Occurrence Mechanism of Dissolved Musty-Odorant 2-Methylisoborneol in the Southern Basin of Lake Biwa during 2007－2017

Musty odor is an unavoidable problem for drinking water. 2－Methylisoborneol (2－MIB) is one of the major musty odorants that has been set as a quality standard for drinking water in Japan since 2004. Between 1988 and 1994, 2－MIB-producing cyanobacteria, like Phormidium tenue and Oscillatoria tenuis, were observed at the intake basin of Keage Water Purification Plant (KWPP), which draws water from the southern basin of Lake Biwa (SBLB). However, 2－MIB occurred in the intake basin of KWPP and SBLB surface water between 2007 and 2017 (except 2013 and 2014) in the absence of 2－MIB-producing cyanobacteria. On average, 92% of total 2－MIB was detected as dissolved between 2007 and 2017, but only 60% was found as dissolved between 1988 and 1994. It is believed that the bottom water of SBLB was more contaminated with 2－MIB than the surface water, and P. tenue was found in the submerged plants at the bottom of the lake. The culture strain of P. tenue PTG, isolated in SBLB, effectively produced dissolved 2－MIB (D－2－MIB) under the light with the same photon flux density at the bottom of the lake. In conclusion, the D－2－MIB that occurred in SBLB between 2007 and 2017 was attributed to P. tenue attached to the plants that sank at the bottom of the lake.