To compare the effects of floating-leaved and emergent vegetation on the flow and dissolved oxygen concentration in the bottom layer of lakes, we measured the density of the plants, average flow, dissolved oxygen concentration, and sediment quality inside and outside the lotus stand at Lake Teganuma. We measured sulfide and methane in the sediment as indicators of the redox condition. The floating leaves covered the water surface to suppress the blowing flow, and the flow decreased in both the bottom layer and the surface layer inside the stand area. Although the petiole cross-sectional area per unit area was lower when the leaves were floating than that at the time of emergent leaves, the resistance was the highest in the stand area when the leaves were floating. The flow velocity during emergent leaves and withering was relatively faster in the surface layer than in the bottom layer, and the resistance increased in proportion to the petiole cross-sectional area per unit area. Dissolved oxygen concentration decreased in the stand area in the presence of floating and emergent leaves. Although the water temperature was the highest at the time of emergent leaves, rate of decrease in dissolved oxygen concentration was significantly higher at the time of floating leaves. From the above, we concluded that the degree of reduction in the flow and dissolved oxygen concentration was greater in the lotus stand area in the presence of floating leaves than in the presence of emergent leaves. Since 95.0 % of the gas generated from the sediment collected in October and 74.5 % of the gas collected in December was methane, the sediment of the lotus stand is highly reduced during the entire year.

　In this study, the structure of microbial communities during a hypoxic water formation period in Nakaumi was revealed using molecular biological techniques. Nakaumi is a meromictic lake in which seawater from the Sea of Japan and fresh and low-salinity water from Shinji Lake flow in. As a result, microorganisms related to the sulfur cycle were dominant across the sampling points of Nakaumi under oxygen-deficient conditions. It was suggested that hydrogen sulfide was produced during the hypoxic water formation period by sulfate-reducing bacteria, which are dominant in Nakaumi. In addition, sulfate-reducing bacteria were also detected in the bottom water, which indicates that the microbial sulfur cycle is present not only in the sludge but also in the bottom water. The abundance ratios of Nitrosopumilus martimus, an ammonia-oxidizing archaeon, and genus Nitrospira, nitrite-oxidizing bacteria were higher in October than in September because of the inflow of oxygen from the water of Sea of Japan. However, the abundance ratio of the genus Nitrospira was very low in Nakaumi. Thus, the accumulation of nitrite in Nakaumi is inferred from the low abundance ratio of the genus Nitrospira in Nakaumi.

　Bluegill (Lepomis macrochirus) was collected from the area around the old port of Tsuchiura to the mouth of the Sakuragawa River from December 2019 to November 2020 (excluding February to April) in Lake Kasumigaura (Nishiura), and the stomach contents of 130 specimens were investigated. The frequency of occurrence (%F) and weight composition (%W) for each food item ingested by the bluegill were calculated. As a result, the prey organisms that showed the highest %F and %W throughout the survey period were shrimp, and the values were relatively high from September to December. Filamentous algae showed the next highest values of %F and %W, which were high in January, May, and June. Based on these results, the main prey organisms of bluegill in the survey area were shrimp and filamentous algae. As a result of calculating %F of the 50-99 mm class, the 100-149 mm class, and the ≥ 150 mm class for the total length, shrimp and filamentous algae showed more than 30 % in three classes, and there was no clear tendency by size class. %W of shrimp showed > 40 % in all classes and increased with size class, whereas filamentous algae were less than 15 % in all classes and decreased with size class.