STUDY ON THE ANALYSIS OF THE CONTROLLING FACTORS OF HYPOXIA IN THE BOTTOM OF LAKE BIWA, JAPAN

Abstract

 Lake Biwa in Japan is a water resource utilized by approximately 14 million people in Kansai region, and its water pollution has a significant impact not only on people but also on the ecosystem, particularly in the bottom layer, where hypoxia can cause an environmental problem. Until the 1980s, eutrophication was the main cause of hypoxia of the lake bottom, and after the measures for eutrophication, it is not clear what the main cause quantitatively is such as climate change and a regime shift in the ecosystem. In the present study, seasonal and secular variations of water quality in Lake Biwa over the past 30 years were analyzed by using vertical profile data of water temperature and dissolved oxygen concentration observed at Imazu-oki, the deepest point of observation and weather data at Hikone. The results of the analysis showed that intensity of stratification increased in the surface layer of Lake Biwa, but conversely stratification weakened in the deeper layers. These changes in the physical environment of the lake have affected the decline of dissolved oxygen in the lake bottom of Lake Biwa. The seasonal changes in dissolved oxygen over 30 years could be classified into approximately three patterns, with the dominant factors being the strengthening of stratification due to the change in air temperature and wind speed, the shortening of the period of the overturning and the vertical mixing by wind during the stratified season and cooling period.