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Seasonal occurrence of benthic algae and changes of subtidal vegetation were studied for their species composition, diversity and biomass during 1982 and 1983 at several selected sites at Juckdo Island (38。12´N, 128。32´E), eastern coast of Korea. Three large brown algae which played a role in change of algal vegetation through their great biomass were investigated with regard to their seasonal growth. Large brown algae such as Undaria pinnatifida, Costaria costata, Laminaria japonica, Agarum cribrosum, Sargassum confusum and S. hornerii constitute the major portion of vegetation in this area throughout the year. Algal vegetation in spring time is characterized by dominance of species U. pinnatifida and C. costata, whereas the summer vegetation by S. confusum and S. hornerii. In autumn large brown algae are shedded and only small algae, such as Chondrus ocellatus and Grateloupia filicina, remain. The vegetation in winter is dominated by the growth of U. pinnatifida and C. costata. Monthly chages in mean length and weight of randomly collected U. pinnatifida, C. costata and S. confusum are as follows; U. pinnatifida occurs from December to June and shows their maximum growth during March (120㎝ in length, 201g/individual in wet weight), its maximum growth rate is 1.4㎝/day, 3.3g/day in this month. The growth season of C. costata is very similar to U. pinnatifida, but their average maximum length(110㎝) and weight (106g/ind.) are lower than U. pinnatifida. The greatest growth rate is during March (1.8㎝/day, 2.0g/day). S. confusum is present throughout the year and reachea the maximum growth (102㎝, 63g/ind.)in July. Maximum growth rate (1.5㎝/day, 1.2g/day) occurs also during this month. U. pinnatifida and C. costata show different months of maximum growth evidently during the two year. This seems to be caused by a considerable damage to the local vegetation followed by heavy storm in February 1983.
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