Taxonomic study of nano-planktonic diatoms in Brackish lakes.

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Brackish Lakes Shinji and Nakaumi are located in Shimane Prefecture, western part of Honshu Island, Japan. These lakes are entrophic brackish lakes lying between freshwater Hii river and Japan Sea. Salinity ranges from 5 to 10 % of seawater in Lake Shinji, and from 20 to 50 % of seawater in Lake Nakaumi. Phytoplankton flora of Lake Shinji mainly comprised freshwater and brackishwater species, that of lake Nakaumi comprised brackishwater and seawater species.Species composition and seasonal variation of planktonic diatoms in these lakes were studied from April 1996 to March 1997. During fiscal year of 1997, taxonomic works of Cyclotella was mainly studied.Through the taxonomic study of nano-planktonic diatoms, 25 taxa of 14 genera were identified from these lakes. Dominant species in Lake Shinji were Cyclotella choctaw hatcheeana, Cyclotella atomus var.gracilis, Cyclotella spp., Thalassiosira pseudonana and Chaetoceros sp. Those in Lake Nakaumi were Cyclotella choctawhatcheeana, Cyclotella sp., Minidiscus comicus, Neodelphineis pelagica and Thalassiosira tenera. Cyclotella choctawhatcheeana, Cyclotella atomus var.gracilis were for the first time recognized from Lake Shinji and Lake Nakaumi. These two species were described as new species from brackish water in 1990, and 1993, respectively.In Lake Shinji, Thalassiosira pseudonana dominated in March and April, Cyclotella atomus var.gracilis dominated May to July, Cyclotella sp.3 dominated August to October. In Lake Nakaumi, Cyclotella sp.2 dominated in May and June, Neodelphineis pelagica dominated from September to October.