WATANABE (1982) developed an equation to obtain the river pollution index (RPI_B) by which he tried to assess the general state of river pollution. This equation is based upon the water quality chart obtained from biological assessment. The water quality chart, however, only indicates the distribution of area with different degrees of water pollution (from 4 to 6 degrees). Accordingly WATANABE (1981) developed the diatom community index (DCI) based on the specific composition of epilithic diatom communities at each station. This index is based on his idea that the degree of water pollution in rivers actually changes gradually and continuously and that they, therefore, can not be classified into some definite areas with numerical degrees in itself.
In this study we developed a numerical index (RPI_D) to assess the river pollution by using a new diatom community index (NDCI) based on the specific composition of epilithic diatom communities.