We reviewed the environmental impacts of microplastics, discussed the dynamics of microplastics in the aquatic environment and their impacts on marine ecosystems, and introduced the latest analytical methods for microplastics, especially for samples with small particle sizes. Regarding the situation of microplastics in land areas and rivers, the numerical and mass concentrations of microplastics in rivers all over Japan and the estimated total amount of plastics discharged from Japan’s land areas to the ocean were determined. Regarding the state of microplastics in the sea, the dynamics of microplastics in Tokyo Bay and the uptake of microplastics by marine life were determined. For the analysis of microplastics, the methods of sampling, pretreatment, and equipment measurement and their ploblems were summarized, and a technical perspective was presented. It is essential to clarify the dynamics of microplastics in the environment and their impact on ecosystems in order to plan effective and efficient measures for the proper use and management of plastics in the social system. We hope that the information introduced in this paper will contribute to these activities.

The water quality of the river basins situated in volcanic areas and exposed to volcanic activity can be affected by the acidic water, resulting in the pollution of surface water. On April 19th, 2018, the first eruption in 250 years of Mt. Iou, one of the Kirishima volcanos, caused the discharge of mud and acidic water into the Nagae River, contaminating the water with arsenic at a high concentration that exceeded the environmental standard (10 μg L^-1) . In this study, we investigated the changes in arsenic concentration in the streamflow process, the relationship between pH and Electrical Conductivity (EC) , and the runoff characteristics through a continuous observation. The results indicate that the river water was diluted and improved during the streamflow process owing to dilution as a result of influx of water from tributaries, the arsenic concentration has a correlation with pH and EC, particularly, in the upstream area and on nonrainy days, and the arsenic concentration correlates with iron, aluminum, and manganese concentrations. Consequently, arsenic accumulates at the bottom of the riverbed on nonrainy days, and its concentration becomes high owing to the resuspension of the river sediment on rainy days.

The degradation of ecosystem services in rivers due to the invasion and expansion of invasive attached diatoms, Cymbella janischii, is a grave concern. The distribution and seasonal growth of C. janischii in the Tamagawa River system were studied. In the upstream area of the Tamagawa River, C. janischii formed visually identifiable clusters resembling “dirty cotton balls”. C. janischii was identified only under a microscope in the midstream area, but was not observed in the downstream area, and differences were observed between water areas. C. janischii was also distributed in a branch river that joined the upper midstream area of the Tamagawa River, but its concentration was negligible. In the upstream area of the Tamagawa River, C. janischii formed clusters from April to May. During this C. janischii bloom period, the water temperature in the upstream area of the Tamagawa River coincided well with the temperature of the rivers in which C. janischii grows in North America.