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Variation in structure of the subcanopy communities associated with southern California Silvetia compressa ( J. Agardh) Serrão, Cho, Boo & Brawley populations was examined at eight sites, including four long-standing intertidal Marine Protected Areas (MPAs). Although sea temperature and salinity showed little variation, maximum wave force and sand influence differed significantly among sites. Seaweed and sessile macroinvertebrate cover and mobile macroinvertebrate densities were determined in 10 quadrats during both autumn 1995 and spring 1996. A total of 111 taxa was distinguished at the eight sites, including 47 macroalgae, 20 sessile macroinvertebrates, and 44 mobile macroinvertebrates; however, only a few species consistently dominated abundances in the subcanopy assemblage. Silvetia compressa cover varied significantly among sites during both sampling periods; cover was significantly greater at all but one site during the autumn. Morphologies of Silvetia compressa thalli were qualitatively similar except at Monarch Bay, where plants were the least densely aggregated and frond lengths were two to three times greater than at other sites. Seaweeds contributed 71.2% of the subcanopy cover averaged over all sites compared with 23.8% sessile macroinvertebrate cover; mobile invertebrate densities averaged 363.9 m-2 over all sites. The three most abundant seaweeds (Pseudolithoderma nigra, Pseudolithophyllum neofarlowii, and Corallina pinnatifolia/ C. vancouveriensis) and macroinvertebrates (Phragmatopoma californica, Mytilus californianus, and Anthopleura elegantissima) accounted for approximately 67% and 20%, respectively, of total understory cover. The three most abundant mobile macroinvertebrates (Littorina scutulata, Lepidochitona hartwegii, and Macclintockia scabra/Lottia conus) accounted for nearly 60% of all mobile animals. An average of 27 macrophytes and sessile macroinvertebrates and 19 mobile macroinvertebrates occurred at a site; site H' diversity based on macrophyte and sessile macroinvertebrate cover averaged 1.91; mobile macroinvertebrate H' diversity based on density averaged 2.03. Neither cluster analysis nor multidimensional scaling produced clear site patterns based on geographic location or sampling period; long-standing MPA sites did not form a distinct group and did not differ significantly in community structure from nonhistorical MPAs based on Analysis of Similarity (ANOSIM) tests. Communities representing autumn and spring were more closely associated with each other than with communities from other sites. Differences in community structure were detected among individual sites in all ANOSIM tests despite strong similarities in abundant taxa. ANOSIM tests also showed that understory communities differed between sampling periods, except for analyses based on cover in recently established MPAs. Significant differences in the cover and density of many abundant subcanopy populations also were found among sites using univariate statistical procedures. Only weak relationships could be established between variations in species types and environmental factors. These results suggest the importance of localized and stochastic histories in generating site variation among rockweedassociated populations and the difficulties in establishing post hoc relationships between environmental patterns and variations in species abundances.