Seasonal variations and dynamics of dissolved carbohydrates in Lake Biwa

The MBTH method was used to determine total dissolved carbohydrate (DCHO) concentrations in Lake Biwa on a monthly basis during a 22 month study. Three fractions of DCHO were analyzed: monosaccharides, HCl-hydrolyzable (HCl-CHO), and HCl-resistant polysaccharides (HR-CHO). The HR-CHO fraction was determined as the concentration difference after hydrolysis by 2 M H2SO4 after swelling with 6 M H2SO4 and hydrolysis by 0.09 M HCl (i.e. HCl-CHO). Neutral aldose composition of samples from the epilimnion and hypolimnion were determined in samples collected in July 1998 and February 1999 by GC/FID and GC/MS. Low recoveries of standard carbohydrates after mild acid hydrolysis suggests that lake DOM likely contains structural polysaccharides or glycoproteins. In field observations, the HR-CHO fraction was produced and consumed rapidly by aquatic organisms. The HCl-CHO fraction was the major fraction among the DCHO in lake water, and consisted of microbially labile and refractory components. The aldose composition of the HCl-CHO fraction indicated modification of the DCHO by heterotrophic consumers. The abundance of the HCl-CHO in the lake water can be maintained by the high activity of heterotrophs.