Humic substances with soluble forms that are typically found in clear natural water were investigated to understand the effects that dissolved pollutants have on the Secchi depth (SD) in lakes and marshes. The relationship between humic substance concentration and color image SD (CISD) was obtained by comparing the calculated color images of the Secchi disk with the background. The calculation was based on first-order approximation of disk radiance and background light, with the incident angle of light on the water surface set to 40° and the known absorption characteristics of humic and fulvic acids used for each wavelength. A CISD of 126 m was obtained for 0 mgC L^-1 humic acid, which decreased to 78 m at 0.0047 mgC L^-1 before rapidly dropping to a local minimum of 60 m at 0.0048 mgC L^-1. The CISD then increased to the local maximum of 68 m at 0.02 mgC L^-1 before decreasing again to 0.3 m at 20 mgC L^-1. The CISD of fulvic acid displayed the same tendencies as those observed for humic acid. The obtained CISD trend may be a result of changes in color causing changes in brightness. The function approximation was performed by region division where the CISD increased, and the root mean square of the relative error in the approximation function for concentration with respect to the increasing CISD was found to be less than 0.03 in all sections.

　The life history of Rhyacophila lezeyi was investigated at a highly acidic mountain stream, the Shibukuro Stream (pH 2.98 on average) of the Tamagawa River, the Omonogawa River System, Akita Prefecture, northern Honshu, from May 2017 to November 2019. Automatic recording of stream temperature was conducted at one hour or two hours intervals from October 2017 to November 2019. The stream receives inputs from a hot spring, and the stream temperature was distinctly higher than that of the neighboring mountain streams lacking hot spring water inputs.The difference in water temperature was more pronounced in the winter. The larvae of Rhyacophila lezeyi mostly pupated and emerged from late May to early July, but pupation continued until mid-October in the present study. Five instars were present almost continuously from April to November, and no clear temporal delineation of instars was observed. It was presumed that this species has a less synchronous (asynchronous) univoltine life cycle in the stream. We estimated the developmental zero temperature of rearing pupae (DZ-P) as 6.8 ℃ from the minimum deviations of effective degree days calculated by tentative DZ. The stream temperature probably exceeded the DZ of larvae (supposed to be close to DZ-P) during some periods of winter due to the input of hot spring water, which facilitated larval growth and development, even in winter. This peculiar stream thermal regime seems to be a major factor resulting in the less synchronous life-cycle observed for R. lezeyi in the stream.

　The objective of this study was to clarify the effect of geology on dissolved inorganic phosphorus (DIP) concentrations in river water during normal water flow conditions. The study was carried out in the Lake Nukabira Dam watershed, located in eastern Hokkaido, which is part of a national park where forests cover most of the land use. We measured phosphorus concentrations by component at 14 river sites in the watershed and statistically analyzed the relationship between DIP concentrations and environmental variables, including surface geology, soil, vegetation, and topography in the catchment area. DIP concentrations in river water showed the strongest positive correlation with the area fraction of Quaternary lacustrine deposits classified as igneous rocks. DIP concentrations were significantly high in rivers located in the northern part of the watershed, where Quaternary lacustrine deposits were distributed in the catchment area. In some of these rivers, the concentration exceeded the environmental standard (0.01 mg L^-1 for total phosphorus) of Lake Nukabira Dam. DIP concentration in river water showed a significant positive correlation with silicate-form silicon (SiO₂-Si) concentration, suggesting the influence of rock weathering. The supply of DIP via groundwater may contribute to the high DIP concentration of the river water in the distribution area of Quaternary lacustrine deposits.

　Lake Shinji in San-in district, Japan, is home to many freshwater, brackish water, diadromous and marine fish species as well as crustaceans. Among these some marine and brackish water species and a crustacean species migrate upward from Lake Nakaumi during the warmer seasons. Catching these fishes and crustaceans by the set net and other methods has been actively practiced. However, there was a decline in the population of major fishes such as Hypomesus nipponensis, etc., decreased after the abnormal weather in the cool, rainy summer of 1993 and hot, dry summer of 1994. A previous study published in 2019 claimed that zooplankton and Meiobenthos such as Chironomus plumosus suddenly decreased in Lake Shinji after the introduction of neonicotinoid insecticides in 1993, these decreases may have caused the decline of H. nipponensis and Anguilla japonica. However, apart from the aforementioned two species and Salangichthys microdon, variation in the catch of other fish and crustacean species was not considered. Therefore, we investigated it using catch data for eight species by the set net practice from April 1979 to March 2009 kept in The Lake Shinji Fishery Cooperative Association. Post March 1995 the catch of H. nipponensis, A. japonica and Gobiidae decreased from September to March every year, and that of Decapoda also decreased from September to December, however, Mysidae increased from January to March, although yearly fluctuations were frequently observed. We considered the variation in these catches comparing with the annual variation in salinity and water temperature and the introduction of neonicotinoid insecticides from 1993.

　The seasonal changes in aquatic vegetation areas of Ibanaiko Lagoon on the east coast of Lake Biwa were investigated using drone and satellite data. From February to December 2021, we continuously photograthed Ibanaiko Lagoon with a drone six times and created a composite,ortho-mosaic image. Based on the image, polygons were formed by QGIS to encircle the aquatic vegetation. The aquatic vegetation area and their coverage ratio on the water surface were estimated. Of the 63 hectares surveyed, the maximum area of aquatic vegetation was 36 hectares, and the ratio was 57 %. The series of vegetation area were also estimated using NDVI of satellite data. The areas estimated by both approaches were positively correlated (r = 0.988, p < 0.01). The aquatic vegetation area expanded from May–August and shrank from August–December. The maximum growth rate was observed in July. The detected aquatic plants were predominantly Trapa L. and water hyacinth Eichhornia crassipes. The most dominant genus was Trapa L. in August. Trapa L. began to decline prior to water hyacinth,shifting the dominant genus to water hyacinth in December.