Denitrification in stream epilithon: Seasonal variation in Gelbæk and Rabis Bæk, Denmark

Seasonal variations of denitrification activity were determined and compared with various environmental parameters in undisrupted epilithon communities (biofilm) from two Danish lowland streams. In the nutrient-rich Gelbæk, denitrification activity followed extensive changes in biofilm thickness, dry weight and chlorophyll a content during the season. The absolute maximum of denitrification (1.4 mmol N m⁻² d⁻¹, dark incubation) was recorded when the biofilm was best developed in the spring (April and May). Activity decreased dramatically after most of the biofilm suddenly disintegrated and peeled off in early summer. Photosynthetic O₂ production was an important controlling factor of denitrification on a diurnal scale, since the activities were always 2–3 fold lower in the light (50 μE m⁻² s⁻¹) than in the dark. In the more nutrient-poor Rabis Bæk, the biofilm was much less developed and denitrification activities were relatively small (maximum of 0.2 mmol N m⁻² d⁻¹ under dark incubation). The results indicate that productivity of the micro-algae regulates both the seasonal and diurnal patterns of denitrification in the biofilms.