Enormous environmental changes have taken place around the Japanese coastal lagoons during these last thousand years. Water pollution, coastal development by industrialization and sea level rise, related to warming up of the world, have cause serious damages to the nature of them. The author has investigated living and fossil diatoms of coastal lagoons in Japan. Diatom is not only one of the best indicator for the measurement of recent environment, but it is also a good key for unlocking the paleo-environment of the areas. A study of the historic change of diatom fossils, which are contained in lake sediments, provide us with several kinds of important information about water quality, water depth, and distribution of lake areas.
Lake Shinji and Lake Nakaumi, low salinity lagoons located along the coast of the Japan Sea, are now being planned to be converted in to fresh water condition by shutting up marine water, to procure fresh water for industry, In view of protection of its nature, strong opposition against this development come from the inhabitants near the both lakes. The author took a gravity core sample at the center part of Lake Shinji in 1989. The lowest part of this sample had been deposited about 500 years ago, as determined by radioisotope method. Diatom fossils in the sample provide detail history of lake environment during this past five hundred years.
About 500 years ago, Lake Shinji had been a coastal lagoon. The salinity of that age was about 20-30 permil, because Grammatophora oceanica, which is a marine littoral diatom, was dominant at the lowest part of this sample. About 400-100 years ago, Lake Shinji had become a fresh water lake, with marine water hardly coming into the lake. Aulacoseira granulata, a fresh water diatom spices, was dominant during that time. Since the last 100 years, salinity of Lake Shinji has increased again and Cyclotella caspia, which is a brackish water diatom, has become dominant. The cause of this environmental change is not made clear yet but it might be related with the sea level rise due to global warming.
Key words: diatom, Holocene, Lake Shinji, Lake Nakaumi, paleolimnology