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We evaluated the seasonal stability of two algal symbiont populations in the temperate intertidal sea anemone Anthopleura elegantissima on San Juan Island, Washington, where the relatively thermally tolerant dinoflagellate Symbiodinium muscatinei coexists with the less thermally tolerant chlorophyte Elliptochloris marina. Random collection of anemones along repeatedly sampled transects over four seasons and three shore heights revealed S. muscatinei to be the dominant symbiont, with E. marina mostly limited to anemones in the lower intertidal zone. At the lowest shore height sampled (+0.2 m), the proportion of E. marina was between 40% and 50% of the total symbiont population throughout the year. Symbiont distribution patterns persisted despite considerable seasonal variation in aerial exposure, temperature, irradiance, nutrients, and phytoplankton concentration, as well as a high potential for symbiont shuffling, with mixed-symbiont assemblages occurring in 51% of all anemones sampled. Symbiont density in anemones also changed little despite three- to fourfold-higher division frequencies of both symbionts during July and November. Although the intertidal zonation of these symbionts was stable over an annual period, we predict that their spatial distributions will be responsive to longer-term environmental change, and anticipate that this anemone symbiosis will be a useful and highly tractable barometer for future climate change, with this study serving as a baseline. [ABSTRACT FROM AUTHOR]