1900年代における東京湾の赤潮と植物プランクトン群集の変遷

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Historical plankton records on red tide events and phytoplankton community in Tokyo Bay published between 1907 and 1997 were reviewed to elucidate long-term changes. Before 1940s, naked flagellates unpreservable in formalin-seawater predominated over the phytoplankton, and were major causative organisms of red tides. Community composition of diatoms and armored dinoflagellates in the bay and the adjoining water, Uraga Channel, showed little change in this period. In 1950s, a red tide by a diatom Thalassiosira mala was recorded for the first time. Eutrophication process developed rapidly between late 1950s and early 1970s, and now the bay is one of the most polluted inlet waters in the world. In the meantime, that by Skeletonema costatum came to occur very frequently. The basic community composition of phytoplankton in recent days was already molded in late 1960s with rapid eutrophication process. Species diversity reduced owing to desappearance of neritic/oceanic species from the bay and the occupation by several dominant species. Clear dissimilarity of phytoplankton community composition between Tokyo Bay and Uraga Channel was also made up in this period, suggesting large environmental differences between the two areas. The biomass of diatoms has been increasing since 1950s, mainly due to S. costatum. Since 1970, Heterosigma akashiwo came to make red tides frequently and occasionally built up exceptionally high biomass(>1000 mg-chl. a m^<-3>). Since early 1980s, nano-and micro-fragellates came to be a major component of phytoplankton. The cell densities of brackish water species have been increasing since 1980s with increase of freshwater inflow caused by a human population growth around the bay. Averaged annual occurrence of red tide events varied as follows ; twice for 1900s-1940s, five times for 1950s, ten times for 1960s, 14 times for 1970s, 19 times for 1980s and 15 times for 1990s. These results suggest that the artificial impacts on the aquatic ecosystem of Tokyo Bay altered the community composition of phytoplankton between 1955 and 1970.
日本海洋学会の論文
1998-06-05

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