Natural variability in zygote dispersal of Ascophyllum nodosum at small spatial scales

1. Dispersal of propagules of marine benthic species may contribute to variation in the structure and dynamics of populations. Ascophyllum nodosum dominates the midintertidal zone of sheltered rocky shores in the north-west Atlantic, and populations often consist of large, mature individuals and few newly recruited thalli. 2. We investigated the dispersal of zygotes of A. nodosum at three sites on Swans Island, ME, USA, to determine its variability under natural conditions and to test whether limited dispersal explains recruitment patterns. 3. At each site we released artificially fertilized, marked zygotes from a single point during the rising tide, and sampled their dispersal using two methods: sampling the water column to track the dispersing cloud of zygotes; and assaying caged ceramic tiles affixed to the shore for the presence of marked zygotes. 4. At all sites, zygotes were diluted to less than or equal to1.3% of the initial concentration within 8-12 min of their release. The mean direction of transport was correlated with both wind and tidal current direction at only one site. The most dense settlement occurred at this site, suggesting greater local reproductive success. 5. Regression analyses based on settlement densities suggest that a quadratic model fit the data best for all sites. Quadratic terms occurred in the direction normal to flow, and are interpreted to represent turbulent transport and diffusion. Both linear and quadratic terms were significant parallel to flow, and we ascribe these to advective transport without and with turbulent mixing, respectively. 6. The range of zygote dispersal exceeded 6 m from the source, and therefore cannot explain the exponential decline of recruits in large patches (approximate to 10 m diameter) within adult stands of A. nodosum. 7. Migration of zygotes between populations is unlikely, but their local dispersal may strongly influence the spatial and temporal dynamics within populations.