湖水環境の人為的改造と底生有孔虫の群集変化(その7) : 珪藻Cyclotellaの産出による堆積年代の推定とその意義

Recent occurrences of diatom Cyclotella spp. are significant in brackish Lake Nakaumi. Based on the surface sediment, Kashima and Noguchi(1983) suggested that these diatom taxa became developed during the time of 1960's to 1980's. If the Cyclotella occurrence can be traced in the sediment core, an age estimation may be possible with some hypotheses.
In order to elucidate this Cyclotella occurrence, I examined the sediment core from the central part of Lake Nakaumi, and found a significant change of the relative abundance at 2.5-3.0cm sub-bottom depths. Since Cyclotella taxa are basically adapted to fresh water or lower saline brackish water, the dominant occurrence of Cyclotella in Lake Nakaumi may be mostly due to the transportation from Lake Shinjiko, Yonago Embayment, and other fresh water areas. A total area of Lake Nakaumi was reduced to about 77% of the original by a land reclamation project of the Honjyo area, and thereby the transported diatom accumulated in more high abundance in main Lake Nakaumi than before. The Ohmisaki embankment completed in 1978 and then controlled the dispersion of Cyclotella occurrence. If this age applies to the sediment core, sedimentation rate can be estimated to 0.0416～0.0526g/cm^2/year. This sedimentation rate is conformable to the previously reported values.