zooplankton species colonized the mesocosms throughout the experiment.  Using a repeated measures ANOVA, total zooplankton abundance and species richness were analyzed.  Sample date and a distance by egg bank interaction were significant in explaining variation in total zooplankton abundance, whereas only sample date significantly explained the variation in zooplankton species richness.  Least square means indicated that distance was important with higher zooplankton abundances in the high spatial dispersal treatments than the low spatial dispersal or control treatments.  The presence or absence of an egg bank had little effect on zooplankton abundance.  Tukey’s post hoc comparisons showed that high versus low spatial dispersal/no egg bank treatments were significantly different, further supporting the importance of proximity to a source pond for spatial dispersal.  This indicates that the effects of spatial dispersal may only be strongly felt by the community when an egg bank is absent.  Thus, both spatial and temporal dispersal have important effects on zooplankton community structure.