We present a study on lakeshore water temperature (WT) in Lake Kasumigaura from July to September 2022. WT was monitored at 17 lakeshore sites using WT loggers. The results show that the lakeshore WT was higher during the daytime and lower during the night than the WT at the center of the lake. The highest lakeshore WT reached 37.4 ℃. Additionally, two factors were examined, the breakwater and the mouth of the river, which may affect WT. It was found that WT at the lakeshore with a breakwater tended to be higher during the daytime and lakeshore WT also seemed to be affected by the existence of the mouth of the inflowing river. Abnormally high WT was observed at the shores of Lake Kasumigaura in summer, which possibly resulted in the decrease in the smelt and mussel (Nodularia douglasiae nipponensis) populations. As global warming continues to cause an increase in average air temperature, lakeshore WT will rise and reach higher temperatures than before. Therefore, continuous monitoring of lakeshore WT is essential to protect the lakeshore ecosystem from the effects of global warming.

The spatial–temporal distributions of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) concentrations and changes in the limiting nutrients of phytoplankton in the Seto Inland Sea since the 1980s were analyzed for clustered areas. The DIN concentration decreased in all areas, whereas the DIP concentration decreased in areas strongly affected by the land and in transition areas where the effect of the open sea is strong. These areas account for approximately 20% of the Seto Inland Sea. On the other hand, no significant decrease was observed in the area strongly affected by the open sea, which accounts for approximately 80% of the Seto Inland Sea. The decrease in DIN concentration was greater than the load reduction rate of total nitrogen from land over the past 40 years in the areas strongly affected by the land. The decrease in DIP concentration was equivalent to the load reduction rate from the land. The decrease in DIN concentration was approximately 30% to 40%, even in the area strongly affected by the open sea. The limiting nutrient was phosphorus in the areas strongly affected by the land, whereas it was nitrogen in the area strongly affected by the open sea. The area with nitrogen as the limiting nutrient increased in autumn and winter.

In Kumihama Bay, Kyoto, which is a brackish lake, hypoxic water is known to damage aquacultural bivalve in winter; however, the dynamics of hypoxic water mass has not been elucidated. To clarify this dynamics, we observed the monthly spatial distributions of water temperature, salinity, and dissolved oxygen concentration. In the breakdown process of hypoxic water mass, a decrease in hypoxic water mass induced by the tidal intrusion of seawater from the bay mouth from December to March and river flow entrainment was observed. Although cooling vertical mixing is generally observed in winter in lakes and bays in a temperate zone, that was not observed in Kumihama Bay. In Kumihama Bay, a brackish lake located in the central Honshu Island (mainland Japan) along the Sea of Japan, watershed precipitation is high in winter, and rainfall and snowfall that melts in a few days immediately flow into the bay. This flow of fresh water causes strong salinity and density stratifications in the upper layer of the bay, obstructing the cooling vertical mixing. As a result, the hypoxic water mass formed in summer remained until winter. In winter, the seawater that intruded from the bay mouth lifted the bottom hypoxic water mass to the middle layer, and the hypoxic water reached the bivalve aquaculture fields.