A survey of the aquatic plant flora in Lake Towada, a caldera lake located in the northern Tohoku region, was conducted in 2021. Twenty taxa of aquatic plants were confirmed; among them, a previously reported species was revised and re-identified and five species were newly recorded. Based on these updated flora data, the conservation importance of 65 lakes was evaluated following a method by Yamanouchi et al. (2016), using the number and rarity of extant species as indicators. The results showed that out of the 65 lakes, Lake Towada improved from the 19th to 12th and from 17th to 8th in terms of the number and rarity of extant species, respectively, indicating that the lake has higher conservation importance compared to what was previously reported. Elodea nuttallii, an invasive alien species, first documented in Lake Towada in 2014, had spread throughout the lake in 2021 and has grown vigorously at several sites.

　We examined the concentrations of neonicotinoid insecticides in lake water, inflowing river water, and tap water sourced from Lake Kasumigaura. Sampling was conducted at the shore near the Kihara water intake on the right bank of Lake Kasumigaura (Nishiura) for lake water, and the Seimei River for river water, which is the closest inflow river to the water intake. Sampling was conducted on 16 days from September 2020 to August 2021, the day after it rained more than 10 mm. Tap water samples were collected seven times in the evening on the days when lake and river water were sampled from September 2020 to March 2021. Of the seven types of neonicotinoids used in Japan, five types were detected in the Seimei River and four types in Lake Kasumigaura. Dinotefuran and clothianidin were detected year-round in both lake and river water. However, neonicotinoids were not detected in tap water during the entire period, likely due to activated carbon treatment at the water purification plant.

　To clarify the fluctuation of salinity, water current, and seawater exchange rates in the brackish water of Lake Obuchi, adjacent to a nuclear fuel reprocessing facility in Rokkasho, Aomori, Japan, we observed the salinity, water currents, inflow of fresh water from the nearby catchment area, and the seawater flux between the lake and sea during 1 August–10 December 2020. Mean salinity at the surface (i.e., 0.5 m depth) and bottom (i.e., 3.5 m depth) layers of the lake were 21.1 psu (range: 12.2-28.0 psu) and 26.6 psu (range: 19.7-31.7 psu), respectively. In addition, the mean water velocity in both the upper and lower layers of the central part of the lake was moderate, at less than 5 cm s^-1, and low velocity (<10 cm s^-1) accounted for >90% in surface and bottom layers of Lake Obuchi. The mean values of freshwater inflow, seawater inflow, and lake water outflow were estimated to be 1.0×10⁵, 4.2×10⁵, and 5.9×10⁵ m³ d^-1, respectively. Outflow from the lake to the sea was generally the inflow of seawater in addition to the inflow of freshwater. Based on these results, the mean ±SD of the daily mean seawater exchange rate in Lake Obuchi’s water supply section was estimated to be 0.58 ± 0.15 d^-1, suggesting that almost half of the seawater that flows into lake is exchanged every day.