The water environment changes of Lake Abashiri since 1983 were investigated by using diatom assemblages in lake sediment core, which collected from the center of Lake Abashiri in 2019. Annual varve was confirmed throughout the core, and the eutrophic environment indicator planktonic diatom taxa have always been dominated since 1983. These suggested that Lake Abashiri has been a eutrophic environment during the period. The relative abundance of the first dominant taxon, Cyclotella atomus, has gradually increased since 1983, while the relative abundance of the third dominant taxon, Cyclostephanos invitatus has gradually decreased. The replacement of these taxa is thought to be due to changes in seasonal water quality by increasing autumn precipitation. Lake Abashiri was rapidly freshened after the temporary gate was installed in the Abashiri River. Both the freshened and eutrophic of the surface water by the construction of temporary gate and increasing precipitation introduced the rapid increasing of the diatom flux after 2008. Previous studies suggested that cyanobacteria have been coming prevalent in recent. The summer temperature, and probable summer lake surface temperature, has been controlled factor whether diatom or cyanobacteria dominated in summer. In Lake Abashiri, the diatom flux increased under the cold summer and high precipitation autumn after the construction and operation of the Ohmagari Weir.

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