KINETIC-STUDIES OF ACETATE IN FRESH-WATER SEDIMENTS - USE OF STABLE ISOTOPIC TRACERS

The kinetics of acetate uptake in the sediment of Lake Vechten, a meso-eutrophic, monomictic freshwater lake, was investigated by monitoring the disappearance of deuterated acetate tracers. Steep acetate concentration gradients were observed in the top cm when the sediment was analyzed at the millimeter scale. The gradients revealed not only the dynamic nature of the acetate pools but also indicated substantial mass transport between sediment layers. Acetate uptake could conveniently be modeled using first order reaction kinetics. Uptake rates ranged from 30 mu M day(-1) (winter) to 546 mu M day(-1) (early spring) in the sulfidogenic horizon (2 cm) and from 30 mu M day(-1) (winter) to 1176 mu M day(-1) (late summer) in the methanogenic horizon (5 cm). The integrated acetate uptake rate in the top 7 cm was 39.8 +/- 20 (2 X SE) mmol acetate m(-2) day(-1) during early spring. More than 40% of the total acetate pool was reversibly adsorbed in oxic sediment slurries. The presence of hydrous Fe(III) oxides appeared to determine the occurrence of acetate adsorption.