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 depth-dependent 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 different fractions indicated an increasing degree of humification 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 profiles also indicated that humification in lake sediments consists of simultaneous degradation and polymerization processes.