We reported on the water quality and diatom community at the shore of Lake Tazawa, which experienced acidification due to water conduction from the Tama River. The Tama and Hinokinai Rivers, which were also impacted by acidification, were also investigated. Despite the weakly acidic to neutral pH of the sampling sites, the diatom community consisted mainly of pH-indifferent and alkaliphile taxa, with Fragilaria crotonensis (up to 40%) and Melosira varians (up to 24%) being the dominant species. Diatom densities were generally low and mainly consisted of periphyton and ephemeral plankton taxa. We did not find the planktonic diatom Cyclostephanos tazawaensis, endemic to Lake Tazawa. These results suggest that the true planktonic diatom community in Lake Tazawa has not yet recovered to its pre-acidification state.
