LIFE HISTORY VARIATIONS IN A PREDOMINANTLY GAMETOPHYTIC POPULATION OV IRIDAEA CORDATA (GIGARTINACEAE, RHODOPHYTA)1

An Iridaea cordata ( Turner) Bory population made up of 83% gametophytes and 17% tetrasporophytes demonstrated little variation in these proportions over three years. These data contrast with an expectation that alternations of generations, as demonstrated by culture studies, should result in equal numbers of isomorphic generations. Analyses of reproductive and demographic attributes of the population were made to better understand the role of alternation of generations in this population .
Growth and mortality rates in the summer could not account for the success of gametophytes. however, size class frequencies demonstrated that tetrasporophytes were smaller individuals. Sporelings recruited into this intertidal region and made up 20% of the spring population numbers. Even with this level of recruitment, gametophyte-to-tetrasporophyte ratios did not change. Perennating holdfasts regenerated 80% of the spring number and 90% of the total blade area. Consequently, perennation and differential survival of gametophyte spores or sporelings are proposed as mechanisms maintaining gatnetophyte dominance in this San Juan Island, Washington population .