Many terrestrial and marine systems are open to immigration. As such, the delivery of reproductive propagules should play a substantial role in determining local diversity in many systems. Here we present the results of a two-year experimental manipulation of subtidal flow regimes and show that flow has a strong positive effect on the assembly and maintenance of epifaunal invertebrate diversity by reducing recruitment limitation in two biogeographic regions. At two sites each in Alaska and Maine, USA, we experimentally manipulated flow speeds and measured the diversity of communities assembling through time and on recruitment panels scraped clean regularly. At all sites, the species richness of established communities, and the richness of recruitment into established communities and onto empty plates was >25% higher in enhanced flow than in control flow treatments. These effects were consistent for two years, and community diversity remained higher despite 30% higher species loss in enhanced flow treatments. Because communities remained open to immigration throughout the experiment, the data suggest that the diversity of epifaunal communities is strongly limited by recruitment and that supply-side effects on diversity in natural communities are strong.

The positive effect of flow on diversity through a decrease in recruitment limitation was robust across scale, biogeographic region, and flow velocities and was consistent in magnitude in communities and on recruitment plates. Consequently, the data strongly suggest that the positive effects of flow on epifaunal diversity are persistent, can operate without diversity-enhancing positive feedback mechanisms, and are driven by increases in propagule delivery. Thus flow plays a large role in establishing and maintaining epifaunal diversity by mediating the delivery of propagules necessary to colonize a patch or to replace species within communities. Although our data do not preclude effects of interspecific interactions, they strongly suggest that flow plays a large and essentially untested role in determining the diversity of benthic marine communities, and they imply that flow is a key mechanism driving recruitment limitation in diverse aquatic systems.