2023 Volume 46 Issue 6 Pages 173-180
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.
