Annual benthic denitnficatlon in a shallow soft bottom lake was estimated in laboratoryIncubated sediment cores, using the isotope pairing technique Rates were < 3 0 % of the estimate calculated from nitrogen mass balance of the lake. To investigate this discrepancy, in situ measurements were performed in flexible enclosures to which 15N03-was added. Accumulation of 29N2 and 30N2 in the sedunent and water column was measured and the loss to the atmosphere was estimated from depletion of argon, added in excess to the water of the enclosures. The calculated denitrification activities in the enclosures were 6 to 26 times higher than the activlty in the incubated cores. We suggest that denitrification in situ was enhanced b y wave forces, increasing the transport of oxygen and nitrate into these soft sediments. Such transport is not simulated by conventional stirring of the water column in laboratory incubations, which consequently may underestimate the in situ activity.