Аннотация:Seasonal variation of chlorophyll content, photosynthesis, O 2 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 O 2 respiration and Chl content in the sediment. Denitrification activity was related to Chl content, NO 3 − availability, and O 2 conditions. Diffusion from the overlying water was always the major NO 3 − source for denitrification. Under lighted conditions, photosynthetic O 2 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 O 2 respiration rates and concentrations of O 2 and NO 3 − 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 NO 3 − to the denitrification zone.
