Seasonal Variation of Benthic Nepheloid Layer in Lake Biwa

Abstract

Seasonal variation and characteristics of the benthic nepheloid layer (BNL) were investigated in Lake Biwa. During the last four years, vertical and horizontal distributions of water temperature, turbidity and electric conductivity have been obtained both in the South Basin and the southern part of the North Basin of Lake Biwa. The BNL developed in the seasons of thermal stratification, and is not detectable in the non-stratification period (winter). Turbidity in the BNL was much affected by the turbid water from rivers after heavy rainfall. In this case, the major component of the suspended substance (SS) in the BNL was inorganic soil. The particulate P concentration in the BNL, which originated from phytoplankton, also increased after a rainfall. This suggests that phytoplankton in the surface layer sinks with clay and silt entering via the rivers. The particulate Al concentration (soil origin) also increased after every heavy rainfall, but decreased soon thereafter. However, the concentration of particulate P increased consistently after heavy rain. These results show that inorganic matter sinks quickly due to its high density, and that the turbidity in the BNL is mainly maintained by organic matter such as phytoplankton under decomposition. From summer to the end of the stratification period, another kind of turbidity appeared in the bottom layer. This is caused by the chemical reaction of Mn under anoxic conditions. Resuspension of bottom sediment by strong currents also occurred, but it was not a major process for maintaining the BNL.