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Seasonal variation of sediment denitrif~cation and nitrification in darkness was measured by the nitrogen isotope pairing and the nitrate isotope dilution techniques in 3 different sediment types: coarse sand, fine sand and muddy sand (S, A and M, respectively) in intertidal Konigshafen, Island of Sylt, Germany. The temporal pattern of denitrification based on nitrate from the overlying water could be explained by variations in the nitrate concentration and oxygen penetration depth in all 3 sediment types. Highest rates (8 to 48 pm01 m-2 h-') were observed in April 1994 when nitrate concentration in the overlying water was 65 to 70 pM and the lowest (0.2 to 1.1 pm01 m-2 h-') in summer and fall when nitrate was depleted from the water phase. The annual mean dark denitrification, of which 69 to 15% was due to overlying water nitrate, increased in the sequence A, S and M in accord with increasing sediment oxygen uptake. AU 3 sediments types exhibited low nitrification in early winter. At S and M, maximum nitrification was measured in spring and late summer (8 to 17 and 27 to 28 pm01 m-"-', respectively), whereas nitrification was repressed in mid summer. Maximum dark nitrification here generally occurred at the time of the highest activity of the benthic microalgae (measured as oxygenbased dally gross primary production). At Stn A, maximum nitrification was measured in mid summer (26 pm01 m-2 h-'), when daily gross primary production was low. The 2 different seasonal patterns of nitrification can be explained by differences in oxygen and ammonium availability. In all sediment types, a low degree of coupling between nitrification and denitrification (in relation to total nitrification) was evident, especially during summer or autumn. Annual mean dark nitrification, of which 11 to 27 % was coupled to denitrification, was higher in the muddy sediment than in the 2 sandy sediments.

aquatic microbial ecology

Nitrification and denitrification in Wadden Sea sediments (Königshafen, Island of Sylt, Germany) as measured by nitrogen isotope pairing and isotope dilution