[A-OS19] Coastal physical processes associated with mixing, eddies, internal waves

[AOS19-P03] Comparison of mixing conditions between Kitaura and Nishiura in Lake Kasumigaura during summer

*kazuki satoh¹, Eiji Masunaga¹, Tatsumi Kitamura² (1.ibaraki university, 2.Ibaraki Kasumigaura Environmental Science Center)

Keywords:Kasumigaura, Mixing, Dissolved oxygen

This study presents vertical mixing conditions during summer in two lakes, Nishiura and Kitaura, in Lake Kasumigaura from field observations using YODA Profiler and Mooring instruments. Hypoxia and eutrophication have been known as pressing issues in the study area. Despite years of efforts to improve water quality, there has been no improvement in water quality in Kitaura. There is a possibility that mixing condition influences water quality. Mooring results showed that stratification developed in both lakes during the daytime due to solar radiation. Intensified stratification disappeared during the nighttime in Nishiura, meanwhile intensified stratification continued for two or three days in Kitaura. The Richardson number was used to evaluate the mixing conditions. The averaged Richardson number in Kitaura was approximately two times higher than in Nishiura. This implies that stronger mixing occurs in Nishiura than in Kitaura. According to the data from the YODA Profiler, there is large differences between Kitaura and Nishiura in the distribution of dissolved oxygen and turbidity. Differences between upper and lower layer of both dissolved oxygen and turbidity in Kitaura were higher than those in Nishiura, which indicates that the less mixing condition in Kitaura suppress vertical transport of oxygen and suspended sediments. The difference in mixing between the two lakes can be simply explained by the size of the lakes. The width (surface area) of Kitaura is much shorter (smaller) than Nishiura. According to the Wedderbuen number, weak winds, ~ 2.5 m/s, can generate vertical mixing in Nishiura. On the other hand, wind speed of 5 m/s is desired to induce vertical mixing in Kitaura. This study suggests that the difference in mixing conditions between Kitaura and Nishiura was caused by the difference in topography (shape of lakes).