Carbohydrates have been shown to attain 25% of the organic matter pool during periods of high productivity in a small eutrophic lake. The chemical nature of the organic matter in the water column, as determined by its source, had an important influence on its role in the coagulation and aggregation of inorganic colloids. During periods of high productivity, inorganic colloids were observed to be depleted at depths of maximum polysaccharide concentrations. Furthermore, in field experiments, depletion rates of colloidal particles were correlated to the presence of aquagenic biopolymers. In controlled laboratory experiments, the coagulation rate of colloidal montmorillonite was increased in the presence of 2.0 or 4.0 mg C liter⁻¹ of aquagenic biopolymers. Sedimentation also increased with increasing biopolymer concentrations (0–2.3 mg C liter⁻¹). On the other hand, 0.1–6.3 mg C liter⁻¹ of pedogenically derived fulvic acids were shown to stabilize colloidal solutions, in agreement with previous studies.