びわ湖底堆積物の古生物学的研究

Abstract

1) This paper reports the quantitative study of microfossils remained in the core sample of Lake Biwa-ko. 2) In the core, 13 species of animal microfossils were found, dominated by Cladocera-9, Protozoa-2, Insecta-1 and Platyhelminthes-1 species in order. The gauntitatively important microfossil species were Turbellaria (egg capsules) in Platyhelminthes, Bosmina, Monospilus in Cladocera and Chironomdiae (larvae) of Insecta. 3) Regarding plant microfossils other than pollen and diatoms, Pediastrum and Staurastrum of green algae were found in the layers lower than 85 m. The quantity of the former (66.0%) was, however, a little more than the latter (34.0%). 4) The changes of vertical distribution of Cladocera agree well with those of the whole animal microfossils, i, e., when the latter decrease, the former also becomes to small numbers. In the other words, the increase of Cladocera brings the increase of total animal microfossils. Turbellaria and Insecta, however, show opposite tendency to the Cladocera in vertical distribution. 5) When Lake Biwa-ko was under warm climatic conditions, several hundred thousand years ago, green algae dominated too much to allow the propagation of animals. Green algae, however, gradually decreased while the warm and cold climatic ages repeated and entirely disappeared in the cold time. On the other hand, the animals increased gradually not only in the species but also in their quantity in the layers upper than 85 m at which no more green algae were found. They showed a tendency of remarkable increase in the layers upper than 15 m. However, the propagation of the green algae was not allowed in the cold climatic conditions and animals were found at least abundance in that age.