Deoxygenation of Lake Ikeda, Japan

Lake Ikeda (Kagoshima prefecture, Japan) is a deep lake with a maximum depth of 233 m. Monitoring data of lake Ikeda exist since 1975. We have analyzed the long-term variability in the water conditions of Lake Ikeda. Recently, Lake Ikeda has exhibited the phenomenon of incomplete overturning because of climate warming. The concentrations of DO (dissolved oxygen) in the deepest parts of the lake have reduced. This phenomenon was observed to have started in the 1980s, and gradually, the deepest parts of the lake became anoxic. Later, the anoxic layer became thicker. Currently, winter mixing in Lake Ikeda reaches to depths of only 100 m. According to our simple estimation, the total volume of oxygen in Lake Ikeda will reduce from approximately 70% in the mid-1980s to 40% by the end of 2010. In addition to this phenomenon, the oxygen concentration appears to vary with several years oscillations. The depths to which mixing occurs depends on the severity of the winter, such as the air temperature during the winter season. The mixing period generally occurs in February; hence, the limnological year is considered to start in February. During our analysis period, the total DO mass showed high values in 1996, 2001, and 2003. Air temperature data obtained for regions near Lake Ikeda (the station name is Ibusuki) are used to clarify the cause of the high DO mass values in the three abovementioned years. During the period prior to the occurrence of the high DO mass in February 1996, i.e., in December 1995 and January 1996, the air temperature was low. Similarly, in 2001 and 2003, the air temperature was low in January (one month before the high DO mass was observed). In January 2001 and 2003, the AO (Atlantic Oscillation) index was negative. When the AO index is negative, there tends to be a greater movement of cold polar air into mid-latitudinal regions including Japan (Yamakawa, 2005). This movement induced a low air temperature in Ibusuki, and consequently, a high DO mass was observed in Lake Ikeda. On the other hand, the AO index was negative in December 1995 and January 1996. In addition, the WP (Western Pacific) index was also negative in the winter of 1995/96. When the WP index is negative, Japan experiences a cold winter (Koide and Kodera, 1999; Yasunaka and Hanawa, 2008). Therefore, the combination of the negative phase of AO and the negative phase of WP led to the occurrence of cold surges near Lake Ikeda, which in turn resulted in the high DO mass in February 1996. When DO concentration in the deep layer of the lake becomes higher caused winter mixing, we observe also a reduction in the DO concentration in the surface layer. The DO concentration in the surface layer sometimes decreased to 70%. In future, once Lake Ikeda will deep mixing during very cold winters, the DO concentration in the surface water might reduce largely.