曾根沼の湖底泥中の硅藻殼

Abstract

The analysis of diatom shells has been made on cores of the bottom deposits of Soné-numa, a lagoon on the east coast of Lake Biwa. The results obtained from the core taken up in water depth of 2 m in the western part of the lake are described below. The core is 450 cm below the mud surface in its total length and may be classified into the eight layers with respect to diatom succession.The first layer, 0-5 cm below the mud surface, is soft ooze containing abundant diatoms and is represented by the Fragilaria-Cyclotella-Cocconeis Association. The second layer, 5-15 cm, is represented by the Pinnulafia-Epithemia-GomPhonema Association, the components of which are rich in numbers of species but few in individuals as a whole. In the third layer, 15-55 cm, the diatoms are hardly found, while the fourth layer is made up by a small amount of diatoms, its flora being characterized by the Eunotia Association. In the fifth layer, 90-100 cm, diatoms are quite absent, as nearly similar to the third layer, but in the next sixth, 170-280 cm, a few diatoms occurs again showing the Eunotia Association as in the fifth layer. The seventh (280-370cm) and the eighth (370-450cm) layers contain abundant diatoms, the former being dominated by the Cocconeis-Amphora-Achnanthes Association and the latter by the Fragilaria-Amphora Association.It is interesting to note that the lowest deposits (seventh and eighth layers) contain species which may be indicated as fossil or relic forms, namely, Amphora delphinea, Amphora ovalis var. libyca, Actinella brasiliensis, Gomphonema lingulatum, Cyclotella nipponica, Cymbella hybrida, Pinnularia lignitica and so on. They are not found in the layer upper than 280 cm of the core. This marked change between the upper and the lower parts of the core has been concerned in rather rapid modification of the environment in the lake during its history. It is also a striking fact that Melosira solida and Stephanodiscus carconensis, both plankton diatoms common to the main basin of Lake Biwa, have never occurred throughout the core.