We investigated the microbial contamination in various swimming pools in elementary schools and sporting facilities under normal conditions without a reported outbreak of pathogens. We further elucidated their contamination factors according to usage and water quality control practice and evaluated the effectiveness of various bacteria and viruses as indicators of fecal contamination in those waters. The results showed that the detection of microorganisms varied depending on usage conditions, water quality control practice, and facility characteristics (e.g. outdoors or indoors) . Both Escherichia coli and F-specific bacteriophages were detected by culture methods, suggesting that their complete inactivation is difficult with current water quality control practices. Novel contamination indicators, such as the pepper mild mottle virus, cross-assembly phage, and human-specific HF183 Bacteroides 16S rRNA gene marker, were detected at relatively high frequencies. In particular, the cross-assembly phage and HF183 were considered as effective indicators of human fecal contamination in swimming pools where water quality was controlled by filtration and chlorination. Human enteric viruses were not detected in all of the surveyed swimming pools; however, when those pathogens are excreted from infected individuals to swimming pool waters, they may be present in the waters without losing infectivity.

Isahaya Bay has suffered from severe red tides, and previous reports revealed that red tides have continued to exist under dissolved-inorganic-nitrogen (DIN) -depleted conditions in the bay. One possible explanation for this phenomenon is that the influx of dissolved organic nitrogen (DON) from a reservoir on reclaimed land facilitates the persistence of red tides in the bay. In this study, DON enrichment bioassays were conducted to test whether the growth of phytoplankton in Isahaya Bay can be stimulated through the addition of DON isolated from the reservoir. Microsized phytoplankton (> 10 μm) growth rates in both the presence and absence of antibiotics, a bacterial inhibitor, were stimulated by DON addition. The results suggest that microsized phytoplankton in Isahaya Bay can be stimulated by DON isolated from the reservoir directly and indirectly. Thus, the influx of DON from the reservoir might be an important process supporting the existence of red tides under DIN-depleted conditions.

A novel sampling method with an analysis for microplastics (MPs) with sizes greater than 20 μm in tap water was developed to reveal their occurrence characteristics, such as their concentrations, polymer compositions and size distributions, with high precision. Micro-Fourier transform infrared spectroscopy followed by density separation could recover 91.0% of MP particles in tap water. Large amounts of water could make the standard deviations of detected MP concentrations much smaller, and the coefficient of variation decreased to 14.9% when the sampling volume was 1000 L. The sampling method for MPs under faucets with unused plankton nets outside collected 16.7 ± 6.4 particles by dry deposition from the atmosphere, but the coverage by lids could reduce the level of deposition. Finally, MPs in 1000 L of tap water originating from groundwater were analyzed. Twelve polymers were detected and their total concentrations ranged from 78 ± 10.4 to 2130 ± 358 particles m⁻³. Polyethylene, alkyd resins and polymethyl methacrylate were dominant and their total concentrations contributed 56.5 to 94.0% to the total MP concentrations. The median sizes of MPs observed in all samples ranged from 30.1 to 60.4 μm and their dominant shape was of the fragment type.

Scum floated by biogas generated in Hirano River sediment has caused difficulties because of the combined sewer overflow. The Osaka Prefectural Government conducted a demonstration project for sediment remediation in 2021–2022 by dosing tablets of calcium nitrate. This study used prokaryotic 16S rRNA gene next-generation sequencing to verify the project performance on the basis of changes in the sediment microflora. Sediment at a test site at a depth of 4 m near the Minamibenten Bridge was dosed with chemical tablets every two months. Methanogenic archaea were found to be dominant, but sulfate-reducing bacteria were minor in the anaerobic sediment. The chloroplast DNA sequences of Streptophyta growing in the river surface and settling in the sediment were also detected in some sediment samples. Although methanogenic archaea continued to be dominant, canonical correlation analysis indicated that the microflora at the test site was characterized by their increased proportions of bacteria belonging to Betaproteobacteria, Methylophilales, and Thiobacillus, and by their increased oxidation-reduction potential and decreased total sulfide content.

Gross primary production (GPP) and ecosystem respiration (ER) , as well as chlorophyll a (Chl a) and nutrient concentrations, were measured in the central part of Lake Teganuma in both 2018 and 2021. The autotrophic and heterotrophic communities showed high activity levels during summer and fall, which were strongly affected by water temperature. Net ecosystem production (NEP = GPP − ER) was positive throughout the study period, with an average GPP/ER ratio of 4.0. The mean GPP value (9.4 mgO₂ L⁻¹ d⁻¹) was approximately six times higher than the threshold GPP (GPP at GPP = ER: 1.6 mgO₂ L⁻¹ d⁻¹) . Therefore, the central part of Lake Teganuma was autotrophic. The variations in GPP and NEP were closely related to changes in Chl a concentration, which explains approximately 40% of the variance in GPP and NEP.