Historical frequency of wind dispersal events and role of topography in the dispersal of anostracan cysts in a semi-arid environment

Abstract

Propagules of several freshwater invertebrates are passively dispersed by wind, but the importance of wind as a dispersal vector remains poorly understood. We examined the historical frequency of wind dispersal events in cysts of Artemia franciscana. A threshold wind speed of ~5 km/h was required to begin dispersing surface-resting cysts, beyond which cyst dispersal increased rapidly with increasing wind speed. The analysis of wind speed and cyst dispersal data from our field experiment and wind data for the last 45 years from a nearby airport revealed that wind events strong enough to disperse most cysts at a wind-exposed site are common, occurring on ~16 snow-free days per year and occurring consistently over long time periods. In a topographically uneven landscape, however, wind speeds vary substantially from one location to another; wind speeds at our exposed site were 69% higher than in a nearby wind-sheltered depression. Accordingly, winds strong enough to disperse most cysts from this sheltered site occurred on only 5 days of the 45 year period. Cysts are thus less likely to disperse from, and more likely to settle in, sheltered depressions than wind-exposed areas. Sheltered areas could thus function as traps for wind-dispersing propagules.