Heat Waves Can Cause Hypoxia in Shallow Lakes.

We assessed how warm air temperatures, high solar radiation, and weak wind speeds might induce hypoxia in a shallow lake during a heat wave. We simulated bottom‐water dissolved oxygen concentrations and compared concentrations in 2022 with the average for the previous 30 years. We found that hypoxia was most sensitive to wind speeds. When the wind speed was low, convection was insufficient to prevent hypoxia, but there was no hypoxia if the wind speed equaled the average speed during the previous 30 years. However, if solar radiation and air temperatures equaled the respective averages during the previous 30 years, hypoxia did not occur, even if wind speeds were low. We conclude that the combined effects of weak winds and either high solar radiation or air temperatures induced hypoxia during the heat wave of 2022. Plain Language Summary: The concentration of dissolved oxygen (DO) is one of the most important characteristics of lake ecosystems. However, a mechanistic understanding of the formation of bottom‐water DO in shallow water during heat waves is still limited by the paucity of relevant data. We combined high‐frequency monitoring via a new buoy system and numerical simulation to analyze how meteorological changes during a heat wave affected bottom‐water DO concentration in a shallow lake. We found that slackening of wind speeds and increases of either air temperatures or solar radiation induced bottom‐water hypoxia by constraining convective mixing. Wind speeds during the night were low, and the supply of DO by convective mixing during the night was insufficient to prevent hypoxia. Key Points: Hypoxia was observed in Lake Kasumigaura during a heat wave in 2022We observed higher solar radiation and air temperatures, and weaker winds in 2022 than the average for the previous 30 yearsInsufficient convection led to hypoxia because of the combined effects of weak winds, high solar radiation, and high air temperatures [ABSTRACT FROM AUTHOR]