Influence of the diffusive boundary layer on solute dynamics in the sediments of a seiche-driven lake: A model study

The diffusive boundary layer (DBL) plays an important role in the transport of electron acceptors for mineralization and oxidation processes in highly reactive sediments. We used transient numerical modeling to characterize the effects of the DBL thickness on solute dynamics in the sediments of Lake Alpnach. Our model study shows that the DBL mainly influences short-term sedimentary denitrification by resisting transport. The DBL also governs the reoxidation of reduced compounds by controlling the oxygen penetration depth in the sediment. An increase of the DBL thickness from 0.25 to 1.5 mm diminished the oxygen flux into the sediment by more than 30% from 15 to 9.5 mmol m(-2) d(-1). At the same time, this change in DBL thickness had contrasting effects on the reoxidation of reduced solutes released in the anoxic sediment layers: While the rates of Fe(II) and Mn(II) oxidation decreased by up to 60%, the oxidation of methane changed by only 2%. Still, the contribution to the total oxygen uptake by these redox processes never exceeded 40%. Denitrification rates under steady state conditions were only 8% slower when the DBL was extended from 0.25 to 1.5 mm. The decreased nitrate supply was partially compensated by a stimulated denitrification process due to the lower oxygen penetration. However, fluxes of nitrogen species periodically deviated by more than 60% when an oscillating DBL thickness with periods of less than 6 h was modeled.