RESPONSE OF EPILITHIC DIATOM ASSEMBLAGES TO WATER-POLLUTION IN RIVERSIN THE TOKYO METROPOLITAN-AREA, JAPAN

1. During the spring of 1992, fifty-two quantitative diatom samples we re collected from twenty-eight rivers located in the Tokyo Metropolita n area, Japan, to study the response of the diatom assemblages to wate r pollution (assessed using physical and chemical data determined mont hly from April 1987 to March 1992). 2. Species composition was analyse d by means of biotic indices (Pantle and Buck's saprobic index) and mu ltivariate analyses [two-way indicator species analysis (TWINSPAN) for classification and canonical correspondence analysis (CCA) for ordina tion]. Species-abundance relationships were analysed using diversity i ndices (species richness, Shannon's diversity index and Pielou's evenn ess index) and rank-abundance patterns (rank-abundance curves). 3. CCA revealed two major gradients. The first corresponded to organic pollu tion and eutrophication. The second corresponded to variables related to geographical location. Four main station groups were determined by TWINSPAN. The location of the indicator species of groups 1-3 along th e CCA axis 1 is consistent with their known pollution tolerance charac teristics. Indicator species for group 4 had larger scores on CCA axis 2, and are representative of brackish water environments. 4. Species richness tended to be higher in the intermediate range of water pollut ion. Pielou's evenness index and Shannon's diversity index followed th e same tendency but only weakly. 5. The rank-abundance patterns of dia tom assemblages were more or less constant in all stations. The curves were very similar in shape, differing only in length and gradient (di rectly related to species richness and evenness, respectively). 6. The results of this study indicate that the response of diatom assemblage s to environmental change can be observed in species compositional var iation. Multivariate analyses and pollution indices revealed this resp onse and are to be preferred to species diversity measures.