In order to clarify the transition of water quality in Lake Suwa from 1948 to 2015, we analyzed the available long-term data focusing on phytoplankton and the trophic state index (TSI) in summer. We classified the transition of the dominant species of phytoplankton and water quality in summer for about 70 years from 1948 into four phases. In the first phase from 1948 to 1969, the dominant phytoplankton type shifted from diatoms to Microcystis, and the lake shifted from the mesotrophic to eutrophic state. In the second phase from 1970 to 1998, the trophic state shifted from eutrophic to hypereutrophic, and Microcystis was dominant. In the third phase from 1999 to 2007, water quality indicated the eutrophic state. The dissolved inorganic phosphorus was 10 μg L^-1 or less, and dissolved inorganic nitrogen was 600 μg L^-1 or less. In the fourth phase, after 2008, TSI indicated eutrophic and mesotrophic state. Bacillariophyceae Synedra and the nitrogen-fixing cyanobacterium Dolichospermum were dominant. From the point of view of the dominant types of phytoplankton and TSI, Lake Suwa after 2008 is in a trasition phase from a eutrophic lake to a mesotrophic lake, while water quality is being improved.

We have investigated the effects of the Great East Japan Earthquake on the water quality of brackish Lake Hinuma located in Ibaraki Prefecture. It was observed that for three years after the earthquake, the chloride ion (Cl⁻) concentration in the lake water was high. This can be attributed to the fact that the bottom of the lake was lowered owing to the ground subsidence due to the earthquake, which facilitated the increase in salinity. Three years after the earthquake, the bottom has recovered and the Cl⁻ concentration in the lake water has decreased. Compared with the water quality three years before the earthquake, the chlorophyll-a concentration decreased during summer. It seems that the amount of Cyanobium, which was predominant in summer, decreased owing to the high Cl⁻ concentration.