Denitrification in nitrate‐rich streams: Diurnal and seasonal variation related to benthic oxygen metabolism

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Abstract
Seasonal variation of chlorophyll content, photosynthesis, O2 respiration, and denitrification was measured under light and dark conditions in the sediment of a nutrient‐rich Danish lowland stream. Exponential growth of benthic microalgae was observed in early spring (April–May) and photosynthetic capacity persisted until fall. The benthic algae were a major C source for heterotrophic activity as indicated by a close correlation between O2 respiration and Chl content in the sediment. Denitrification activity was related to Chl content, NO3− availability, and O2 conditions. Diffusion from the overlying water was always the major NO3− source for denitrification. Under lighted conditions, photosynthetic O2 production increased the oxic zone and reduced denitrification activity by up to 85% in spring. A simple diffusion‐reaction model allowed denitrification rates to be estimated from O2 respiration rates and concentrations of O2 and NO3− in the stream water. Throughout the season, estimated denitrification rates correlated well with those actually measured. The model demonstrated that denitrification activity was controlled primarily by the thickness of the oxic surface layer which served as a diffusion barrier for NO3− to the denitrification zone.