We measured diurnal variations in oxidative stress conditions of cyanobacteria utilizing field observations and laboratory experiments in order to evaluate photoinhibition effects. On clear summer days, transparent bottles filled with surface water were set up at several depths and were collected every three hours together with the measurement of the photosynthetically active radiation (PAR). In the laboratory experiment, two cyanobacterial species were exposed to gradually increasing and then decreasing light intensities. The samples were analyzed with the PAR-induced (H₂O₂), along with the total hydrogen peroxide concentrations (total H₂O₂), the catalase activities (CAT), OD₇₃₀, protein (Protein), and chlorophyll a (Chl a) contents, and so on. Protein was significantly proportionate with OD₇₃₀ and Chl a, and was used as an indicator of cell biomass. Increasing PAR, H₂O₂ concentration increased proportionately with the PAR intensity. Then, an oxidative stress indicator in a cell, H₂O₂/Protein is given by the PAR divided by cell volume, evaluated by Protein. CAT activity in a cell, far largest among antioxidant activities, solely followed total H₂O₂/Protein. The prediction model for H₂O₂/Protein was developed with the sufficient agreement with the experimental and field observation results. The model elucidated that the maximum H₂O₂/Protein in a day was larger with lower cell density even at the water surface, indicating that the higher photoinhibition was imposed at low density, in addition to the lower attenuation of PAR. These results indicate that H₂O₂/Protein is an effective biomarker to indicate the stress level of cyanobacteria; the observed levels of H₂O₂ to freshwater may prove useful in designing the criteria for cyanobacteria management.

Humans and other organisms are adversely affected by cyanobacterial blooming. This study aims to investigate the long-term effects of light intensities and different temperatures on Phormidium ambiguum and Pseudanabaena foetida. Enough P. ambiguum and P. foetida cells were acclimated for 24 days at 30 °C, 20 °C, and 10 °C in separate incubators. The starting day sample was collected after 24 days of acclimatization, and a second sample was collected seven days later at light intensities of 10, 30, 50, 200, and 600 µmol m⁻² s⁻¹ for each temperature. The optical density (OD₇₃₀), hydrogen peroxide (H₂O₂) concentration, protein content, chlorophyll-a (Chl-a) concentration, and catalase (CAT) activity were measured. Light intensity changes soon after collection resulted in nearly identical starting day samples at each individual temperature. The H₂O₂ concentration and algal biomass increased until a light intensity of 200 µmol m⁻² s⁻¹ was reached and decreased afterward in each temperature for both species after seven days. In association with an increasing H₂O₂ concentration, the Chl-a concentration decreased after 50 µmol m⁻²s⁻¹ of light intensity, affecting the protein content. The algal biomass was significantly lower at 10 °C compared to 30 °C. The CAT activity increased proportionately with the H₂O₂ concentration and algal biomass. Therefore, water bodies in the field can be illuminated with long-term high light intensities in different temperatures to reduce algal biomass.

The Miyanaka Intake Dam fishway underwent improvements in 2012, and we established a new rock-ramp fishway called the Seseragi Fishway, cognizant of its utility as a passage and a habitat for bottom-dwelling and small fish with weak swimming ability. However, the fishway is occasionally submerged by floods, causing sediment accumulation that leads to changes in the vegetation composition. In addition, the arrival and inflow of seeds from upstream and the surrounding areas result in vegetation changes. In this study, the inside and outside of the rock-ramp fishway were divided into eight areas, and the vegetation succession after 2012 was determined. A correlation was observed between the results of fish catch surveys during the same period and the vegetation. Based on these results, we reported on the process of steadily operating the rock-ramp fishway while devising and improving specific management methods. Changes in vegetation, such as an increase in upright vegetation and a decrease in flow-obstructing vegetation, contributed to an increase in the population of bottom-dwellers, weak swimmers, and juvenile fish. The existence and management of appropriate vegetation are important for maintaining fishways inhabited by a variety of fish species.

Pseudanabaena foetida, a filamentous cyanobacterium, is highly sensitive to temperature and light intensity. This study explores its oxidative stress responses under diurnal light intensities in temperature variations, utilizing hydrogen peroxide (H₂O₂) normalized to optical density (OD₇₃₀) levels (H₂O₂/OD₇₃₀) as a biomarker. The adequate P. foetida cells were distributed to three incubators for 22 days at 30 °C, 20 °C, and 10 °C. Light intensity varied over the course of the day, increasing from morning to a maximum at noon, followed by a gradual decrease until evening. Measurements of H₂O₂, protein, chlorophyll a (Chl a), and catalase (CAT) activity were carried out every three hours, starting at 6:00 and ending at 21:00. Protein concentration and Chl a increased between 12:00 and 15:00 for every temperature. H₂O₂, CAT, and H₂O₂/OD₇₃₀ increased until 15:00 and decreased afterwards for each temperature. The CAT was directly proportional to P. foetida’s H₂O₂ concentration or cell biomass (H₂O₂/OD₇₃₀), which indicates oxidative stress responses and a defense mechanism. The reduced temperature (30 °C to 20 °C and 30 °C to 10 °C) was significantly impactful on H₂O₂ concentration, protein concentration, and Chl a content. The model, based on P. foetida biomass, provides valuable insights into oxidative stress responses under different temperatures, with implications for understanding climate change.

Native and invasive plants of the riverain region undergo a range of environmental stresses that result in excess reactive oxygen species (ROS). Hydrogen peroxide (H₂O₂) is a relatively stable and quickly quantifiable way among different ROS. The herbaceous species including Artemisia princeps, Sicyos angulatus, and Solidago altissima were selected. The H₂O₂ and photosynthetic pigment of leaves were measured, soil samples were analyzed to quantify macronutrients such as total nitrogen (TN), total phosphorus (TP), and soil moisture, and photosynthetic photon flux density (PPFD) was also recorded at different observed sites of Arakawa Tarouemon, Japan. The H₂O₂ concentration of S. altissima significantly increased with high soil moisture content, whereas A. Princeps and S. angulatus significantly decreased with high soil moisture. In each species, H₂O₂ was negatively correlated with chlorophyll a (chl a) and chlorophyll b (chl a). When comparing different parameters involving TN, TP, PPFD, and soil moisture content with H₂O₂ utilizing the general additive model (GAM), only soil moisture content is significantly correlated with H₂O₂. Hence, this study suggests that H₂O₂ would be an effective biomarker for quantifying environmental stress within a short time, which can be applied for riparian native and invasive plant species vegetation regulation.