Diatom Assemblages in the Surface Sediments of Lake Shiji and Lake Nakaumi, Shimane Prefecture, Japan.

Abstract

This study examined the diatom assemblages in the surface sediments of two brackish lagoons, Lake Shinji and Lake Nakaumi. The results of this paper are summerized, as follows.
(1) There are distinct difference in composition of diatom assemblages between the two lakes. In Lake Shinji, Cyclotella caspia is dominant, and share s more than 80%. On the contrary, in Lake Nakaumi Cyclotella caspia, Thalas s ionema nitzschioides, Thalassiosira spp. and Fragilaria flavovirens appear together. Paralia sulcata, one of typical diatom species, which commonl y appears in inner bays and brackish lagoons, occurs very rarely only in Lake Nakaumi.
(2) Skeletonema costatum and Chaetoceros sp., which are dominant species in the living planktonic diatom floras of these lakes (AKiyAmA1,976-1985; Date,1976-1985), are not found in sediments. The diatom valves of those species are broken easily soon after their death and hardly deposit in lake floors.
(3) Most of fresh water diatoms which are brought into lakes by rivers are deposited within 2km apart from mouths of rivers. However some species (for example, Synedra ulna) are sometimes brought far from mouths of rivers.