Dispersal and recruitment of a canopy-forming intertidal alga: the relative roles of propagule availability and post-settlement processes

The daily settlement of eggs and zygotes of the monoecious brown alga Pelve tia compressa (J. Agardh) De Toni was measured on artificial substrata in a reas inside and outside patches of adults in the high intertidal zone of ce ntral California. Settlement was generally 1-2 orders of magnitude higher u nder the adult canopy. This pattern seems to be due to the synchronous rele ase of gametes during the daytime low tide. The release of gametes also app ears periodic over longer time scales (e.g., 3- and 14-day cycles). In spit e of the high availability of propagules under the adult canopy, juveniles were most abundant outside patches, where propagule availability was lower. In both areas, juveniles were disproportionately associated with patches o f a red algal turf [primarily Endocladia muricata (Postels & Ruprecht) J. A gardh and Masticarpus papillata (C. Agardh) Kutzing]. The turf, which is le ss common under the P. compressa canopy, may offer protection from dislodgm ent, grazing, and/or desiccation and thus facilitate recruitment at this si te. Overall, post-settlement processes appear more important in determining population structure than does the availability of propagules in areas in and around patches of adults. However, the apparent small range of dispersa l of P. compressa may make propagule availability an important limitation t o the establishment of new populations and may restrict gene flow between p opulations.