抄録
Lake Kojima is one of the most eutrophicated lakes in Japan. This artificial lake is located in southern area of Okayama prefecture, and was made in 1962 as a reservoir for many paddy fields. This study was conducted to make clear species composition of phytoplankton and its seasonal changes in Lake Kojima in a period from April 1993 to March 1998, and obtain the basic informations which will be supplied for the environmental protection plan for Lake Kojima.
The results obtained can be concluded as follows :
1) 36 genera phytoplankton, that is, 5 genera of Cyanophyceae, 9 genera of Bacillariophyceae, 18 genera of Chlorophyceae, 1 genus of Phytoflagellata and 3 genera of Euglenophyceae, were observed in Lake Kojima from April 1993 to March 1998 (FY1993-1997).
2) Chlorophyceae was dominant in species composition and Bacillariophyceae was dominant in cell number, and Auracoseira italica, Melosira varians and Cyctotella spp. were recognized as dominant species in Lake Kojima through a year.
3) The diversity index (H') showed a mode of spring ≥autumn>summer≥winter.
4) Cyanophyceae showed negative correlationship, Bacillariophyceae and Phytoflagellata showed positive correlationships, and Chlorophyceae and Euglenophyceae did not show clear correlationships with rise of T-N/T-P mass ratio.
5) The T-N/T-P mass ratio lowers in summer because of effect of inflow loading from paddy field, but Lake Kojima has enough condition for water bloom because the absolute values of nutrient salts are high.
6) Main Cyanophyceae which formed water bloom were Microcystis aeruginosa and Anabaeba spiroides, and the dominant species for water bloom was recognized as Oscillatoria in 1970s, Microcystis in 1980s, and Anabaena in 1990s.
