Vertical transport caused by mixing is essential for understanding physical processes in lakes. However, mixing processes in shallow lakes are not well understood because of the lack of turbulence measurements. This study presents observations of vertical mixing and oxygen flux in a shallow lake, Lake Kitaura of the Lake Kasumigaura continuous lake system, which is located along the central eastern coast of the Japanese mainland. Mooring and microstructure surveys were conducted in August 2020. The vertical eddy diffusivity was estimated from the Ellison scale using high-frequency sampled temperature data from the mooring location, and the estimations were consistent with those observed from the microstructure profiler. The estimated eddy diffusivity revealed a mixing structure and oxygen flux in the lake during the study period. The daily cycle of stratification and mixing was caused by daily heating due to solar radiation and by winds from daily sea breezes, respectively. The daily stratification maximum occurred around noon, which suppressed vertical mixing. Vertical mixing was intensified due to sea breezes in the afternoon, which led to movement of oxygen from the surface layer to the bottom layer. The maximum vertical mixing was observed at 18:00. The oxygen concentration did not increase during nighttime when the negative surface heat flux was observed; thus, nighttime cooling may not primarily contribute to the vertical oxygen supply.