We studied fossil diatom assemblages from the uppermost part of the Gamo Formation of the Plio-Pleistocene Kobiwako Group, and reconstructed its shallow lake paleoenvironment at 1.8–1.9 Ma. Samples were collected from 11 horizons of the strata at a field site in Taga Town, eastern Shiga Prefecture, Japan. At least 495 diatom valves were counted and identified from each sample using optical microscopy. The dominant species was usually Staurosira construens or other species belonging to Fragilaria sensu lato, although there was one horizon dominated by Aulacoseira ambigua. Therefore, we inferred that the waterbody was shallow and rich in aquatic plants, but it became slightly deeper when A. ambigua was dominant. We estimated pH and total phosphorus concentration based on the diatom count data by weighted averaging, which indicated that the water was meso-to eutrophic and slightly alkaline throughout the study period. Although, specific estimates were not confident, and especially estimates of total phosphorus concentration varied substantially depending on the literature used for reference. The paleoenvironment reconstructed using fossil diatom assemblages was consistent with sedimentological findings and fossil plant evidence from the same strata.

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.

We conducted a survey of diatom flora in a channel located on the campus of the University of Shiga Prefecture (Hikone, Shiga, Japan). This channel receives water from Lake Biwa via fish farming ponds operated by the Shiga Prefectural Fishery Experiment Station. Diatom samples were collected from 10 sites during 2022 and 2023, and examined using both light and scanning electron microscopes. In total, 174 species were recorded, including 19 unidentified taxa, representing 62 genera. The genus with most taxa was Gomphonema, followed by Nitzschia and Navicula. More than a dozen taxa formerly classified within the genus Fragilaria, such as Staurosira, Pseudostaurosira, and Nanofrustulum were observed. These observed species suggested eutrophic and slightly alkaline conditions. The diatom taxa occurred in this study had commonly been reported from the littoral zone of Lake Biwa. Additionally, several potentially introduced species were detected, indicating possible anthropogenic introductions associated with human-mediated transfer of fishes and mussels during field coursework or research activities.