ASSESSMENT OF THE IMPACT OF STRATIFICATION DEVELOPMENT BY CLIMATE CHANGE DURING THE SUMMER SEASON IN LAKE KASUMIGAURA

Abstract

 This study presents effects of global warming and climate change on water quality in Lake Kasumigaura located in the southeastern part of Ibaraki Prefecture, Japan. Meteorological and water quality data between 2005 and 2022 (18 years) were analyzed to investigate variations of vertical mixing and oxygen distributions in the lake. Air temperature increased with a ratio of approximately 0.1 degrees per year in summer and the increase in air temperature was clearly correlated with water temperature in the lake. Stratification increases as air temperatures increases due to surface heat flux toward the lake, which weakens vertical mixing/transport leading to hypoxia in the bottom boundary layer. The combination of shear due to wind stress and surface heat fluxes due to solar radiation (shortwave radiation) explains stratification resulting in hypoxia. In order to investigate effects of increase in temperature due to climate change, we examined differences in water quality between the hottest(2018) and the coolest( 2007) summer in the data set. Stratification in 2018 was stronger than that in 2007. Dissolved oxygen (DO) in the bottom layer in 2018 was smaller than in 2007. In addition, a comparison of hot summers(6 years) and cool summers(6 years) showed the same trend appeared in the comparison of the hottest and coolest summers. These findings imply that the water quality in Lake Kasumigaura would likely modulated by the increase in temperature owing to global warming. Furthermore, there is concern that increase in temperatures associated with future global warming may alter the water quality in lake Kasumigaura.