Diatom Assemblages in Lacustrine Sediments of Lake Shibu-ike, L. Misumi-ike, L. Naga-ike, L. Kido-ike in Shiga Highland and a New Biotic Index based on the Diatom Assemblage for the Acidity of Lake Water.

Abstract

Diatom taxa occurring in freshwater with low pH values were classified into the following two categories according to their tolerance of increasing acidity (Table 2).
1) Extremely tolerant taxa-They are often dominant in the freshwater below 5.5 in pH value.
2) Tolerant taxa-These taxa occur in water below 6.0 in pH value, but are not dominant in the water below 5.5 in pH value.
The value computed from the following formula was proposed as a new biotic index based on diatom assemblage in lacustrine sediments for the water acidity in average. The formula is
DIA=_??_
where DIA is the diatoms in ex for the water acidity, _??_ is the total of the relative abundances of all extremely tolerant taxa in the sample, _??_ Tj that of all tolerant taxa.
The values of DIA as calculated from the formula by means of the diatom assemblages in lacustrine sediments of four small lakes in Shiga Highland (Table 3) tended to show a clear relationship with the average pH value in each lake water (Table 1).