Recruitment of Ascophyllum nodosum: wave action as a source of mortality

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.