Denitrification and photosynthesis in stream sediment studied with microsensor and wholecore techniques

The effect of light on benthic photosynthesis, denitrification, and assimilation of NH 4 + and NO 3 − in stream sediments was studied with whole‐core techniques and with O 2 and N 2 O microsensors. Photosynthetic oxygen production increased the thickness of the aerobic surface layer from 1.5 mm in the dark ∼3.5 mm at a light intensity saturating photosynthesis. The O 2 flux changed concurrently from net uptake to net release and the overall rate of denitrification was reduced by 70%. Denitrification was always restricted to a narrow zone immediately below the aerobic‐anaerobic interface. Calculated NO 3 − microprofiles showed that overall denitrification was primarily dependent on the thickness of the aerobic layer which acted as a barrier for diffusion of NO 3 − from the overlying water. In the light, algal NO 3 − assimilation could exceed NO 3 − consumption by denitrification when availability of NH 4 + was low. Assimilation of NO 3 − , however, had no influence on the flux of NO 3 − to the denitrification zone, since assimilation occurred relatively close to the sediment surface.