Eddy-covariance measurements of turbulent fluxes across the oxygen-depleted benthic layer in a shallow stratified lake

Seasonal stratification hinders the ventilation of hypolimnion water, leading to the formation of an anaerobic benthic layer in shallow lakes. This study quantifies the processes of hypoxia formation and recovery in Lake Kasumigaura, Japan, under conditions of developing metalimnion during early summer, specifically from early May to mid-July 2022. We conducted direct observations of vertical turbulent fluxes of dissolved oxygen (DO), as well as heat and momentum fluxes using an eddy-covariance system (ECS), enabling simultaneous measurements of various variables near a sampling volume. Additionally, autonomous acquisition of vertical profiles for DO, temperature, and Chlorophyll a in the lake's vicinity was performed. Our observations revealed that thermal stratification temporarily developed, leading to hypoxia near the bottom when near-surface air temperature exceeded 25 degrees C. During the metalimnetic period under stable stratification, the ECS demonstrated downward transport of heat and DO, resulting in daily averages of approximately 30-40 W m-2 and 1-1.5 x 10-5 mg L-1 m s-1 (equivalent to 1.1-1.7 mmol m-2 h-1), respectively. Using ECS observations, we estimated a turbulent diffusivity of approximately 5 x 10-6 m2 s-1 during enhanced stratification, consistent with previous reports. Under weak stratification in mid-May, the ECS indicated enhanced vertical fluxes, with occasional upward DO flux. We propose the possibility that phytoplankton acted as oxygen suppliers through photosynthesis activity, promoting vertical DO transfer.