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Changes in benthic marine macrophyte community structure in the Strait of Georgia: long-term and grazing responses Manson, Murray


Benthic marine macrophyte community structure at two sites near Bath and Sear Islands in the Strait of Georgia, British Columbia, was quantified from the high tide level on the shore to about 10 m below zero tide level. Data obtained from the sites during the summers of 1972 through 1975 and in 1992 was used to make several comparisons of macrophyte community structure. 1) Changes in community structure along the depth gradient from intertidal to subtidal were investigated. 2) At Bath Island, a period of intensive grazing by sea urchins, Strongy locentrotus droebachiensis, during 1973 altered the community structure and provided an opportunity for a comparative study of macrophyte community change and regeneration at the two sites. 3) Community structure at both sites in 1992 was compared to that in the 1970's. A multivariate approach was taken for the analyses, using principal component analysis, canonical variate analysis (CVA), and a modification of CVA including depth as an environmental variable. Several macrophyte associations were consistently found by the analyses. 1) An association of Fucus spp., Mastocarpus papillatus, Neorhodomela larix, Porphyra spp., and Ulva spp. defined the community structure of intertidal quadrats. 2) A subtidal foliose red and kelp species association was found in ungrazed areas. 3) A grazed form of the subtidal association was characterized by low variability and high abundances of Calliarthron tuberculosum and Ulvaria obscure.4) After grazing pressure was removed, community structurewas characterized by initial increases in Ulva spp. And Sargassum muticum during the first year, followed by regrowth of foliose red and kelp species. 5) A 1992 upper subtidal association characterized by high abundances of S. muticumand Lomentaria hakodatensis, Gigartina exasperate, Corallina officinalis, and Gelidium spp. contrasted with a 1970'sassociation of such species as Callophyllis spp., Cryptopleura ruprechtiana, Polyneura latissima, and Desmarestia viridis.

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