Multi-month prediction of summertime hypoxia occurrence in the bottom of Funka Bay, Japan, with a focus on the wintertime surface heat flux

Hypoxic water (< 2 mL of O₂ per L), which can cause mass mortality of demersal fishes, is reported to occur in the enclosed coastal area around the world. Funka Bay, a semi-enclosed and cone-shaped bay with a maximum depth of 107 m, located in the southern part of Hokkaido, Japan, also suffers summertime bottom hypoxic water. Long-term monitoring reveals that the occurrence is greatly variable year by year, yet the controlling factor is still unknown. The present study examined the interannual variability of summertime bottom dissolved oxygen (DO) with the focus on the wintertime heat loss of the water and subsequent convection overturning using year-round moored/ship-board DO observation for the period of 2012–2019. A common feature found in the 8 years was the continuous recovery of bottom DO in winter from the previous summer, which peaked February or March. The first half of the analysis period ceased the DO recovery until February, while one month extension of the DO recovery by March in the second half. The latter tended to show less hypoxia in the following summer as its peak is higher and its DO reduction phase starts later. Wintertime surface heat flux in February created the difference; more heat was lost in the second half because of warmer water temperature under the winter atmosphere. We were successful to predict summertime bottom DO with the accuracy of 0.26 mL/L considering the wintertime heat flux and stratification. This multi-month prediction will be of great help for future fishery activity in Funka Bay.