Author(s): Yueyi Wang; Masayasu Irie; Yusuke Nakatani; Ayato Kohzu; Takao Ouchi; Miyuki Yuzawa

Linked Author(s): Yueyi Wang, Masayasu Irie

Keywords: Shallow Lake; Hypoxia; Diurnal stratification; SCHISM; Unstructured grid

Abstract: Shallow lakes, which are highly susceptible to wind, usually do not form significant seasonal stratification but diurnal stratification. Lake Kasumigaura is a large lake in Japan with an average depth of only 4m, and it is highly eutrophic. Although declining dissolved oxygen (DO) in the bottom of the lake during the weak wind period has been reported by field observations in previous studies, however, the nature of the hypoxia generation and recovery process is unclear. In order to reveal this process in the lake, an integrated approach, which combines data acquisition and numerical modeling, was followed. The analysis was conducted with the three-dimensional coastal ocean hydrodynamic model coupled with an ecological numerical model, SCHISM-ICM. The horizontal resolution of the unstructured grid is approximately 200m, and 44 layers of the LSC² grid are split vertically. Water quality data monitored at 4 automatic observation stations were used in the analyses, which indicates a time-scale correlation between temperature stratification and hypoxia water masses at the shallow lake. The observation data revealed that hypoxia events occurred a few times during summer even in the shallow area. Under the condition of weak wind (about 2 m/s), the lake water frequently shows continuous diurnal stratification. During this period, DO in the lake has obvious diurnal variation, but generally tends to decrease. And in a few days of sustained diurnal stratification, the bottom layer may enter a state of hypoxia. The simulation results demonstrated that the model has the ability to represent both the spatial and temporal patterns of observed variables. Additionally, it could have a good performance for modeling the diurnal temperature stratification and the trend of observed DO. Comparing the results of different wind scenarios supported the importance of wind field on the change of DO in shallow lakes. Simulation results indicated that the diurnal temperature stratification under the weak wind forcing reduced the vertical mixing of water masses. It leads to continuous depletion of DO in the bottom layer, inducing the occurrence of hypoxia.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221012

Year: 2022