The brown alga Ascophyllum nodosuni (L.) Le Jolis is a dominant rocky intertidal organlsm throughout much of the North Atlantic Ocean, yet its inabilily 10 colonize exposed or denuded shores is well recognized. Our experimental data show that wave action is a major source of mortality to recently settled zygotes. Artificially recniited zygotes consistently exhibited a Type IV survivorsh~p curve in the presence of moving water. As few as 10, bul olten only 1 reLalively low energy wave removed 85 to 99'% of recenlly setlled zygotes. increasing the setting time for attachment of zygotes (prior to distilrbance from water movemenl) had a positive effect on survival. However, survival was significantly lower at high densities, and decreased at long (24 h) setting times, probably as a result of bacleria 011 the surface of zygotes. Spatial refuges provided significant protection from gentle water movement but relatively little protectjon from waves. These data indicate that zygotes are maladapted lor attachment in moving water and suggest that water movement is the primary faclor conlrolliny recruitment and distributional patterns of A. nodosum. These and earlier observations on the long-term lack of colonizat~on of denuded shores suggest that successful recruitment is highly epjsodic on all but the most sheltered shores. Because of the widespread don~ina~~ce of A. nodosum, disturbance b y waves or currents, and stochastic events may play major roles in structuring intertidal comrnunilies In the Northwest Allantic.

Summary1. 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 ≤ 1·3% 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 ( ≈ 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.

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