Environmental changes recorded in coastal lagoon sediments-the local effects and the global phenomena-

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The purpose of this research project is to make clear the environmental characteristics and the forming processes of the representative coastal lagoons by extracting the local effects and the global phenomena, and to present the fundamental data for discussing on the influences of the sea level raising and the predicting artificial change of environment. For this purpose, we carried out the collaborative works in such coastal lagoons as Saroma-ko, Abashiri-ko, Hanama-ko, Nakaumi, Shinji-ko, etc.In Lagoon Saroma-ko facing the Sea of Okhotsk, the change in water quality caused by the artificial cutting of sand bar is clearly indicated by the change in diatom assemblages, which affected the organic matter content of the bottom sediment as well as the assemblages of benthic fauna. Dredging the bottom of River Abashiri for expanding the port area accelerated the upstreaming of high saline waters to Lake Abashiri-ko. Consequently it raised the level of halocline and expanded anoxic environment in bottom waters.In Lagoon Hamana-ko, the opening of Imagireguchi strait, which is believed to be caused by a big earthquake in 1498, is recorded by the change of diatom assemblages in the core samples. The abnormal bottom topography in the northern area of this Lake found by echo sounding is probably related to that event. These phenomena suggest that the lagoons facing the Pacific Ocean are strongly effected by the tectonic movements as well as the eustatic sea level change.In case of the lagoons facing the Sea of Japan, such as Lake Shinji-ko, Lake Nakaumi, etc., the change in water quality and sedimentary facies are totally controlled by the meteorological effects of heavy rains, drastic change of atmospheric pressure, and seasonal strong wind. The detection and differentiation of these effects in core samples are important subject in the discussion on the global climatic change in the past to the future.