ABSTRACT: Denitrification in exposed intertidal sediment was measured for the first time at locations in the Tagus Estuary, using a new ammonium spray method. With this method, ¹⁵NH₄⁺ was sprayed on to the sediment surface, and nitrification-denitrification was subsequently estimated from accumulation of ²⁹N₂ and ³⁰N₂ in gas-tight glass chambers placed on the ¹⁵N-labeled sediment. Coupled nitrification-denitrification ranged from 8.5 to 11.5 µmol N m^-2 h^-1 at night to below 2 µmol N m^-2 h^-1 during the day. Salt stress, light inhibition and depletion of NH₄⁺ by benthic diatoms could contribute to the low daytime activity. The nighttime rates were confirmed by an independent estimate based on potential nitrification-denitrification and oxygen uptake. Coupled nitrification-denitrification activity at night during exposure was not significantly different from the activity measured when the sediment was inundated, but was 3 times lower than rates measured at a subtidal station in the estuary.

KEY WORDS: Intertidal mud flats · Exposed sediment · ¹⁵N · Denitrification

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Published in MEPS Vol. 209 (2001) on January 5
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 2001