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

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.