The effects of hydroperiod duration and timing on California vernal pool communities

Background/Question/Methods

Climate change has been found to be important in structuring many freshwater ecosystems.  In addition to temperature, variation in precipitation can determine the hydroperiod gradient, which is among the most important factors determining the composition, structure, and function of these ecosystems.  Less understood is how the timing of hydroperiod and accompanying temperature variation interacts to also affect community structure and function.  Temporary waters, such as vernal pools, are the result of highly variable climate within and among years, resulting in variable hydroperiod patterns.  In California, vernal pools exhibit an annual cycle of inundation during the winter months and desiccation by late spring.  High endemism and species diversity characterize these ecosystems.  However, this habitat has also been greatly reduced and therefore understanding how climatic variation affects this system is critical.  I manipulated hydroperiod duration and timing in mesocosms lined with vernal pool soil, and a microcosm experiment was also conducted to separate the confounding effects of hydroperiod and temperature.  Abiotic and biotic variables were measured over the course of sixteen weeks.  I also used several years of data from natural pools to compare to the experimental results. 

Results/Conclusions

Hydroperiod duration, timing, and temperature all had significant effects on community composition and functioning.  Abiotic variables (conductivity, dissolved oxygen, nitrates, and turbidity) and species composition were significantly different among the hydroperiod-timing treatments, but not always among hydroperiod-duration treatments. Unimodal relationships between time and species richness and density were found in all hydroperiod treatments.  There were important differences in species-specific responses to treatments.  Some species (e.g., fairy shrimp species) dominated in early hydroperiod treatments, while others (e.g., mosquitos and chironomids) dominated in late hydroperiod treatments.  Species also differed in response to hydroperiod duration.  Some species (e.g., tadpole shrimp and copepods) required longer hydroperiods. Interestingly, mosquitos could not invade long hydroperiod duration or early hydroperiod timing treatments.  Decomposition responded to both duration and temperature effects.  Natural pools did not exhibit identical patterns to mesocosms, but general patterns in species composition and water characteristics were found.  With anticipated climate change, these results indicate that vernal pool species composition and functioning may be greatly altered.   Future management and restoration approaches should take into account hydrology and landscape heterogeneity to maintain high levels of species diversity and endemism.