Dynamics in composition and size-class distribution of humic substances in profundal sediments of Lake Constance

Organic matter from profundal lake sediments was fractionated and analyzed with high depth resolution, and the size-class distribution of humic substances was determined by gel-permeation chromatography. Together with a depthdependent increase of the thermal stability of the organic matter and an increasing C/N atomic ratio of the humic acids, the size-distribution patterns of di€erent fractions indicated an increasing degree of humi®cation with depth. The main stabilization processes occurred in the surface layer (0±2 cm depth), which is clearly distinguished from the deeper sediment. The depth pro®les also indicated that humi®cation in lake sediments consists of simultaneous degradation and polymerization processes. # 2001 Elsevier Science Ltd. All rights reserved.