Abstract
Lake Osoresan, or Usoriko, Aomori Prefecture. is well known for its strong acidity (pH ca 3-4), the fauna and flora met with in it are consequently limited. The phytoplankton is generally poor through all seasons. The zooplankton is, however, found in great abundance, being dominated by a certain species from year to year.
The fact that the dominant species in this lake vary according to year deserves special comments. A cladoceran, Eimocephalus vetualus (O. F. Müller), which dominated until 1933 showed a marked decrease in 1934, remaining so during 1935-1936. Chydorue aphaeriew O. F. Müller, too, showed a similar change. A copepod, Cyclops fuecue Jurine, on the contrary, showed a marked increase in later years. As to the' explanation of such a change the writer recalls the fact that in 1931 and 1932 the lake water showed but little stagnation, while in 1935-1936 the stratification was very distinct.
The vertical distribution and diurnal migration were studied using wing pump. On June 2829th, bulk of the nauplius and juvenile of C. fuseue inhabited the thermoeline through a day, though slight nocturnal and twilight migration were likely to occur. Contrasted with the above case C. fuacus, on August 4-5 th, tended to migrate nocturnally, though majority of nauplius remained in the hypolimnion. A rotifer, Brachionus uroeolaria O. F. Müller, mainly inhabited the surface layer. As regards the last two species the layer of maximun population sank deeper in in the early morning. This may be considered as being due to the heavy rain of that morning.
