Japan Geoscience Union Meeting 2022

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[J] Oral

A (Atmospheric and Hydrospheric Sciences ) » A-CG Complex & General

[A-CG45] Water and sediment dynamics from land to coastal zones

Thu. May 26, 2022 9:00 AM - 10:30 AM Exhibition Hall Special Setting (2) (Exhibition Hall 8, Makuhari Messe)

convener:Dai Yamazaki(Institute of Industrial Sciences, The University of Tokyo), convener:Shinichiro Kida(Research Institute for Applied Mechanics, Kyushu University), Yuko Asano(The University of Tokyo), convener:Keiko Udo(International Research Institute of Disaster Science, Tohoku University), Chairperson:Shinichiro Kida(Research Institute for Applied Mechanics, Kyushu University), Keiko Udo(International Research Institute of Disaster Science, Tohoku University)

9:15 AM - 9:30 AM

[ACG45-02] Observations of vertical mixing and associated transport in a shallow lake, Lake Kasumigaura, Japan

★Invited Papers

*Eiji Masunaga1, Tastumi Kitamura2, Sachihiko Itoh3 (1.Ibaraki University, 2.Ibaraki Kasumigaura Environmental Science Center, 3.The University of Tokyo)

Keywords:Mixing, Lakes, dissolved oxygen, nutrient

This study presents field observations of vertical mixing and associated transport in Lake Kasumigaura during summer. We conducted field campaign in the lake using acoustic doppler current profilers (ADCPs), high-frequency sampling thermistor chain, tow-yo instrument (YODA Profiler) and microstructure profiler (VMP-250) in summer, 2020 and 2021. In addition to the measurements in physical structures, the concentration of nutrients (N and P) was obtained from water samples from the lake. The stratification was enhanced by day time solar radiation, which results to less mixing and less vertical oxygen supply into the bottom layer. The vertical eddy diffusivity was quite low under stable stratified conditions, Kr = O(10-6) m2 s-1 and reached O(10-3) m2 s-1 under strong wind conditions. The wind speed was enhanced in the afternoon (12:00-18:00) due to daily-cycle sea breeze, which results in strong vertical mixing. The conventional mixing/stability parameters of Richardson number and Monin-Obukhov length scale well reparent mixing conditions. The phosphorus concentration (TP and PO4P)increased as dissolved oxygen (DO) decreases in the bottom layer. This implies that hypoxia under less mixing leads to phosphorus release from sediments. On the other hand, the nitrate concentration did not related with the DO concentration. This study suggest that vertical mixing plays an important role in oxygen and nutrient supply in the lake.