DIURNAL-VARIATION OF DENITRIFICATION AND NITRIFICATION IN SEDIMENTS COLONIZED BY BENTHIC MICROPHYTES

Diurnal variation of denitrification in sediments with benthic microph ytes was investigated by a N-15 isotope-pairing technique in order to distinguish between coupled nitrification-denitrification and denitrif ication of nitrate supplied from the water column. Sediments were incu bated in a continuous flow-through system and exposed to diurnal light and dark cycles. Illumination of both limnetic and estuarine sediment s doubled the rate of coupled nitrification-denitrification but reduce d the rate of denitrification of nitrate supplied from the water colum n by approximately 50%. Photosynthesis in the uppermost sediment layer s during illumination led to deeper oxygen penetration and a resultant stimulation of nitrification. This stimulation may explain the increa sed rate of coupled nitrification-denitrification during illumination. The rate of denitrification of nitrate supplied from the water column was reduced during illumination due to a longer diffusion path betwee n water and the anoxic denitrification zone.