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Title: Variation in Structure of the Subcanopy Assemblage Associated with Southern California Populations of the Intertidal Rockweed Silvetia compressa (Fucales) 
Author: Sapper, Stephanie A; Murray, Steven N
Date: 2003-10
Publisher: University of Hawai'i Press
Citation: Sapper SA, Murray SN. 2003. Variation in structure of the subcanopy assemblage associated with Southern California populations of the intertidal rockweed Silvetia compressa (Fucales). Pac Sci 57(4): 433-462.
Abstract: Variation in structure of the subcanopy communities associated with
southern California Silvetia compressa (J. Agardh) Serrao, 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 pinnatifolial
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
scabralLottia 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
ISSN: 0030-8870
URI: http://hdl.handle.net/10125/2696

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